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2.605-2.656.167-.429.414-1.055.644-1.517a10.195 10.195 0 0 1-.052-1.086c0-1.331.282-2.499 1.132-3.368.397-.406.89-.717 1.474-.952 1.399-1.136 3.392-2.093 6.122-2.093 2.731 0 4.767.957 6.166 2.093.584.235 1.077.546 1.474.952.85.869 1.132 2.037 1.132 3.368 0 .368-.014.733-.052 1.086.23.462.477 1.088.644 1.517 1.258.364 2.233 1.721 2.605 2.656a.832.832 0 0 1 .053.22v2.869a.641.641 0 0 1-.078.256ZM12.172 11h-.344a4.323 4.323 0 0 1-.355.508C10.703 12.455 9.555 13 7.965 13c-1.725 0-2.989-.359-3.782-1.259a2.005 2.005 0 0 1-.085-.104L4 11.741v6.585c1.435.779 4.514 2.179 8 2.179 3.486 0 6.565-1.4 8-2.179v-6.585l-.098-.104s-.033.045-.085.104c-.793.9-2.057 1.259-3.782 1.259-1.59 0-2.738-.545-3.508-1.492a4.323 4.323 0 0 1-.355-.508h-.016.016Zm.641-2.935c.136 1.057.403 1.913.878 2.497.442.544 1.134.938 2.344.938 1.573 0 2.292-.337 2.657-.751.384-.435.558-1.15.558-2.361 0-1.14-.243-1.847-.705-2.319-.477-.488-1.319-.862-2.824-1.025-1.487-.161-2.192.138-2.533.529-.269.307-.437.808-.438 1.578v.021c0 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1-.427.177L13.03 4.03 9.28 7.78a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042l3.75-3.75-1.543-1.543A.25.25 0 0 1 10.604 1Z"></path> </svg> </a></li> </ul> </div> </div> </div> </li> <li class="HeaderMenu-item position-relative flex-wrap flex-justify-between flex-items-center d-block d-lg-flex flex-lg-nowrap flex-lg-items-center js-details-container js-header-menu-item"> <button type="button" class="HeaderMenu-link border-0 width-full width-lg-auto px-0 px-lg-2 py-lg-2 no-wrap d-flex flex-items-center flex-justify-between js-details-target" aria-expanded="false"> Solutions <svg opacity="0.5" aria-hidden="true" height="16" viewBox="0 0 16 16" version="1.1" width="16" data-view-component="true" class="octicon octicon-chevron-down HeaderMenu-icon ml-1"> <path d="M12.78 5.22a.749.749 0 0 1 0 1.06l-4.25 4.25a.749.749 0 0 1-1.06 0L3.22 6.28a.749.749 0 1 1 1.06-1.06L8 8.939l3.72-3.719a.749.749 0 0 1 1.06 0Z"></path> </svg> </button> <div class="HeaderMenu-dropdown dropdown-menu rounded 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communication","anchor":"channels-for-communication","htmlText":"Channels for communication"},{"level":3,"text":"Kick-off meetings","anchor":"kick-off-meetings","htmlText":"Kick-off meetings"},{"level":2,"text":"Implementation and testing","anchor":"implementation-and-testing","htmlText":"Implementation and testing"},{"level":3,"text":"Feature flag","anchor":"feature-flag","htmlText":"Feature flag"},{"level":4,"text":"How to add a new language feature flag","anchor":"how-to-add-a-new-language-feature-flag","htmlText":"How to add a new language feature flag"},{"level":3,"text":"Language testing","anchor":"language-testing","htmlText":"Language testing"},{"level":4,"text":"To enable the feature in a language test","anchor":"to-enable-the-feature-in-a-language-test","htmlText":"To enable the feature in a language test"},{"level":4,"text":"To disable the feature in a language test","anchor":"to-disable-the-feature-in-a-language-test","htmlText":"To disable the feature in a language test"},{"level":3,"text":"Google3 testing","anchor":"google3-testing","htmlText":"Google3 testing"},{"level":3,"text":"Implementation work","anchor":"implementation-work","htmlText":"Implementation work"},{"level":2,"text":"Migrations","anchor":"migrations","htmlText":"Migrations"},{"level":2,"text":"Shipping","anchor":"shipping","htmlText":"Shipping"},{"level":3,"text":"After shipping","anchor":"after-shipping","htmlText":"After shipping"}],"lineInfo":{"truncatedLoc":"350","truncatedSloc":"270"},"mode":"file"},"image":false,"isCodeownersFile":null,"isPlain":false,"isValidLegacyIssueTemplate":false,"issueTemplate":null,"discussionTemplate":null,"language":"Markdown","languageID":222,"large":false,"planSupportInfo":{"repoIsFork":null,"repoOwnedByCurrentUser":null,"requestFullPath":"/dart-lang/language/blob/main/doc/life_of_a_language_feature.md","showFreeOrgGatedFeatureMessage":null,"showPlanSupportBanner":null,"upgradeDataAttributes":null,"upgradePath":null},"publishBannersInfo":{"dismissActionNoticePath":"/settings/dismiss-notice/publish_action_from_dockerfile","releasePath":"/dart-lang/language/releases/new?marketplace=true","showPublishActionBanner":false},"rawBlobUrl":"https://github.com/dart-lang/language/raw/refs/heads/main/doc/life_of_a_language_feature.md","renderImageOrRaw":false,"richText":"\u003carticle class=\"markdown-body entry-content container-lg\" itemprop=\"text\"\u003e\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch1 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eLifecycle of a language feature\u003c/h1\u003e\u003ca id=\"user-content-lifecycle-of-a-language-feature\" class=\"anchor\" aria-label=\"Permalink: Lifecycle of a language feature\" href=\"#lifecycle-of-a-language-feature\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis document describes the process from creation to the shipping and release of\na language feature. This document serves as a walkthrough for what's needed to\nbuild a language feature.\u003c/p\u003e\n\u003col dir=\"auto\"\u003e\n\u003cli\u003e\u003ca href=\"https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#user-issues-and-feature-requests\"\u003eUser issues and feature requests\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#design-feedback-and-iteration\"\u003eDesign, feedback, and iteration\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#acceptance\"\u003eAcceptance\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#team-communication\"\u003eTeam communication\u003c/a\u003e (eg. kick-off meetings)\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#implementation-and-testing\"\u003eImplementation and testing\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#migrations\"\u003eMigrations\u003c/a\u003e\u003c/li\u003e\n\u003cli\u003e\u003ca href=\"https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#shipping\"\u003eShipping\u003c/a\u003e\u003c/li\u003e\n\u003c/ol\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eUser issues and feature requests\u003c/h2\u003e\u003ca id=\"user-content-user-issues-and-feature-requests\" class=\"anchor\" aria-label=\"Permalink: User issues and feature requests\" href=\"#user-issues-and-feature-requests\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eFeatures arise from perceived user issues or feature requests. These issues are\ndocumentation of the user issue to be solved.\u003c/p\u003e\n\u003cdiv class=\"markdown-alert markdown-alert-note\" dir=\"auto\"\u003e\u003cp class=\"markdown-alert-title\" dir=\"auto\"\u003e\u003csvg class=\"octicon octicon-info mr-2\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z\"\u003e\u003c/path\u003e\u003c/svg\u003eNote\u003c/p\u003e\u003cp dir=\"auto\"\u003eThese feature requests are filed in the \u003ca href=\"https://github.com/dart-lang/language/issues/new?labels=request\"\u003elanguage repo\u003c/a\u003e,\nand are labelled \u003ca href=\"https://github.com/dart-lang/language/labels/request\"\u003e\u003ccode\u003erequest\u003c/code\u003e\u003c/a\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWe may close issues that we believe we won't address for whatever reason. We may\nalso keep issues open indefinitely if we believe they are something that we\nmight want to address in the future.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eHow does the team choose which feature request to accept?\u003c/h3\u003e\u003ca id=\"user-content-how-does-the-team-choose-which-feature-request-to-accept\" class=\"anchor\" aria-label=\"Permalink: How does the team choose which feature request to accept?\" href=\"#how-does-the-team-choose-which-feature-request-to-accept\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThere are many factors that determine why we might choose to do one feature over\nanother, such as feasibility, difficulty, or popularity. Often, we will take a\nlook through the most 👍-reacted issues to see what our users are interested in.