Add `x.py check --stage 1`

[Diggsey]

When modifying the interface between platform-specific/non-platform-specific parts of the codebase, proper testing requires being able to build for every supported target, which is very impractical. Ideally x.py would support cargo check such that platform specific code could still be validated and type checked without needing the corresponding cross-compiling C toolchains.

This requires at least #44737 to be fixed to be useful.

[pietroalbini]

Does #47460 do everything you wanted?

[Diggsey]

I think it would need to be extended to support stage 1 to be useful for the case I outlined above.

[pickfire]

Is this fixed yet?

[jyn514]

I think it would need to be extended to support stage 1 to be useful for the case I outlined above.

This is sort of a hack, but you can do this by

1. Building a stage1 compiler (x.py build --stage 1 library/std)
2. Adding a new git worktree (git worktree add ../rust2)
3. Setting the newly built compiler as stage 0 for the new worktree (rustc = "/path/to/rust1/build/$TARGET/stage1/bin/rustc")
4. Running x.py check like normal.

I agree it would be great to support x.py check --stage 1 or something like that.

[jyn514]

This was added in #81064.