rustc_llvm need to be rebuilt after ./x.py check

[matthiaskrgr]

repo @ edc0258
running

./x.py build
./x.py check

invalidates the build cache for the rustc_llvm crate.
This means that after

./x.py build
./x.py check
./x.py build

the last x.py build will have to rebuild rustc_llvm, rustc_codegen_llvm, rustc_interface, rustc_driver and rustc-main which is less than optional given no code was changed.

[cuviper]

I think it's because of this:

rust/src/bootstrap/builder.rs

Lines 767 to 771 in edc0258

	// Set a flag for `check`/`clippy`/`fix`, so that certain build
	// scripts can do less work (e.g. not building/requiring LLVM).
	if cmd == "check" || cmd == "clippy" || cmd == "fix" {
	cargo.env("RUST_CHECK", "1");
	}

rust/src/librustc_llvm/build.rs

Lines 18 to 22 in edc0258

	if env::var_os("RUST_CHECK").is_some() {
	// If we're just running `check`, there's no need for LLVM to be built.
	println!("cargo:rerun-if-env-changed=RUST_CHECK");
	return;
	}

[Mark-Simulacrum]

Interesting. Ideally we'd have two separate caches for rustc_llvm and the other crates such that x.py check doesn't affect this... I think.

[jonas-schievink]

Is there any workaround for this other than not using ./x.py check?

[Mark-Simulacrum]

Fixed in #71815