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eDesign, feedback, and iteration\u003c/h2\u003e\u003ca id=\"user-content-design-feedback-and-iteration\" class=\"anchor\" aria-label=\"Permalink: Design, feedback, and iteration\" href=\"#design-feedback-and-iteration\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eInterested parties may propose several (competing) language features to a\nsingle request/problem.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eA \"language feature\" issue (labelled\n\u003ca href=\"https://github.com/dart-lang/language/labels/feature\"\u003e\u003ccode\u003efeature\u003c/code\u003e\u003c/a\u003e)\nwill be created for tracking the solution to the user issue/problem or feature\nunder consideration.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eProposal checklist\u003c/h3\u003e\u003ca id=\"user-content-proposal-checklist\" class=\"anchor\" aria-label=\"Permalink: Proposal checklist\" href=\"#proposal-checklist\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cul class=\"contains-task-list\"\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e An issue labelled \u003ccode\u003efeature\u003c/code\u003e for discussion of a proposed solution.\n\u003cul class=\"contains-task-list\"\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Links to and from the \u003ccode\u003erequest\u003c/code\u003e issue they are trying to address.\u003c/li\u003e\n\u003c/ul\u003e\n\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e A link to an initial feature specification writeup of the proposed\nsolution in the \u003ccode\u003efeature\u003c/code\u003e issue.\n\u003cul class=\"contains-task-list\"\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Located in a sub-directory with the name of the feature (\u003ccode\u003e/ [feature-name]/\u003c/code\u003e), inside the\n\u003ca href=\"https://github.com/dart-lang/language/tree/master/working\"\u003e\u003ccode\u003eworking\u003c/code\u003e\u003c/a\u003e\ndirectory of the language repository.\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e File is named \u003ccode\u003efeature-specification.md\u003c/code\u003e.\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003eAll written plans, specs, and materials must be written using \u003ca href=\"https://guides.github.com/features/mastering-markdown/#GitHub-flavored-markdown\"\u003eGitHub\nMarkdown format\u003c/a\u003e format, and must use the \u003ccode\u003e.md\u003c/code\u003e extension.\u003c/li\u003e\n\u003c/ul\u003e\n\u003c/li\u003e\n\u003c/ul\u003e\n\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp dir=\"auto\"\u003eProposals may be iterated on in-place.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor smaller and non-controversial features, we will sometimes skip this step and\nproceed directly to the \u003ca href=\"#acceptance\"\u003eAcceptance\u003c/a\u003e phase.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eExternal (outside of the language team) feedback\u003c/h3\u003e\u003ca id=\"user-content-external-outside-of-the-language-team-feedback\" class=\"anchor\" aria-label=\"Permalink: External (outside of the language team) feedback\" href=\"#external-outside-of-the-language-team-feedback\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWe use the Github issue tracker as the primary place for accepting feedback on\nproposals, solicited or unsolicited.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eIf we solicit feedback, we will open an issue for discussion and feedback. We\nhighly recommend filing and splitting off different issues for different threads\nof discussion. We generally expect to formally solicit at least one round of\nfeedback on significant changes.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eAcceptance\u003c/h2\u003e\u003ca id=\"user-content-acceptance\" class=\"anchor\" aria-label=\"Permalink: Acceptance\" href=\"#acceptance\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIf consensus is reached on a specific proposal and we decide to accept it, a\nmember of the language team will be chosen to shepherd the implementation.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe implementation will be tracked via two artifacts:\u003c/p\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003eThe feature specification document\u003c/li\u003e\n\u003cli\u003eThe feature project\u003c/li\u003e\n\u003c/ul\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eFeature specification\u003c/h3\u003e\u003ca id=\"user-content-feature-specification\" class=\"anchor\" aria-label=\"Permalink: Feature specification\" href=\"#feature-specification\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe 'feature specification' is \u003cstrong\u003ea single canonical writeup of the language\nfeature\u003c/strong\u003e which serves as the implementation reference. The feature\nspecification should follow the \u003ca href=\"https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#proposal-checklist\"\u003eProposal Checklist\u003c/a\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eOnce we've chosen a specification, we'll update it throughout the implementation\nprocess. For example, we could add new information to the specification if we\nencounter edge-cases we didn't think of initially or remove parts of it if a\ncertain behavior isn't feasible to implement.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eFeature project\u003c/h3\u003e\u003ca id=\"user-content-feature-project\" class=\"anchor\" aria-label=\"Permalink: Feature project\" href=\"#feature-project\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWe'll \u003ca href=\"https://github.com/orgs/dart-lang/projects\"\u003ecreate a project\u003c/a\u003e in\ndart-lang with the title \u003ccode\u003e[Language] \u0026lt;feature-name\u0026gt;\u003c/code\u003e. This project will link to\nand from all the Github issues that we need to track to complete and ship the\nfeature.\u003c/p\u003e\n\u003cdiv class=\"markdown-alert markdown-alert-tip\" dir=\"auto\"\u003e\u003cp class=\"markdown-alert-title\" dir=\"auto\"\u003e\u003csvg class=\"octicon octicon-light-bulb mr-2\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"M8 1.5c-2.363 0-4 1.69-4 3.75 0 .984.424 1.625.984 2.304l.214.253c.223.264.47.556.673.848.284.411.537.896.621 1.49a.75.75 0 0 1-1.484.211c-.04-.282-.163-.547-.37-.847a8.456 8.456 0 0 0-.542-.68c-.084-.1-.173-.205-.268-.32C3.201 7.75 2.5 6.766 2.5 5.25 2.5 2.31 4.863 0 8 0s5.5 2.31 5.5 5.25c0 1.516-.701 2.5-1.328 3.259-.095.115-.184.22-.268.319-.207.245-.383.453-.541.681-.208.3-.33.565-.37.847a.751.751 0 0 1-1.485-.212c.084-.593.337-1.078.621-1.489.203-.292.45-.584.673-.848.075-.088.147-.173.213-.253.561-.679.985-1.32.985-2.304 0-2.06-1.637-3.75-4-3.75ZM5.75 12h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1 0-1.5ZM6 15.25a.75.75 0 0 1 .75-.75h2.5a.75.75 0 0 1 0 1.5h-2.5a.75.75 0 0 1-.75-.75Z\"\u003e\u003c/path\u003e\u003c/svg\u003eTip\u003c/p\u003e\u003cp dir=\"auto\"\u003eGenerate all the issues for the project automatically with\n\u003ca href=\"https://github.com/itsjustkevin/aviary\"\u003ehttps://github.com/itsjustkevin/aviary\u003c/a\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThis script generates all the issues that need to be tracked and added to the\nproject. If there are any issues or implementation areas that are unnecessary,\nyou can close them out immediately, but each issue is worth thinking through.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eMeta implementation issue\u003c/h4\u003e\u003ca id=\"user-content-meta-implementation-issue\" class=\"anchor\" aria-label=\"Permalink: Meta implementation issue\" href=\"#meta-implementation-issue\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe meta implementation issue (labelled \u003ccode\u003eimplementation\u003c/code\u003e) will be filed in the\nlanguage repository for tracking the implementation process.