coverage: Regression test for functions with unreachable bodies

[Zalathar]

This is a regression test for the coverage issue that was addressed temporarily by #116166, and is tracked by #116171.

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If we instrument a function for coverage, but all of its counter-increment statements are removed by MIR optimizations, LLVM will think it isn't instrumented and it will disappear from coverage maps and coverage reports.

Most MIR opts won't cause this because they tend not to remove statements from bb0, but UnreachablePropagation can do so if it sees that bb0 ends with TerminatorKind::Unreachable.

Currently we have worked around this by turning off UnreachablePropagation when coverage instrumentation is enabled, which is why this test is able to pass.

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@rustbot label +A-code-coverage

[rustbot]

r? @Mark-Simulacrum

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[Zalathar]

Currently I've added two copies of the test, one for tests/coverage-map and one for tests/run-coverage.

I don't like having to do this, which is why I started looking into ways to let both suites share the same directory of test files (Zulip thread). But that turned out to be non-trivial, so for now I'm just adding both copies.

[cjgillot]

@bors r+ rollup

[bors]

📌 Commit 3de0824 has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3de0824 with merge 51ddc74...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 51ddc74 to master...

[rust-timer]

Finished benchmarking commit (51ddc74): 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)	3.3%	[3.3%, 3.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
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: 628.185s -> 627.563s (-0.10%)
Artifact size: 273.33 MiB -> 273.33 MiB (-0.00%)