Add convert_for_to_iter_for_each assist

[mattyhall]

Implements one direction of #7681

I wonder if this tries to guess too much at the right thing here. A common pattern is:

let col = vec![1, 2, 3];
for v in &mut col {
  *v *= 2;
}
// equivalent to:
col.iter_mut().for_each(|v| *v *= 2);

I've tried to detect this case by checking if the expression after the in is a (mutable) reference and if not inserting iter()/iter_mut(). This is just a convention used in the stdlib however, so could sometimes be wrong. I'd be happy to make an improvement for this, but not sure what would be best. A few options spring to mind:

1. Only allow this for types that are known to have iter/iter_mut (ie stdlib types)
2. Try to check if iter/iter_mut exists and they return the right iterator type
3. Don't try to do this and just add .into_iter() to whatever is after in

[Veykril]

You'll have to rebase as the assists crate has been renamed to ide_assists

[Veykril]

I think the best approach here is to just wrap the iterable in (_).into_iter() and special case it to iter{_mut} when we know those functions exist and are applicable.

Edit: Actually we should be able to check if an iter{_mut} function exists on a type with code_model::Type::iterate_method_candidates. That seems better than special casing.

[Veykril]

2 more things aside from that it looks good to me 👍
bors d+

[bors]

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[mattyhall]

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)

[Veykril]

[masklinn]

FWIW this seems to somewhat unhelpfully trigger on for loops using flow control (continue/break/return) yielding broken code. Is that known / intended?

I think in that case the assist either should not trigger, or it should use try_for_each.

[Veykril]

That is not intended and was an oversight, could you open an issue for that?

[masklinn]

That is not intended and was an oversight, could you open an issue for that?

Gladly, sorry I wasted such a fun issue number as #11111 for it.