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThis issue must contain links to:\u003c/p\u003e\n\u003cul class=\"contains-task-list\"\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e The related \u003ccode\u003erequest\u003c/code\u003e and \u003ccode\u003efeature\u003c/code\u003e issues (if they exist)\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e A link to the feature specification\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e All implementation issues\u003c/li\u003e\n\u003c/ul\u003e\n\u003cdiv class=\"markdown-alert markdown-alert-note\" dir=\"auto\"\u003e\u003cp class=\"markdown-alert-title\" dir=\"auto\"\u003e\u003csvg class=\"octicon octicon-info mr-2\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z\"\u003e\u003c/path\u003e\u003c/svg\u003eNote\u003c/p\u003e\u003cp dir=\"auto\"\u003eOnce you're done making the implementation issue, copy the link of the issue and add it to the \u003ca href=\"https://github.com/orgs/flutter/projects/82/views/1\"\u003eDash Feature Tracker board\u003c/a\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eTeam communication\u003c/h2\u003e\u003ca id=\"user-content-team-communication\" class=\"anchor\" aria-label=\"Permalink: Team communication\" href=\"#team-communication\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eOne of the most important parts of the language feature process is communicating\nwith all the implementation teams and any other stakeholders.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eChannels for communication\u003c/h3\u003e\u003ca id=\"user-content-channels-for-communication\" class=\"anchor\" aria-label=\"Permalink: Channels for communication\" href=\"#channels-for-communication\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWe'll send out an announcement email to the entire team, letting them know that\nwork for the feature is beginning.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eWe may create a mailing list or a Google chat group to organize communication\nbetween the different teams working on the language feature.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eKick-off meetings\u003c/h3\u003e\u003ca id=\"user-content-kick-off-meetings\" class=\"anchor\" aria-label=\"Permalink: Kick-off meetings\" href=\"#kick-off-meetings\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWe need to get sign-off from all of the relevant implementation teams indicating\nthat they understand the proposal, believe that it can reasonably be\nimplemented, and feel that they have sufficient details to proceed. We do this\nin the form of kick-off meetings.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eSince feature specification documents are usually long and very detailed, we\nlike to begin this sign-off process with a set of kick-off meetings, typically\none for each affected implementation team. These meetings are on opportunity for\na language team representative to present a broad outline of the new feature,\ngive some concrete examples, and collect insights from the implementation teams\nabout what aspects of the implementation might need special attention.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eGet sign-off from the following teams:\u003c/p\u003e\n\u003cul class=\"contains-task-list\"\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Analyzer team\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e CFE team\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Back-end teams:\n\u003cul class=\"contains-task-list\"\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Dart VM Team\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Dart Web Team\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Dart2Wasm Team\u003c/li\u003e\n\u003c/ul\u003e\n\u003c/li\u003e\n\u003c/ul\u003e\n\u003cdiv class=\"markdown-alert markdown-alert-note\" dir=\"auto\"\u003e\u003cp class=\"markdown-alert-title\" dir=\"auto\"\u003e\u003csvg class=\"octicon octicon-info mr-2\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z\"\u003e\u003c/path\u003e\u003c/svg\u003eNote\u003c/p\u003e\u003cp dir=\"auto\"\u003eIf a feature consists entirely of a new piece of syntactic sugar, it can be\ntempting to assume that it's not necessary to consult with any back-end teams\nsince the CFE will lower the new syntax into a kernel form that is already\nsupported, but this can be a dangerous assumption. It's easy to forget that\neven in features that appear to consist purely of syntactic sugar, some\nback-end support may be needed in order to properly support single-step\ndebugging or hot reload.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAlso, some work may need to be done on back-end code generators in order to\nmake sure that the new feature is lowered into a form that can be well\noptimized.\u003c/p\u003e\n\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eTo avoid missing things, we prefer to err on the side of asking teams whether\nthey're affected, rather than assuming they won't be.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eImplementation and testing\u003c/h2\u003e\u003ca id=\"user-content-implementation-and-testing\" class=\"anchor\" aria-label=\"Permalink: Implementation and testing\" href=\"#implementation-and-testing\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eFeature flag\u003c/h3\u003e\u003ca id=\"user-content-feature-flag\" class=\"anchor\" aria-label=\"Permalink: Feature flag\" href=\"#feature-flag\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eMost new language features should be implemented using a feature flag. The\nfeature flag serves several purposes:\u003c/p\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003eIt allows the feature to be implemented over a series of CLs without\ndestabilizing the SDK. If it comes time to ship a new release of the SDK\nbefore the feature is ready, the feature flag can remain off, so users won't\nbe exposed to a partially-implemented or buggy feature before it's ready.\u003c/li\u003e\n\u003cli\u003eIt allows users to opt in to trying out features that have not yet been\nreleased, by turning on the feature flag locally.\u003c/li\u003e\n\u003cli\u003eOnce the feature is enabled, the feature flag logic ensures that it will only\naffect users who have set their language version to a version that includes\nthe new feature. This is especially important for package developers who may\nwant to support a range of versions of the Dart SDK.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cdiv class=\"markdown-alert markdown-alert-note\" dir=\"auto\"\u003e\u003cp class=\"markdown-alert-title\" dir=\"auto\"\u003e\u003csvg class=\"octicon octicon-info mr-2\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z\"\u003e\u003c/path\u003e\u003c/svg\u003eNote\u003c/p\u003e\u003cp dir=\"auto\"\u003eImplementing a language feature using a feature flag is frequently\nmore work than implementing it without a feature flag, since the compiler and\nanalyzer must faithfully implement both the old and new behaviors.\nOccasionally the language team may decide that the benefits of using a feature\nflag don't justify this extra work. But this is a rare scenario. If you are\nworking on a feature and believe you don't need to use a feature flag, please\nconsult with the language team to be sure.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eHow to add a new language feature flag\u003c/h4\u003e\u003ca id=\"user-content-how-to-add-a-new-language-feature-flag\" class=\"anchor\" aria-label=\"Permalink: How to add a new language feature flag\" href=\"#how-to-add-a-new-language-feature-flag\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cul class=\"contains-task-list\"\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Add an entry to \u003ccode\u003etools/experimental_features.yaml\u003c/code\u003e describing the feature, in\nthe top section (above the line that says \u003ccode\u003eFlags below this line are shipped\u003c/code\u003e).\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Run \u003ccode\u003edart pkg/front_end/tool/fasta.dart generate-experimental-flags\u003c/code\u003e to update\n\u003ccode\u003epkg/_fe_analyzer_shared/lib/src/experiments/flags.dart\u003c/code\u003e and\n\u003ccode\u003epkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart\u003c/code\u003e.\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Run \u003ccode\u003edart pkg/analyzer/tool/experiments/generate.dart\u003c/code\u003e to update\n\u003ccode\u003epkg/analyzer/lib/src/dart/analysis/experiments.g.dart\u003c/code\u003e.\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Add a static final declaration to the \u003ccode\u003eFeature\u003c/code\u003e class in\n\u003ccode\u003epkg/analyzer/lib/dart/analysis/features.dart\u003c/code\u003e.