Add support for MC/DC coverage

[aytey]

MC/DC coverage recently landed in rustc:

• Implement Modified Condition/Decision Coverage rust-lang/rust#123409
• coverage: Prepare for improved branch coverage rust-lang/rust#124217

but cargo-llvm-cov doesn't yet pass the -show-mcdc flag to llvm-cov, so we cannot get reports showing MC/DC coverage.

This PR adds support for -show-mcdc to llvm-cov when working with cargo-llvm-cov.

I checked: if RUSTFLAGS contains -Zcoverage-options=branch (i.e., MC/DC is not enabled) then the reports are the same as if you didn't pass -show-mcdc (i.e., you don't get "empty" MC/DC tables).

[aytey]

Oh, I hadn't realise it would fail if you used a non-nightly with that flag.

Is there a way to check if the current version of rustc supports MCDC?

[taiki-e]

Is there a way to check if the current version of rustc supports MCDC?

I think you can check rustc version like:

cargo-llvm-cov/src/main.rs

Lines 205 to 211 in 550dc19

	// Workaround for https://github.com/rust-lang/rust/issues/91092.
	// Unnecessary since https://github.com/rust-lang/rust/pull/111469.
	let needs_atomic_counter_workaround = if cx.ws.rustc_version.nightly {
	cx.ws.rustc_version.major_minor() <= (1, 71)
	} else {
	cx.ws.rustc_version.major_minor() < (1, 71)
	};

[taiki-e]

The actual error here is due to the version of llvm-cov command (-show-mcdc requires LLVM 18+), so a check like cmd!(&cx.llvm_cov, "show", "--help").read()?.contains("-show-mcdc") might be better.