`rust.codegen-backends` interacts confusingly with paths

[jyn514]

I tried this code:

x build rustc_codegen_cranelift

I expected this to happen: Bootstrap builds cranelift, since I named it explicitly.
Instead, this happened: Nothing, since I didn't have cranelift configured in codegen-backends:

rust/src/bootstrap/compile.rs

Lines 998 to 1009 in ce6adcc

	impl Step for CodegenBackend {
	type Output = ();
	const ONLY_HOSTS: bool = true;
	// Only the backends specified in the `codegen-backends` entry of `config.toml` are built.
	const DEFAULT: bool = true;
	fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
	run.paths(&["compiler/rustc_codegen_cranelift", "compiler/rustc_codegen_gcc"])
	}
	fn make_run(run: RunConfig<'_>) {
	for &backend in &run.builder.config.rust_codegen_backends {

At a minimum, I would expect this to give an error or warning that I need to configure codegen-backends. It would be even nicer if we could treat codegen-backends as only controlling the defaults, instead of as a hard-off switch, like how rust.compiler-docs works. That should be possible to do by looking at the run.paths we're passed.

cc @bjorn3

[lenko-d]

@rustbot claim

[bjorn3]

The first element in codegen-backends is used as default backend. In addition even if the CodegenBackend step were to directly use the passed in paths, assembling the toolchain did still only add the backends in codegen-backends to the sysroot. Having path specifiers for one step affect another step doesn't feel right. For check builds I do agree this is an improvement. Although I personally actually use this behavior to have ./x.py check compiler/rustc_codegen_cranelift build all backends, not just cranelift (by enabling all in codegen-backends) It did be nice to have a replacement for this. In addition for ./x.py build at the very least a warning makes sense.

[jyn514]

Having path specifiers for one step affect another step doesn't feel right.

Does it make sense to separate cg_clif and cg_gcc into different steps in that case?

cc @antoyo

[bjorn3]

That doesn't have any effect on if specifying a backend as path will add it to the sysroot.