\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Increment the value of \u003ccode\u003eAnalysisDriver.DATA_VERSION\u003c/code\u003e in\n\u003ccode\u003epkg/analyzer/lib/src/dart/analysis/driver.dart\u003c/code\u003e.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp dir=\"auto\"\u003eExample CL: \u003ca href=\"https://dart-review.googlesource.com/c/sdk/+/365545\" rel=\"nofollow\"\u003ehttps://dart-review.googlesource.com/c/sdk/+/365545\u003c/a\u003e\u003c/p\u003e\n\u003cdiv class=\"markdown-alert markdown-alert-tip\" dir=\"auto\"\u003e\u003cp class=\"markdown-alert-title\" dir=\"auto\"\u003e\u003csvg class=\"octicon octicon-light-bulb mr-2\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"M8 1.5c-2.363 0-4 1.69-4 3.75 0 .984.424 1.625.984 2.304l.214.253c.223.264.47.556.673.848.284.411.537.896.621 1.49a.75.75 0 0 1-1.484.211c-.04-.282-.163-.547-.37-.847a8.456 8.456 0 0 0-.542-.68c-.084-.1-.173-.205-.268-.32C3.201 7.75 2.5 6.766 2.5 5.25 2.5 2.31 4.863 0 8 0s5.5 2.31 5.5 5.25c0 1.516-.701 2.5-1.328 3.259-.095.115-.184.22-.268.319-.207.245-.383.453-.541.681-.208.3-.33.565-.37.847a.751.751 0 0 1-1.485-.212c.084-.593.337-1.078.621-1.489.203-.292.45-.584.673-.848.075-.088.147-.173.213-.253.561-.679.985-1.32.985-2.304 0-2.06-1.637-3.75-4-3.75ZM5.75 12h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1 0-1.5ZM6 15.25a.75.75 0 0 1 .75-.75h2.5a.75.75 0 0 1 0 1.5h-2.5a.75.75 0 0 1-.75-.75Z\"\u003e\u003c/path\u003e\u003c/svg\u003eTip\u003c/p\u003e\u003cp dir=\"auto\"\u003eKeep a WIP CL which enables your feature flag by default in the latest\nversion. Rebase it occasionally to get a good idea of how your feature breaks\nthe SDK and other dependencies. This is very helpful for getting insight on\nwhat migrations and breaking changes you'll be making.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eLanguage testing\u003c/h3\u003e\u003ca id=\"user-content-language-testing\" class=\"anchor\" aria-label=\"Permalink: Language testing\" href=\"#language-testing\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe language team will generally write a preliminary set of language tests for a\nfeature, in the SDK's \u003ccode\u003etests/language\u003c/code\u003e subdirectory. These tests are not\nintended to be exhaustive, but should illustrate and exercise important and\nnon-obvious features. Implementation teams are encouraged to write additional\nlanguage or unit tests. The language team may also coordinate the writing of\nadditional tests.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAn important use case for any new language feature is to ensure that the feature\nisn't accidentally used by package authors that have not yet opted into a\nlanguage version that supports it. So in addition to testing that the new\nlanguage feature works when the feature flag is turned on, the tests added to\n\u003ccode\u003etests/language\u003c/code\u003e should verify that when the feature flag is turned off, the\nprevious behavior of the SDK is preserved.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eOne important exception to preserving previous SDK behavior is that it's\npermissible (and even encouraged) to improve error messages so that if the user\ntries to use a disabled language feature, they receive an error explaining that\nthey need to enable it, rather than, say, a confusing set of parse errors.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eTo enable the feature in a language test\u003c/h4\u003e\u003ca id=\"user-content-to-enable-the-feature-in-a-language-test\" class=\"anchor\" aria-label=\"Permalink: To enable the feature in a language test\" href=\"#to-enable-the-feature-in-a-language-test\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"// SharedOptions=--enable-experiment=$FEATURE\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c\"\u003e// SharedOptions=--enable-experiment=$FEATURE\u003c/span\u003e\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eInclude this comment near the top of the test (before the\nfirst directive), where \u003ccode\u003e$FEATURE\u003c/code\u003e is replaced with the feature name.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eTo disable the feature in a language test\u003c/h4\u003e\u003ca id=\"user-content-to-disable-the-feature-in-a-language-test\" class=\"anchor\" aria-label=\"Permalink: To disable the feature in a language test\" href=\"#to-disable-the-feature-in-a-language-test\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cdiv class=\"highlight highlight-source-dart notranslate position-relative overflow-auto\" dir=\"auto\" data-snippet-clipboard-copy-content=\"// @dart=$VERSION\"\u003e\u003cpre\u003e\u003cspan class=\"pl-c\"\u003e// @dart=$VERSION\u003c/span\u003e\u003c/pre\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eInclude this comment near the top of the test (before the first directive),\nwhere \u003ccode\u003e$VERSION\u003c/code\u003e is the current stable release of Dart (and therefore is the\nlargest version that is guaranteed \u003cem\u003enot\u003c/em\u003e to contain the feature).\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eGoogle3 testing\u003c/h3\u003e\u003ca id=\"user-content-google3-testing\" class=\"anchor\" aria-label=\"Permalink: Google3 testing\" href=\"#google3-testing\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eNew features should be tested in Google's internal code base before they are\nswitched on.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eDetails of how to do this will be described in a separate (Google internal)\ndocument.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eImplementation work\u003c/h3\u003e\u003ca id=\"user-content-implementation-work\" class=\"anchor\" aria-label=\"Permalink: Implementation work\" href=\"#implementation-work\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis is the bulk of creating a new language feature and this stage will take the\nmost time.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eHere are some tips for the implementation stage:\u003c/p\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003eAll implementation code should be under the experiment flag.\u003c/li\u003e\n\u003cli\u003eUse the language tests as a pseudo-checklist for what's left to\nimplement.\u003c/li\u003e\n\u003cli\u003eSome teams may have their own checklist for what needs to be considered in a\nnew language feature implementation. For example,\n\u003ca href=\"https://github.com/dart-lang/sdk/blob/main/pkg/analyzer/doc/process/new_language_feature.md\"\u003ethis analyzer doc\u003c/a\u003e\nand \u003ca href=\"https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server/doc/process/new_language_feature.md\"\u003ethis analysis server doc\u003c/a\u003e.\u003c/li\u003e\n\u003cli\u003eAs we work through an implementation, we may encounter problems or holes in\nthe spec. If so, create a new issue, add the feature label to it, and cc\n@dart-lang/language-team to discuss if we should make any changes in the\nspecification.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cdiv class=\"markdown-alert markdown-alert-note\" dir=\"auto\"\u003e\u003cp class=\"markdown-alert-title\" dir=\"auto\"\u003e\u003csvg class=\"octicon octicon-info mr-2\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z\"\u003e\u003c/path\u003e\u003c/svg\u003eNote\u003c/p\u003e\u003cp dir=\"auto\"\u003eRemember that making a language feature is a two way street between the spec\nwriters and the implementers. The implementation may change because the spec\nchanged, or the spec might change after finding out the implementation is too\ncomplex.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eMigrations\u003c/h2\u003e\u003ca id=\"user-content-migrations\" class=\"anchor\" aria-label=\"Permalink: Migrations\" href=\"#migrations\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eOnce the implementation is finished, you should start a CL that enables the\nfeature by default, if you haven't already. Use this CL to check what code your\nfeature will break and determine if you need any other lints or quick-fixes to\nhelp these migrations.