Auto complete on variable destruction can be improved

[FMorschel]

Originally opened at Dart-Code/Dart-Code#5379.

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Is your feature request related to a problem? Please describe.
When you have code like this:

void foo(Iterable iterable) {
  if (iterable case List()) {
    
  }
}

When inside List(^) you get prompted by auto-complete for all proprieties the class has. But when you auto-complete, it simply fills the propriety name and not :var isEmpty which is a valid syntax for getting that value (in for in loops you also can do this but the var is then before the Type name). We already have the rename refactor for changing the variable name to a new one if we want to, but I feel like this would help more than harm most of the time.

Describe the solution you'd like
For it to auto-complete with :var before the propriety name (if you know what lints are available maybe we can look at the ones that ask for the final keyword or the type name) on if cases and without the var keyword on for in loops.

[FMorschel]

@srawlins can you think of any other case where we can have variable destruction like this? Did I forget anything?

[FMorschel]

I also thought about code like destructing records:

var (: myVar) = record;

[FMorschel]

Similar to #60118, lots of invalid things here:

({int value1, String myString}) func() => (value1: 0, myString: 'string');

void main() {
  var (:^) = func();
}

But I'd also argue that the following (without :) should also show the same values as above and auto-add the : on completion. Things like A() and other classes are not valid here I don't think. Can someone confirm?

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Edit: Also, adding hashCode and whatever it showed there would make the assignment invalid so we sould only get the valid field names.

[FMorschel]

Can someone point me to where are extra code added on completion (do you know @DanTup)? I've already fixed the comment above locally, just trying to get it all in one go, it was really simple.

[DanTup]

Can someone point me to where are extra code added on completion (do you know @DanTup)?

Do you mean things like the (...)? If so, a lot of that comes from the LSP mapping (which has to build the different fields that are used in the completion) around here:

sdk/pkg/analysis_server/lib/src/lsp/completion_utils.dart

Line 527 in f276d3c

CompletionDetail _getCompletionDetail(

[FMorschel]

Do you mean things like the (...)?

No, sorry for not being more clear. My idea is that for:

void f(({int myInt,} r) {
  if (r case (:^) {}
}

It would show myInt but when selecting that option, since we have not written var/final it would add this keyword (or int, depending on active lints).