{"action":"opened","number":3819,"pull_request":{"url":"https://api.github.com/repos/apple/swift/pulls/3819","id":79137659,"html_url":"https://github.com/apple/swift/pull/3819","diff_url":"https://github.com/apple/swift/pull/3819.diff","patch_url":"https://github.com/apple/swift/pull/3819.patch","issue_url":"https://api.github.com/repos/apple/swift/issues/3819","number":3819,"state":"open","locked":false,"title":"[stdlib] Add ContiguousArray -> Array non-copying initialiser","user":{"login":"karwa","id":5254025,"avatar_url":"https://avatars.githubusercontent.com/u/5254025?v=3","gravatar_id":"","url":"https://api.github.com/users/karwa","html_url":"https://github.com/karwa","followers_url":"https://api.github.com/users/karwa/followers","following_url":"https://api.github.com/users/karwa/following{/other_user}","gists_url":"https://api.github.com/users/karwa/gists{/gist_id}","starred_url":"https://api.github.com/users/karwa/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/karwa/subscriptions","organizations_url":"https://api.github.com/users/karwa/orgs","repos_url":"https://api.github.com/users/karwa/repos","events_url":"https://api.github.com/users/karwa/events{/privacy}","received_events_url":"https://api.github.com/users/karwa/received_events","type":"User","site_admin":false},"body":"<!-- Please complete this template before creating the pull request. -->\r\n#### What's in this pull request?\r\nAs I understand it, Array<T> may be backed by contiguous storage (if native) or it may be backed by some exotic NSArray subclass. We provide a related type with stronger backing guarantees: ContiguousArray<T> (and indeed, in my algorithmic code, I can see a total execution time halve by switching from one to the other -- which may be a bug, but in any case there are differences which is why we have the type in the first place).\r\n\r\nUnfortunately, there is no way to vend a ContiguousArray<T> as a regular Array<T>. If you wish to make use of the stronger guarantees of ContiguousArray<T> in your algorithm, everybody who uses those results must also use ContiguousArray<T> or else you will have to copy the data.\r\n\r\nI know it's late for Swift 3. I literally only noticed this today, otherwise I would have mentioned it earlier.\r\n\r\nIt seems like a reasonably large hole with a simple fix. Going from ContiguousArray<T> to Array<T> is a kind-of upcasting conversion and should be allowed -- it seems like that's almost the whole point of the type.\r\n\r\n#### Related bug number: ([SR-2196](https://bugs.swift.org/browse/SR-2196))\r\n(Not resolved: related)\r\n\r\n* * * *\r\n\r\n<!-- This selection should only be completed by Swift admin -->\r\nBefore merging this pull request to apple/swift repository:\r\n- [ ] Test pull request on Swift continuous integration.\r\n\r\n<details>\r\n  <summary>Triggering Swift CI</summary>\r\n\r\nThe swift-ci is triggered by writing a comment on this PR addressed to the GitHub user @swift-ci. Different tests will run depending on the specific comment that you use. The currently available comments are:\r\n\r\n**Smoke Testing**\r\n\r\n        Platform     | Comment\r\n        ------------ | -------------\r\n        All supported platforms     | @swift-ci Please smoke test\r\n        All supported platforms     | @swift-ci Please smoke test and merge\r\n        OS X platform               | @swift-ci Please smoke test OS X platform\r\n        Linux platform              | @swift-ci Please smoke test Linux platform\r\n\r\nA smoke test on macOS does the following:\r\n\r\n1. Builds the compiler incrementally.\r\n2. Builds the standard library only for macOS. Simulator standard libraries and\r\n   device standard libraries are not built.\r\n3. lldb is not built.\r\n4. The test and validation-test targets are run only for macOS. The optimized\r\n   version of these tests are not run.\r\n\r\nA smoke test on Linux does the following:\r\n\r\n1. Builds the compiler incrementally.\r\n2. Builds the standard library incrementally.\r\n3. lldb is built incrementally.\r\n4. The swift test and validation-test targets are run. The optimized version of these\r\n   tests are not run.\r\n5. lldb is tested.\r\n\r\n**Validation Testing**\r\n\r\n        Platform     | Comment\r\n        ------------ | -------------\r\n        All supported platforms     | @swift-ci Please test\r\n        All supported platforms     | @swift-ci Please test and merge\r\n        OS X platform               | @swift-ci Please test OS X platform\r\n        OS X platform               | @swift-ci Please benchmark\r\n        Linux platform              | @swift-ci Please test Linux platform\r\n\r\n\r\n**Lint Testing**\r\n\r\n        Language     | Comment\r\n        ------------ | -------------\r\n        Python       | @swift-ci Please Python lint\r\n\r\nNote: Only members of the Apple organization can trigger swift-ci.\r\n</details>\r\n<!-- Thank you for your contribution to Swift! 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