Inlay hints for patterns

[Zekfad]

Is your feature request related to a problem? Please describe.
It would be helpful to easily see (and navigate) to inferred types for patterns (switch, if-case, destructuring, etc.)
For now it's only possible to see inferred type from hover popup.

Describe the solution you'd like
Make inlay hints for patterns behave like for generics (like #4366)

Describe alternatives you've considered
Not really.

Additional context
Also inferred patterns are not navigable, in example hint Box<int> allows to navigate to int, but Box<(int, int)> both ints are not navigable.
And probably inferred single element pattern should be (Type,) instead of (Type).

final class Box<T> {
  const Box(this.value);
  final T value;
}
void main() {
  {
    { // actual
      final (i, j) = (1, 2);
    }
    { // expected
      final (int i, int j) = (1, 2);
    }
  }
  {
    final pattern = (test: (10,), 2);
    { // actual
      if (pattern case final p) {}

      final (_, :test,) = pattern;

      final Null _switch = switch (pattern) {
        (:final test, var i) => null,
      };
    }
    { // expected
      if (pattern case final (int, {(int,) test}) p) {}

      final (_, :(int,) test,) = pattern;

      final Null _switch = switch (pattern) {
        (:final (int,) test, var /*int*/ i) => null,
      };
    }
  }
  {
    const simple = Box(2);
    const pattern = Box((1,2));

    { // actual
      if (simple case Box(:final value)) {}
      if (pattern case Box(:final value)) {}
      if (pattern case Box(value: final test)) {}
    }
    { // expected
      if (simple case Box<int>(:final int value)) {}
      if (pattern case Box<(int, int)>(:final (int, int) value)) {}
      if (pattern case Box<(int, int)>(value: final (int, int) test)) {}
    }
  }
}

Screenshot of current behavior

[DanTup]

Working on most of this (patterns + breaking up records hints) at https://dart-review.googlesource.com/c/sdk/+/312460.

And probably inferred single element pattern should be (Type,) instead of (Type).

@bwilkerson I think this makes sense, and I started a change to do this both here (inlay hints) and also in ElementDisplayStringBuilder.writeRecordType so hovers etc. display the same too. However, that affects lots of Analyzer tests that have record types written as strings for example here:

staticType: (String)

https://github.com/dart-lang/sdk/blob/6a4ef6c93fca0870e914760c9e50d84fc75820c0/pkg/analyzer/test/src/dart/resolution/binary_expression_test.dart#L125

I think it makes sense to update these, but I wanted a second opinion before I went ahead.

[bwilkerson]

Yes, if we're displaying a record type with a single field, then I believe we should include a trailing comma because that's the syntax used by the language, and being consistent will be less confusing.

[DanTup]

Thanks!

The inlay hints changes landed in dart-lang/sdk@0835375. I've opened a CL for trailing commas at https://dart-review.googlesource.com/c/sdk/+/312421.

With both changes, the example above looks like this:

And the types within records are split up to support individual navigation:

[Zekfad]

That is great! Thank you for fast work, now it would be much easier to inspect pattern switches!

[DanTup]

np! I forgot to mention here (and it's not always obvious).. These changes are in the language server so they'll show up in an future SDK release (and not a Dart VS Code extension release).