Add trailing comma support in cases missing from Swift 6.1

[calda]

Corresponding swift-syntax PR: swiftlang/swift-syntax#3080

---

In Swift 6.1, there are many cases where trailing commas should be supported but currently are not:

• Trailing comma not supported in tuple types or closure types despite SE-0439 #81485
• Trailing comma not allowed in generic argument lists despite SE-0439 #81474

This includes cases like:

let foo: (
  bar: String,
  quux: String, // unexpectedly not supported in Swift 6.1
)

let closure: (
  String,
  String, // unexpectedly not supported in Swift 6.1
) -> (
  bar: String,
  quux: String, // unexpectedly not supported in Swift 6.1
)

let foo: Foo<
  String,
  Int,
  Boo, // unexpectedly not supported in Swift 6.1
>

let foo = Foo<
  String,
  Int,
  Boo, // unexpectedly not supported in Swift 6.1
>()

typealias Bar<
  T1,
  T2
> = Foo<
  T1,
  T2,
  Bool, // unexpectedly not supported in Swift 6.1
>

protocol Baaz<
  T1,
  T2, // unexpectedly not supported in Swift 6.1
> {
  associatedtype T1
  associatedtype T2
}

[rintaro]

Thank you Cal for looking into this!

I think builtin attributes (and availability spec list?) were deliberately NOT implemented in the original PR ( #74522 (comment) .) @xwu What was the verdict for this? Should the compiler accept them?

[calda]

Thanks for the link, @xwu's here discussion make sense:

If the proposal had been accepted with the original scope that everything that's a comma-separated list with an unambiguous terminator should support a trailing comma, then we would need to puzzle over which of these attributes have such a regular list, but we don't have to do that now :)

As I was working through the changes it was indeed unclear which attributes should support trailing commas vs which attributes shouldn't. Excluding all built-in attributes appears to be an intentional decision as a part of the acceptance. I removed that part from the PR.

The other cases are more clearly just bugs that should be fixed.

[rintaro]

The code changes look good to me. Thank you!

[rintaro]

Would you mind adding test cases for generic arguments in expression positions to prevent future regressions? Like let _ = Generic<Int, Bool,>.self and let _ = Generic<Int, Bool,>(). It also uses parseGenericArguments() (currently) so it should work with your changes.

[calda]

Thanks for remembering to check that! It didn't work originally, we also had to update the lookahead code that disambiguated the < between a generic list and a less than operator. Updated, working now :)

[rintaro]

Ah canParseGenericArguments(). That makes sense. Thank you for fixing it!

[rintaro]

@swift-ci Please smoke test

[mateusrodriguesxyz]

@calda Thanks for fixing these cases that I missed!

[calda]

The CI job failed with:

error: unexpected error produced: new Swift parser generated errors for code that C++ parser accepted

@rintaro, can you trigger CI again with the Swift Syntax changes from swiftlang/swift-syntax#3080 included? Thanks!

[rintaro]

swiftlang/swift-syntax#3080
@swift-ci Please smoke test

[rintaro]

(closed/reopened by mistake)

[rintaro]

swiftlang/swift-syntax#3080
@swift-ci Please smoke test macOS

[rintaro]

swiftlang/swift-syntax#3080
@swift-ci Please smoke test Linux

[rintaro]

@calda Everything LGTM, and I will merge this and the swift-syntax PR as soon as the CI passes.
Could you open cherry-pick PRs for release/6.2?

[calda]

Thanks! Here you go:

• [6.2] Add trailing comma support in cases missing from Swift 6.1 #81653
• [6.2] Add trailing comma support in cases missing from Swift 6.1 swift-syntax#3081

[rintaro]

swiftlang/swift-syntax#3080
@swift-ci Please smoke test macOS

[rintaro]

swiftlang/swift-syntax#3080
@swift-ci Please smoke test Linux

[rintaro]

swiftlang/swift-syntax#3080
@swift-ci Please smoke test macOS

[rintaro]

swiftlang/swift-syntax#3080
@swift-ci Please smoke test macOS

[rintaro]

@swift-ci Please smoke test macOS