coverage: Be more strict about what counts as a "visible macro"

[Zalathar]

This is a follow-up to the workaround in #117827, and I believe it now properly fixes #117788.

The old code treats a span as having a “visible macro” if it is part of a macro-expansion, and its parent callsite's context is the same as the body span's context. But if the body span is itself part of an expansion, the macro in question might not actually be visible from the body span. That results in the macro name's length being meaningless as a span offset.

We now only consider spans whose parent callsite is the same as the source callsite, i.e. the parent has no parent.

---

I've also included some related cleanup for the code added by #117827. That code was more complicated than normal, because I wanted it to be easy to backport to stable/beta.

[rustbot]

r? @TaKO8Ki

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[Zalathar]

@rustbot label +A-code-coverage

[TaKO8Ki]

Thanks. @bors r+

[bors]

📌 Commit 242bff3 has been approved by TaKO8Ki

It is now in the queue for this repository.

[bors]

⌛ Testing commit 242bff3 with merge 6316ac8...

[bors]

☀️ Test successful - checks-actions
Approved by: TaKO8Ki
Pushing 6316ac8 to master...

[rust-timer]

Finished benchmarking commit (6316ac8): 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)	1.2%	[1.2%, 1.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	3.7%	[3.7%, 3.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 674.182s -> 675.976s (0.27%)
Artifact size: 314.16 MiB -> 314.13 MiB (-0.01%)