linker: Remove special case for rust-lld in detect_self_contained_mingw

[petrochenkov]

rust-lld is supposed to live inside sysroot, so it doesn't change the behavior of the function, only removes a potential micro-optimization.

#149178 (comment)
r? @mati865

[mati865]

Shouldn't hurt.

@bors r+

[bors]

📌 Commit 388c42e has been approved by mati865

It is now in the queue for this repository.

[petrochenkov]

@bors rollup

[bjorn3]

Below only the sysroot from which we get the standard library is checked, not the fallback sysroot that rustc itself is contained in. rust-lld is likely to be contained only in the latter if the former is overriden.

[mati865]

I'm fairly sure from my tests the sysroot would point at ~/.rustup/toolchains/<toolchain>.

If I read

rust/compiler/rustc_session/src/filesearch.rs

Line 190 in 8202d11

fn default_from_rustc_driver_dll() -> Result<PathBuf, String> {

right, it would indeed stirp all the subdirectories.

[bjorn3]

The code below only strips sess.opts.sysroot.path(), which only returns the value explicitly passed to --sysroot if passed, only returning the fallback sysroot when it isn't passed.

rust/compiler/rustc_session/src/config.rs

Lines 1345 to 1348 in 8202d11

	/// Return explicit sysroot if it was passed with `--sysroot`, or default sysroot otherwise.
	pub fn path(&self) -> &Path {
	self.explicit.as_deref().unwrap_or(&self.default)
	}

You need sess.opts.sysroot.all_paths() to check both.

[mati865]

Good point, but is --sysroot ever passed to rustc when building natively (host == target)?

[bjorn3]

There are legitimate reasons to pass it when building natively. For example miri does for it's copy of the standard library that contains MIR for all functions (though it doesn't need a linker). cargo build -Zbuild-std doesn't use --sysroot currently, but I think it should pass --sysroot /dev/null or something like that to prevent accidentally picking up libraries from the sysroot. The only thing that leads to is lang item conflicts. --sysroot /dev/null would give nicer error messages.

[mati865]

--sysroot /dev/null would break all cases of implicit self-contained detection, not just rust-lld. I mean, there is no obvious issue caused by this PR, not that there is no general issue with implicit self-contained detection.

[bjorn3]

Right. The self-contained components are themself only looked up in --sysroot. Didn't think of that.