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eSee \u003ca href=\"#google3-testing\"\u003eGoogle3 testing\u003c/a\u003e for more information on fixing breakages\nwithin Google3.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eShipping\u003c/h2\u003e\u003ca id=\"user-content-shipping\" class=\"anchor\" aria-label=\"Permalink: Shipping\" href=\"#shipping\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis is a general checklist for shipping a feature:\u003c/p\u003e\n\u003cul class=\"contains-task-list\"\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Make sure all pre-migrations are complete.\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Submit a CL that enables the feature by default in the upcoming version.\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Follow up on documentation changes which should come out when the stable\nversion with your feature comes out.\u003c/li\u003e\n\u003cli class=\"task-list-item\"\u003e\u003cinput type=\"checkbox\" id=\"\" disabled=\"\" class=\"task-list-item-checkbox\"\u003e Communicate with the team that there's a new feature and invite everyone\nto test it out.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp dir=\"auto\"\u003eCongratulations! You've now shipped the feature.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eKeep an eye out for breakages and any new QOL requests that surface from the\nusage of the feature. Sometimes people use features in ways we don't expect and\nwe can learn a lot from seeing how the feature is used in the real world.\u003c/p\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eAfter shipping\u003c/h3\u003e\u003ca id=\"user-content-after-shipping\" class=\"anchor\" aria-label=\"Permalink: After shipping\" href=\"#after-shipping\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAfter a feature has been shipped, the documents pertaining to the feature should\nbe moved into subfolders carrying the name of the release in which they were\nshipped (e.g. \u003ccode\u003e2.1\u003c/code\u003e).\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor example, moving \u003ccode\u003esuper-mixins\u003c/code\u003e to \u003ccode\u003e2.1\u003c/code\u003e after shipping:\u003c/p\u003e\n\u003cdiv class=\"snippet-clipboard-content notranslate position-relative overflow-auto\" data-snippet-clipboard-copy-content=\"/language\n\n /working\n super-mixins.md\n super-mixins-extra.md\n mikes-mixins.md\n spread-operator.md\n mega-constructors/\n proposal.md\n alternate.md\n illustration.png\n\n /accepted\n 2.0/\n 2.1/\n super-mixins/\n feature-specification.md\n future-releases/\n spread-operator/\n feature-specification.md\n /resources/\n [various supporting documents and resources]\"\u003e\u003cpre class=\"notranslate\"\u003e\u003ccode\u003e/language\n\n /working\n super-mixins.md\n super-mixins-extra.md\n mikes-mixins.md\n spread-operator.md\n mega-constructors/\n proposal.md\n alternate.md\n illustration.png\n\n /accepted\n 2.0/\n 2.1/\n super-mixins/\n feature-specification.md\n future-releases/\n spread-operator/\n feature-specification.md\n /resources/\n [various supporting documents and resources]\n\u003c/code\u003e\u003c/pre\u003e\u003c/div\u003e\n\u003c/article\u003e","renderedFileInfo":null,"shortPath":null,"symbolsEnabled":true,"tabSize":8,"topBannersInfo":{"overridingGlobalFundingFile":false,"globalPreferredFundingPath":null,"showInvalidCitationWarning":false,"citationHelpUrl":"https://docs.github.com/github/creating-cloning-and-archiving-repositories/creating-a-repository-on-github/about-citation-files","actionsOnboardingTip":null},"truncated":false,"viewable":true,"workflowRedirectUrl":null,"symbols":{"timed_out":false,"not_analyzed":false,"symbols":[{"name":"Lifecycle of a language feature","kind":"section_1","ident_start":2,"ident_end":33,"extent_start":0,"extent_end":15361,"fully_qualified_name":"Lifecycle of a language feature","ident_utf16":{"start":{"line_number":0,"utf16_col":2},"end":{"line_number":0,"utf16_col":33}},"extent_utf16":{"start":{"line_number":0,"utf16_col":0},"end":{"line_number":350,"utf16_col":0}}},{"name":"User issues and feature 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hGyMdv"><section aria-labelledby="file-name-id-wide file-name-id-mobile" class="Box-sc-g0xbh4-0 fGqKFv"><div class="Box-sc-g0xbh4-0 eoaCFS js-snippet-clipboard-copy-unpositioned undefined" data-hpc="true"><article class="markdown-body entry-content container-lg" itemprop="text"><div class="markdown-heading" dir="auto"><h1 tabindex="-1" class="heading-element" dir="auto">Lifecycle of a language feature</h1><a id="user-content-lifecycle-of-a-language-feature" class="anchor" aria-label="Permalink: Lifecycle of a language feature" href="#lifecycle-of-a-language-feature"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">This document describes the process from creation to the shipping and release of a language feature. This document serves as a walkthrough for what's needed to build a language feature.</p> <ol dir="auto"> <li><a href="https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#user-issues-and-feature-requests">User issues and feature requests</a></li> <li><a href="https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#design-feedback-and-iteration">Design, feedback, and iteration</a></li> <li><a href="https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#acceptance">Acceptance</a></li> <li><a href="https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#team-communication">Team communication</a> (eg. kick-off meetings)</li> <li><a href="https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#implementation-and-testing">Implementation and testing</a></li> <li><a href="https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#migrations">Migrations</a></li> <li><a href="https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#shipping">Shipping</a></li> </ol> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">User issues and feature requests</h2><a id="user-content-user-issues-and-feature-requests" class="anchor" aria-label="Permalink: User issues and feature requests" href="#user-issues-and-feature-requests"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Features arise from perceived user issues or feature requests. These issues are documentation of the user issue to be solved.</p> <div class="markdown-alert markdown-alert-note" dir="auto"><p class="markdown-alert-title" dir="auto"><svg class="octicon octicon-info mr-2" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"></path></svg>Note</p><p dir="auto">These feature requests are filed in the <a href="https://github.com/dart-lang/language/issues/new?labels=request">language repo</a>, and are labelled <a href="https://github.com/dart-lang/language/labels/request"><code>request</code></a>.</p> </div> <p dir="auto">We may close issues that we believe we won't address for whatever reason. We may also keep issues open indefinitely if we believe they are something that we might want to address in the future.</p> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">How does the team choose which feature request to accept?</h3><a id="user-content-how-does-the-team-choose-which-feature-request-to-accept" class="anchor" aria-label="Permalink: How does the team choose which feature request to accept?" href="#how-does-the-team-choose-which-feature-request-to-accept"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">There are many factors that determine why we might choose to do one feature over another, such as feasibility, difficulty, or popularity. Often, we will take a look through the most 👍-reacted issues to see what our users are interested in.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Design, feedback, and iteration</h2><a id="user-content-design-feedback-and-iteration" class="anchor" aria-label="Permalink: Design, feedback, and iteration" href="#design-feedback-and-iteration"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Interested parties may propose several (competing) language features to a single request/problem.</p> <p dir="auto">A "language feature" issue (labelled <a href="https://github.com/dart-lang/language/labels/feature"><code>feature</code></a>) will be created for tracking the solution to the user issue/problem or feature under consideration.