Give a better diagnostic for missing parens in Fn* bounds

[clubby789]

Fixes #108109

It would be nice to try and recover here, but I'm not sure it's worth the effort, especially as the bounds on the recovered function would be incorrect.

[rustbot]

r? @TaKO8Ki

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[clubby789]

Follow-up PR #117298 to address the other case mentioned in the issue of fn main -> () {}

[TaKO8Ki]

@bors r+ rollup

[bors]

📌 Commit 904acee has been approved by TaKO8Ki

It is now in the queue for this repository.

[bors]

⌛ Testing commit 904acee with merge 187d1af...

[bors]

☀️ Test successful - checks-actions
Approved by: TaKO8Ki
Pushing 187d1af to master...

[rust-timer]

Finished benchmarking commit (187d1af): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results

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)	4.1%	[0.6%, 7.5%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.7%	[-0.8%, -0.5%]	2
All ❌✅ (primary)	-	-	0

Cycles

Results

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)	0.6%	[0.6%, 0.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results

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)	-0.0%	[-0.0%, -0.0%]	6
All ❌✅ (primary)	-	-	0

Bootstrap: 662.103s -> 663.805s (0.26%)
Artifact size: 308.97 MiB -> 308.97 MiB (-0.00%)

[jsenkpiel-godaddy]

Nice looks like the fix to my issue was pretty simple, maybe I'll be able to follow that next time