Dot Shorthand for Object Destructuring

[albinpk]

Summary

Introduce a shorthand for object destructuring where the type name can be omitted when it can be inferred from the right-hand side, using a leading dot (.).

---

Motivation

Object destructuring currently requires repeating the type name even when the matched expression already provides it, leading to unnecessary verbosity:

final Point(:x, :y) = Point(1, 2);

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Proposal

Allow using a dot-prefixed object pattern whose type is inferred from the value being matched:

final .(:x, :y) = Point(1, 2);

This behaves the same as explicitly writing Point(:x, :y) but without repeating the type.

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Examples

// current
final Size(:width, :height) = size;

// proposed
final .(:width, :height) = size;

---

// current
LayoutBuilder(
  builder: (context, constraints) {
    final BoxConstraints(:maxWidth, :maxHeight) = constraints;
    // ...
  },
);

// proposed
LayoutBuilder(
  builder: (context, constraints) {
    final .(:maxWidth, :maxHeight) = constraints;
    // ...
  },
);

---

// current
GestureDetector(
  onPanUpdate: (details) {
    final DragUpdateDetails(delta: Offset(:dx, :dy)) = details;
    // ...
  },
);

// proposed
GestureDetector(
  onPanUpdate: (details) {
    final .(delta: .(:dx, :dy)) = details;
    // ...
  },
);

[Reprevise]

I think this is better than what we have currently (writing the type out fully), but I think we can go all the way and removing any prefix entirely. (Related: https://github.com/dart-lang/language/issues/4124 https://github.com/dart-lang/language/issues/2563)

[munificent]

My preferred approach for this is still #2563 because it's maximally terse. You're right that we could use a leading . similar to dot shorthands, but that feels like a weird fit here. With dot shorthands, the syntax that you are abbreviating has a dot in it. When you turn Color.red into .red, it makes sense for there to be a dot there because there was in the longer code. You just eliminated the preceding identifier.

But in a pattern, going from Color(red: _) to .(red: _) makes less sense. There was never a . there, so why summon one in the abbreviated form?