</p> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Proposal checklist</h3><a id="user-content-proposal-checklist" class="anchor" aria-label="Permalink: Proposal checklist" href="#proposal-checklist"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <ul class="contains-task-list"> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> An issue labelled <code>feature</code> for discussion of a proposed solution. <ul class="contains-task-list"> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Links to and from the <code>request</code> issue they are trying to address.</li> </ul> </li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> A link to an initial feature specification writeup of the proposed solution in the <code>feature</code> issue. <ul class="contains-task-list"> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Located in a sub-directory with the name of the feature (<code>/ [feature-name]/</code>), inside the <a href="https://github.com/dart-lang/language/tree/master/working"><code>working</code></a> directory of the language repository.</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> File is named <code>feature-specification.md</code>. <ul dir="auto"> <li>All written plans, specs, and materials must be written using <a href="https://guides.github.com/features/mastering-markdown/#GitHub-flavored-markdown">GitHub Markdown format</a> format, and must use the <code>.md</code> extension.</li> </ul> </li> </ul> </li> </ul> <p dir="auto">Proposals may be iterated on in-place.</p> <p dir="auto">For smaller and non-controversial features, we will sometimes skip this step and proceed directly to the <a href="#acceptance">Acceptance</a> phase.</p> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">External (outside of the language team) feedback</h3><a id="user-content-external-outside-of-the-language-team-feedback" class="anchor" aria-label="Permalink: External (outside of the language team) feedback" href="#external-outside-of-the-language-team-feedback"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">We use the Github issue tracker as the primary place for accepting feedback on proposals, solicited or unsolicited.</p> <p dir="auto">If we solicit feedback, we will open an issue for discussion and feedback. We highly recommend filing and splitting off different issues for different threads of discussion. We generally expect to formally solicit at least one round of feedback on significant changes.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Acceptance</h2><a id="user-content-acceptance" class="anchor" aria-label="Permalink: Acceptance" href="#acceptance"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">If consensus is reached on a specific proposal and we decide to accept it, a member of the language team will be chosen to shepherd the implementation.</p> <p dir="auto">The implementation will be tracked via two artifacts:</p> <ul dir="auto"> <li>The feature specification document</li> <li>The feature project</li> </ul> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Feature specification</h3><a id="user-content-feature-specification" class="anchor" aria-label="Permalink: Feature specification" href="#feature-specification"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">The 'feature specification' is <strong>a single canonical writeup of the language feature</strong> which serves as the implementation reference. The feature specification should follow the <a href="https://github.com/dart-lang/language/blob/main/doc/life_of_a_language_feature.md#proposal-checklist">Proposal Checklist</a>.</p> <p dir="auto">Once we've chosen a specification, we'll update it throughout the implementation process. For example, we could add new information to the specification if we encounter edge-cases we didn't think of initially or remove parts of it if a certain behavior isn't feasible to implement.</p> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Feature project</h3><a id="user-content-feature-project" class="anchor" aria-label="Permalink: Feature project" href="#feature-project"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">We'll <a href="https://github.com/orgs/dart-lang/projects">create a project</a> in dart-lang with the title <code>[Language] <feature-name></code>. This project will link to and from all the Github issues that we need to track to complete and ship the feature.</p> <div class="markdown-alert markdown-alert-tip" dir="auto"><p class="markdown-alert-title" dir="auto"><svg class="octicon octicon-light-bulb mr-2" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="M8 1.5c-2.363 0-4 1.69-4 3.75 0 .984.424 1.625.984 2.304l.214.253c.223.264.47.556.673.848.284.411.537.896.621 1.49a.75.75 0 0 1-1.484.211c-.04-.282-.163-.547-.37-.847a8.456 8.456 0 0 0-.542-.68c-.084-.1-.173-.205-.268-.32C3.201 7.75 2.5 6.766 2.5 5.25 2.5 2.31 4.863 0 8 0s5.5 2.31 5.5 5.25c0 1.516-.701 2.5-1.328 3.259-.095.115-.184.22-.268.319-.207.245-.383.453-.541.681-.208.3-.33.565-.37.847a.751.751 0 0 1-1.485-.212c.084-.593.337-1.078.621-1.489.203-.292.45-.584.673-.848.075-.088.147-.173.213-.253.561-.679.985-1.32.985-2.304 0-2.06-1.637-3.75-4-3.75ZM5.75 12h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1 0-1.5ZM6 15.25a.75.75 0 0 1 .75-.75h2.5a.75.75 0 0 1 0 1.5h-2.5a.75.75 0 0 1-.75-.75Z"></path></svg>Tip</p><p dir="auto">Generate all the issues for the project automatically with <a href="https://github.com/itsjustkevin/aviary">https://github.com/itsjustkevin/aviary</a>.</p> <p dir="auto">This script generates all the issues that need to be tracked and added to the project. If there are any issues or implementation areas that are unnecessary, you can close them out immediately, but each issue is worth thinking through.</p> </div> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Meta implementation issue</h4><a id="user-content-meta-implementation-issue" class="anchor" aria-label="Permalink: Meta implementation issue" href="#meta-implementation-issue"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">The meta implementation issue (labelled <code>implementation</code>) will be filed in the language repository for tracking the implementation process.</p> <p dir="auto">This issue must contain links to:</p> <ul class="contains-task-list"> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> The related <code>request</code> and <code>feature</code> issues (if they exist)</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> A link to the feature specification</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> All implementation issues</li> </ul> <div class="markdown-alert markdown-alert-note" dir="auto"><p class="markdown-alert-title" dir="auto"><svg class="octicon octicon-info mr-2" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"></path></svg>Note</p><p dir="auto">Once you're done making the implementation issue, copy the link of the issue and add it to the <a href="https://github.com/orgs/flutter/projects/82/views/1">Dash Feature Tracker board</a>.</p> </div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Team communication</h2><a id="user-content-team-communication" class="anchor" aria-label="Permalink: Team communication" href="#team-communication"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">One of the most important parts of the language feature process is communicating with all the implementation teams and any other stakeholders.</p> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Channels for communication</h3><a id="user-content-channels-for-communication" class="anchor" aria-label="Permalink: Channels for communication" href="#channels-for-communication"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">We'll send out an announcement email to the entire team, letting them know that work for the feature is beginning.</p> <p dir="auto">We may create a mailing list or a Google chat group to organize communication between the different teams working on the language feature.