Adjust UI tests for unit_bindings lint

[jieyouxu]

• Explicitly annotate let x: () = expr; where x has unit type, or remove the unit binding to leave only expr; instead.
• Use let () = init; or let pat = (); where appropriate.
• Fix disjoint-capture-in-same-closure test which wasn't actually testing a closure: tests/ui/closures/2229_closure_analysis/run_pass/disjoint-capture-in-same-closure.rs.

Note that unfortunately there's a lot of UI tests, there are a couple of places where I may have left something like let (): () (this is not needed but is left over from an ealier version of the lint) which is bad style.

This PR is to help with the unit_bindings lint at #112380.

[rustbot]

r? @compiler-errors

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[Nilstrieb]

r? Nilstrieb
@bors r+
changes look fine to me

[bors]

📌 Commit edafbaf has been approved by Nilstrieb

It is now in the queue for this repository.

[bors]

⌛ Testing commit edafbaf with merge 2ca8d35...

[bors]

☀️ Test successful - checks-actions
Approved by: Nilstrieb
Pushing 2ca8d35 to master...

[rust-timer]

Finished benchmarking commit (2ca8d35): 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)	4.1%	[4.1%, 4.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.1%	[4.1%, 4.1%]	1

Cycles

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Binary size

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Bootstrap: 647.719s -> 647.795s (0.01%)