Use parenthetical notation for Fn traits

[estebank]

Always use the Fn(T) -> R format when printing closure traits instead of Fn<(T,), Output = R>.

Address #67100:

error[E0277]: expected a `Fn()` closure, found `F`
 --> file.rs:6:13
  |
6 |     call_fn(f)
  |     ------- ^ expected an `Fn()` closure, found `F`
  |     |
  |     required by a bound introduced by this call
  |
  = note: wrap the `F` in a closure with no arguments: `|| { /* code */ }`
note: required by a bound in `call_fn`
 --> file.rs:1:15
  |
1 | fn call_fn<F: Fn() -> ()>(f: &F) {
  |               ^^^^^^^^^^ required by this bound in `call_fn`
help: consider further restricting this bound
  |
5 | fn call_any<F: std::any::Any + Fn()>(f: &F) {
  |                              ++++++

[rustbot]

r? @jieyouxu

rustbot has assigned @jieyouxu.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[compiler-errors]

r=me when ci

[compiler-errors]

To be clear, since it was not acknowledged in the PR description, this is not a general fix for #67100. Specifically, the suggestion is not going to be right when the trait bound suggestion needs a return type, since all we have is a trait ref (no return type) and not a projection predicate (has a return type).

Slightly modifying the test from the issue:

fn call_fn<F: Fn() -> i32>(f: &F) {
    f()
}

fn call_any<F: std::any::Any>(f: &F) {
    call_fn(f)
}

fn main() {
    call_any(&|| { 1 });
}

...which suggests Fn() instead of Fn() -> i32. I would prefer if you keep that issue open or file a new issue to capture this, though for the purposes of error messages I'm fine if this PR goes in as-is.

[estebank]

@compiler-errors you're absolutely right. I missed that because I looked at the suggestion to turn Fn<Args, Output = Ret> to Fn(Arg, Arg) -> Ret which does handle that correctly (because we do have the whole info then) and failed to notice that of course didn't apply here. Furthermore, the follow up errors aren't great either.

[estebank]

@bors r=compiler-errors

[bors]

📌 Commit e6bd6c2 has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit e6bd6c2 with merge 1d52972...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 1d52972 to master...

[rust-timer]

Finished benchmarking commit (1d52972): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.6%	[1.6%, 1.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -3.4%, secondary -3.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.4%	[-4.4%, -2.1%]	3
Improvements ✅ (secondary)	-3.2%	[-3.8%, -2.2%]	9
All ❌✅ (primary)	-3.4%	[-4.4%, -2.1%]	3

Cycles

Results (secondary -2.7%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.7%	[-2.7%, -2.7%]	1
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 670.518s -> 669.197s (-0.20%)
Artifact size: 318.94 MiB -> 318.98 MiB (0.01%)