</p> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Kick-off meetings</h3><a id="user-content-kick-off-meetings" class="anchor" aria-label="Permalink: Kick-off meetings" href="#kick-off-meetings"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">We need to get sign-off from all of the relevant implementation teams indicating that they understand the proposal, believe that it can reasonably be implemented, and feel that they have sufficient details to proceed. We do this in the form of kick-off meetings.</p> <p dir="auto">Since feature specification documents are usually long and very detailed, we like to begin this sign-off process with a set of kick-off meetings, typically one for each affected implementation team. These meetings are on opportunity for a language team representative to present a broad outline of the new feature, give some concrete examples, and collect insights from the implementation teams about what aspects of the implementation might need special attention.</p> <p dir="auto">Get sign-off from the following teams:</p> <ul class="contains-task-list"> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Analyzer team</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> CFE team</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Back-end teams: <ul class="contains-task-list"> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Dart VM Team</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Dart Web Team</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Dart2Wasm Team</li> </ul> </li> </ul> <div class="markdown-alert markdown-alert-note" dir="auto"><p class="markdown-alert-title" dir="auto"><svg class="octicon octicon-info mr-2" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"></path></svg>Note</p><p dir="auto">If a feature consists entirely of a new piece of syntactic sugar, it can be tempting to assume that it's not necessary to consult with any back-end teams since the CFE will lower the new syntax into a kernel form that is already supported, but this can be a dangerous assumption. It's easy to forget that even in features that appear to consist purely of syntactic sugar, some back-end support may be needed in order to properly support single-step debugging or hot reload.</p> <p dir="auto">Also, some work may need to be done on back-end code generators in order to make sure that the new feature is lowered into a form that can be well optimized.</p> </div> <p dir="auto">To avoid missing things, we prefer to err on the side of asking teams whether they're affected, rather than assuming they won't be.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Implementation and testing</h2><a id="user-content-implementation-and-testing" class="anchor" aria-label="Permalink: Implementation and testing" href="#implementation-and-testing"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Feature flag</h3><a id="user-content-feature-flag" class="anchor" aria-label="Permalink: Feature flag" href="#feature-flag"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Most new language features should be implemented using a feature flag. The feature flag serves several purposes:</p> <ul dir="auto"> <li>It allows the feature to be implemented over a series of CLs without destabilizing the SDK. If it comes time to ship a new release of the SDK before the feature is ready, the feature flag can remain off, so users won't be exposed to a partially-implemented or buggy feature before it's ready.</li> <li>It allows users to opt in to trying out features that have not yet been released, by turning on the feature flag locally.</li> <li>Once the feature is enabled, the feature flag logic ensures that it will only affect users who have set their language version to a version that includes the new feature. This is especially important for package developers who may want to support a range of versions of the Dart SDK.</li> </ul> <div class="markdown-alert markdown-alert-note" dir="auto"><p class="markdown-alert-title" dir="auto"><svg class="octicon octicon-info mr-2" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"></path></svg>Note</p><p dir="auto">Implementing a language feature using a feature flag is frequently more work than implementing it without a feature flag, since the compiler and analyzer must faithfully implement both the old and new behaviors. Occasionally the language team may decide that the benefits of using a feature flag don't justify this extra work. But this is a rare scenario. If you are working on a feature and believe you don't need to use a feature flag, please consult with the language team to be sure.</p> </div> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">How to add a new language feature flag</h4><a id="user-content-how-to-add-a-new-language-feature-flag" class="anchor" aria-label="Permalink: How to add a new language feature flag" href="#how-to-add-a-new-language-feature-flag"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <ul class="contains-task-list"> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Add an entry to <code>tools/experimental_features.yaml</code> describing the feature, in the top section (above the line that says <code>Flags below this line are shipped</code>).</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Run <code>dart pkg/front_end/tool/fasta.dart generate-experimental-flags</code> to update <code>pkg/_fe_analyzer_shared/lib/src/experiments/flags.dart</code> and <code>pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart</code>.</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Run <code>dart pkg/analyzer/tool/experiments/generate.dart</code> to update <code>pkg/analyzer/lib/src/dart/analysis/experiments.g.dart</code>.</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Add a static final declaration to the <code>Feature</code> class in <code>pkg/analyzer/lib/dart/analysis/features.dart</code>.</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Increment the value of <code>AnalysisDriver.DATA_VERSION</code> in <code>pkg/analyzer/lib/src/dart/analysis/driver.dart</code>.</li> </ul> <p dir="auto">Example CL: <a href="https://dart-review.googlesource.com/c/sdk/+/365545" rel="nofollow">https://dart-review.googlesource.com/c/sdk/+/365545</a></p> <div class="markdown-alert markdown-alert-tip" dir="auto"><p class="markdown-alert-title" dir="auto"><svg class="octicon octicon-light-bulb mr-2" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="M8 1.5c-2.363 0-4 1.69-4 3.75 0 .984.424 1.625.984 2.304l.214.253c.223.264.47.556.673.848.284.411.537.896.621 1.49a.75.75 0 0 1-1.484.211c-.04-.282-.163-.547-.37-.847a8.456 8.456 0 0 0-.542-.68c-.084-.1-.173-.205-.268-.32C3.201 7.75 2.5 6.766 2.5 5.25 2.5 2.31 4.863 0 8 0s5.5 2.31 5.5 5.25c0 1.516-.701 2.5-1.328 3.259-.095.115-.184.22-.268.319-.207.245-.383.453-.541.681-.208.3-.33.565-.37.847a.751.751 0 0 1-1.485-.212c.084-.593.337-1.078.621-1.489.203-.292.45-.584.673-.848.075-.088.147-.173.213-.253.561-.679.985-1.32.985-2.304 0-2.06-1.637-3.75-4-3.75ZM5.75 12h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1 0-1.5ZM6 15.25a.75.75 0 0 1 .75-.75h2.5a.75.75 0 0 1 0 1.5h-2.5a.75.75 0 0 1-.75-.75Z"></path></svg>Tip</p><p dir="auto">Keep a WIP CL which enables your feature flag by default in the latest version. Rebase it occasionally to get a good idea of how your feature breaks the SDK and other dependencies. This is very helpful for getting insight on what migrations and breaking changes you'll be making.</p> </div> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Language testing</h3><a id="user-content-language-testing" class="anchor" aria-label="Permalink: Language testing" href="#language-testing"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">The language team will generally write a preliminary set of language tests for a feature, in the SDK's <code>tests/language</code> subdirectory. These tests are not intended to be exhaustive, but should illustrate and exercise important and non-obvious features. Implementation teams are encouraged to write additional language or unit tests. The language team may also coordinate the writing of additional tests.