Logic to detect rustc version is incomplete

[taiki-e]

The current way does not handle it correctly when the version is specified in a cargo or rustup specific way (e.g., cargo +nightly-..., rustup run +nightly-...).

cargo-llvm-cov/src/rustc.rs

Lines 16 to 22 in eb7c3f3

	let path = env.rustc.as_deref().unwrap_or_else(|| OsStr::new("rustc"));
	let version = cmd!(path, "--version").dir(workspace_root).read()?;
	let nightly = version.contains("-nightly") || version.contains("-dev");
	let mut cmd = if nightly { cmd!(path) } else { cmd!("rustup", "run", "nightly", "rustc") };
	cmd.args(&["--version", "--verbose"]);
	let verbose_version = cmd.read()?;

Currently, the following heuristic is the only one that uses the retrieved rustc information, so it does not affect the coverage results, but it may accidentally remove dependency's build artifacts.

cargo-llvm-cov/src/main.rs

Line 100 in eb7c3f3

// - If there are old artifacts by a different version of rustc: remove all artifacts (handled in Context::new)

related(?): rust-lang/cargo#2833

[taiki-e]

In a build script, the RUSTC environment variable can be used for this purpose, but in a subcommand, the RUSTC environment variable has a different meaning.

If we need the major-minor-patch version of the toolchain (e.g., cargo-hack's --version-range), cargo --version --verbose is fine, but if we need the exact toolchain date of nightly rustc, cargo --version --verbose is not enough.

So I think the correct action here is just to remove this heuristic. (IICU, the way introduced in #79 should work well without rustc version-based heuristics.)

[taiki-e]

So I think the correct action here is just to remove this heuristic. (IICU, the way introduced in #79 should work well without rustc version-based heuristics.)

In fact, it relies on information from rustc here as well, but that can be avoided.

cargo-llvm-cov/src/context.rs

Line 100 in eb7c3f3

let rustlib = sysroot.join(format!("lib/rustlib/{}/bin", rustc.host));

[taiki-e]

Oh, actually I can find the rustc binary based on the sysroot.