Don't suggest adding return type for closures with default return type

[wafarm]

Follow up of #129223

r? @compiler-errors

[compiler-errors]

There's some redundancy between this check and the can_suggest boolean above. Can you find out if we can just reuse can_suggest in this position, rather than making the check from within the function?

[wafarm]

I did some investigations and I found that

rust/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

Line 940 in bf662eb

hir::ClosureKind::Closure => Some((def_id, fn_decl, true)),

If I set can_suggest to false for closures here, then for tests/ui/closures/add_semicolon_non_block_closure.rs

rust/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

Lines 829 to 835 in bf662eb

	match &fn_decl.output {
	&hir::FnRetTy::DefaultReturn(span) if expected.is_unit() && !can_suggest => {
	// `fn main()` must return `()`, do not suggest changing return type
	err.subdiagnostic(errors::ExpectedReturnTypeLabel::Unit { span });
	return true;
	}
	&hir::FnRetTy::DefaultReturn(span) if expected.is_unit() => {

code here will go to the upper branch. I think we still need some way to distinguish main and closures or we will get something like this:

 --> testfile.rs:8:12
  |
8 |     foo(|| bar())
  |           -^^^^^ expected `()`, found `i32`
  |           |
  |           expected `()` because of default return type
  |

I don't expect the last line to be printed.

I've thought of some possible solutions:

• The above result is acceptable?
• Keep my original solution, which indeed adds to complexity
• Add is_main to the return values of get_fn_decl, which seems (I think) a bit overkill and may impact performance if it's getting called a lot
• Refactor some of the code to see if we can find a better solution

[compiler-errors]

Would appreciate testing out the suggestion I had.

[wafarm]

I removed can_suggest from get_fn_decl as the only usage of it is to be passed to suggest_missing_return_type. Instead, I do the check in a new function can_add_return_type.

This also fixed not suggesting adding return type in non-trait impl blocks.

@rustbot ready

[compiler-errors]

@bors r+

[bors]

📌 Commit 736ab66 has been approved by compiler-errors

It is now in the queue for this repository.