</p> <p dir="auto">An important use case for any new language feature is to ensure that the feature isn't accidentally used by package authors that have not yet opted into a language version that supports it. So in addition to testing that the new language feature works when the feature flag is turned on, the tests added to <code>tests/language</code> should verify that when the feature flag is turned off, the previous behavior of the SDK is preserved.</p> <p dir="auto">One important exception to preserving previous SDK behavior is that it's permissible (and even encouraged) to improve error messages so that if the user tries to use a disabled language feature, they receive an error explaining that they need to enable it, rather than, say, a confusing set of parse errors.</p> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">To enable the feature in a language test</h4><a id="user-content-to-enable-the-feature-in-a-language-test" class="anchor" aria-label="Permalink: To enable the feature in a language test" href="#to-enable-the-feature-in-a-language-test"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="// SharedOptions=--enable-experiment=$FEATURE"><pre><span class="pl-c">// SharedOptions=--enable-experiment=$FEATURE</span></pre></div> <p dir="auto">Include this comment near the top of the test (before the first directive), where <code>$FEATURE</code> is replaced with the feature name.</p> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">To disable the feature in a language test</h4><a id="user-content-to-disable-the-feature-in-a-language-test" class="anchor" aria-label="Permalink: To disable the feature in a language test" href="#to-disable-the-feature-in-a-language-test"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <div class="highlight highlight-source-dart notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="// @dart=$VERSION"><pre><span class="pl-c">// @dart=$VERSION</span></pre></div> <p dir="auto">Include this comment near the top of the test (before the first directive), where <code>$VERSION</code> is the current stable release of Dart (and therefore is the largest version that is guaranteed <em>not</em> to contain the feature).</p> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Google3 testing</h3><a id="user-content-google3-testing" class="anchor" aria-label="Permalink: Google3 testing" href="#google3-testing"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">New features should be tested in Google's internal code base before they are switched on.</p> <p dir="auto">Details of how to do this will be described in a separate (Google internal) document.</p> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Implementation work</h3><a id="user-content-implementation-work" class="anchor" aria-label="Permalink: Implementation work" href="#implementation-work"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">This is the bulk of creating a new language feature and this stage will take the most time.</p> <p dir="auto">Here are some tips for the implementation stage:</p> <ul dir="auto"> <li>All implementation code should be under the experiment flag.</li> <li>Use the language tests as a pseudo-checklist for what's left to implement.</li> <li>Some teams may have their own checklist for what needs to be considered in a new language feature implementation. For example, <a href="https://github.com/dart-lang/sdk/blob/main/pkg/analyzer/doc/process/new_language_feature.md">this analyzer doc</a> and <a href="https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server/doc/process/new_language_feature.md">this analysis server doc</a>.</li> <li>As we work through an implementation, we may encounter problems or holes in the spec. If so, create a new issue, add the feature label to it, and cc @dart-lang/language-team to discuss if we should make any changes in the specification.</li> </ul> <div class="markdown-alert markdown-alert-note" dir="auto"><p class="markdown-alert-title" dir="auto"><svg class="octicon octicon-info mr-2" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"></path></svg>Note</p><p dir="auto">Remember that making a language feature is a two way street between the spec writers and the implementers. The implementation may change because the spec changed, or the spec might change after finding out the implementation is too complex.</p> </div> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Migrations</h2><a id="user-content-migrations" class="anchor" aria-label="Permalink: Migrations" href="#migrations"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Once the implementation is finished, you should start a CL that enables the feature by default, if you haven't already. Use this CL to check what code your feature will break and determine if you need any other lints or quick-fixes to help these migrations.</p> <p dir="auto">See <a href="#google3-testing">Google3 testing</a> for more information on fixing breakages within Google3.</p> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Shipping</h2><a id="user-content-shipping" class="anchor" aria-label="Permalink: Shipping" href="#shipping"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">This is a general checklist for shipping a feature:</p> <ul class="contains-task-list"> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Make sure all pre-migrations are complete.</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Submit a CL that enables the feature by default in the upcoming version.</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Follow up on documentation changes which should come out when the stable version with your feature comes out.</li> <li class="task-list-item"><input type="checkbox" id="" disabled="" class="task-list-item-checkbox"> Communicate with the team that there's a new feature and invite everyone to test it out.</li> </ul> <p dir="auto">Congratulations! You've now shipped the feature.</p> <p dir="auto">Keep an eye out for breakages and any new QOL requests that surface from the usage of the feature. Sometimes people use features in ways we don't expect and we can learn a lot from seeing how the feature is used in the real world.</p> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">After shipping</h3><a id="user-content-after-shipping" class="anchor" aria-label="Permalink: After shipping" href="#after-shipping"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">After a feature has been shipped, the documents pertaining to the feature should be moved into subfolders carrying the name of the release in which they were shipped (e.g. <code>2.1</code>).</p> <p dir="auto">For example, moving <code>super-mixins</code> to <code>2.1</code> after shipping:</p> <div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="/language /working super-mixins.md super-mixins-extra.md mikes-mixins.md spread-operator.md mega-constructors/ proposal.md alternate.md illustration.png /accepted 2.0/ 2.1/ super-mixins/ feature-specification.md future-releases/ spread-operator/ feature-specification.md /resources/ [various supporting documents and resources]"><pre class="notranslate"><code>/language /working super-mixins.md super-mixins-extra.md mikes-mixins.md spread-operator.md mega-constructors/ proposal.md alternate.md illustration.png /accepted 2.0/ 2.1/ super-mixins/ feature-specification.md future-releases/ spread-operator/ feature-specification.md /resources/ [various supporting documents and resources] </code></pre></div> </article></div><button hidden=""></button></section></div></div></div> <!-- --> <!-- --> </div></div></div><div class="Box-sc-g0xbh4-0"></div></div></div></div></div><div id="find-result-marks-container" class="Box-sc-g0xbh4-0 cCoXib"></div><button hidden="" data-testid="" data-hotkey-scope="read-only-cursor-text-area"></button><button hidden=""></button></div> <!-- --> <!-- --> <script type="application/json" id="__PRIMER_DATA_:R0:__">{"resolvedServerColorMode":"day"}</script></div> </react-app> </turbo-frame> </div> </turbo-frame> </main> </div> </div> <footer class="footer pt-8 pb-6 f6 color-fg-muted p-responsive" role="contentinfo" > <h2 class='sr-only'>Footer</h2> <div class="d-flex flex-justify-center flex-items-center flex-column-reverse flex-lg-row flex-wrap flex-lg-nowrap"> <div class="d-flex 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