coverage: Consistently remove unused counter IDs from expressions/mappings

[Zalathar]

If some coverage counters were removed by MIR optimizations, we need to take care not to refer to those counter IDs in coverage mappings, and instead replace them with a constant zero value. If we don't, llvm-cov might see a too-large counter ID and silently discard the entire function from its coverage reports.

Fixes #117012.

[rustbot]

r? @petrochenkov

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[Zalathar]

@rustbot label +A-code-coverage

[Zalathar]

Once this is fixed, it should hopefully be feasible to make run-coverage tests use -Copt-level=2 by default, which should help with detecting any future problems between optimizations and coverage.

[Zalathar]

@rustbot ready

[petrochenkov]

@bors r+

[bors]

📌 Commit 230dd5b has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit 230dd5b with merge 6b78377...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 6b78377 to master...

[rust-timer]

Finished benchmarking commit (6b78377): comparison URL.

Overall result: ✅ improvements - 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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	2
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)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	3.0%	[2.0%, 4.6%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[2.4%, 2.4%]	1

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)	2.8%	[1.4%, 5.0%]	15
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 637.097s -> 634.236s (-0.45%)
Artifact size: 304.41 MiB -> 304.41 MiB (-0.00%)