Quick assist to convert switch statements to switch expressions

[munificent]

With the in-progress work on patterns, we're also adding a new switch expression. It would be very cool if we could support a quick fix that would allow you to convert switch statements to switch expressions when it makes sense. I think that's feasible when:

• The switch is exhaustive over its type:
  • It's covering all cases in an enum, or
  • It has a default case.
• And all of the cases (including default) have the same structure:
  • A single return statement, or
  • An assignment to the same variable or setter, or
  • Maybe a call to the same function but with a different argument.
• While also allowing cases that abort too.

Then in that case, the shared structure in the cases can be hoisted out and the switch statement turned into a switch expression whose result is then applied to the shared structure. So, for example:

// Convert return:
switch (color) {
  case Color.red:
    return 'red';
  case Color.blue:
    return 'blue';
  case Color.green:
    throw 'Green is bad';
  case Color.yellow:
    return 'yellow';
}

return switch (color) {
  Color.red => 'red',
  Color.blue => 'blue',
  Color.green => throw 'Green is bad',
  Color.yellow => 'yellow'
};

// Convert assignment:
switch (color) {
  case Color.red:
    name = 'red';
    break;
  case Color.blue:
    name = 'blue';
    break;
  case Color.green:
    throw 'Green is bad';
  case Color.yellow:
    name = 'yellow';
    break;
}

name = switch (color) {
  Color.red => 'red',
  Color.blue => 'blue',
  Color.green => throw 'Green is bad',
  Color.yellow => 'yellow'
};

// Convert argument:
switch (color) {
  case Color.red:
    showName('red');
    break;
  case Color.blue:
    showName('blue');
    break;
  case Color.green:
    throw 'Green is bad';
  case Color.yellow:
    showName('yellow');
    break;
}

showName(switch (color) {
  Color.red => 'red',
  Color.blue => 'blue',
  Color.green => throw 'Green is bad',
  Color.yellow => 'yellow'
});

[bwilkerson]

The question, in my mind, is whether this should be tied to a 'prefer_switch_expressions' lint, or whether this should just be available as a quick assist in places where it could be applied. (I'd be hesitant to say that all such switch statements are marked as a problem without users explicitly choosing that style.)

[munificent]

If it were me, I wouldn't make this a "prefer" lint. Just an assist users can manually invoke for specific switch statements if they want to. I agree with you that I don't think all switch statements should be expressions just because they could be. Sometimes a statement is just easier to read.

[srawlins]

CC @pq @keertip

[bwilkerson]

We probably shouldn't convert a statement that isn't exhaustive, so we'll need to wait for that piece to be done before we implement the assist.

[pq]

If it were me, I wouldn't make this a "prefer" lint.

Proposal here: #58862

[pq]

Implemented w/ b68719c

[pq]

See also #51826 that adds support for case clauses w/ implicit breaks.