Consolidate staging for rustc_private tools

[Kobzol]

This PR continues bootstrap refactoring, this time by consolidating staging for Mode::ToolRustc tools. This refactoring was in the critical path of refactoring test/dist/clippy/doc steps, and getting rid of the rmeta/rlib sysroot copy, because tools are pervasive and they are being used for a lot of things in bootstrap.

The main idea is to explicitly model the fact that a stage N Mode::ToolRustc tool always works with two different compilers:

• Stage N-1 rustc (build_compiler) builds stage N rustc (target_compiler)
• Rlib artifacts from stage N rustc are copied to the sysroot of stage N-1 rustc
• Stage N-1 rustc builds the (stage N) tool itself, the tool links to the rlib artifacts of the stage N rustc

Before, the code often used compiler, which meant sometimes the build compiler, sometimes the target compiler, and sometimes neither (looking at you, download-rustc). This is especially annoying when you get to a situation where you have an install step that invokes a dist step that invokes a tool build step, where some compiler is being propagated through, without it being clear what does that compiler represent. This refactoring hopefully makes that clearer and more explicit. It also gets rid of a few builder.ensure(Rustc(...)) calls within bootstrap, which is always nice.

Rustdoc needs to be handled a bit specially, because it acts as a compiler itself, I documented that in the changes.

It wasn't practical to do these refactorings in multiple PRs, so I did it all in one PR. The meat of the change is 9ee6d1c.

I tested manually that x build rustdoc and x build miri still works even with download-rustc, although I cannot promise any extra support for download-rustc, IMO we will just have to reimplement it from scratch in a different way.

As usually, I did some drive-by refactorings to bootstrap, trying to document and clarify things, add more step metadata and tests.

Best reviewed commit-by-commit (note that I renamed link_compiler to target_compiler, in accordance to the rest of bootstrap, in the last commit).

r? @jieyouxu

[rustbot]

jieyouxu is not on the review rotation at the moment.
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[bors]

☔ The latest upstream changes (presumably #143412) made this pull request unmergeable. Please resolve the merge conflicts.

[jieyouxu]

Will start looking at this today, might take me some time to get through.

[jieyouxu]

Thanks, looks good overall. You can r=me after comment nit.

[jieyouxu]

Discussion (non-blocking): we still have the problem where cg backends fit "weirdly" into builds, right?

[Kobzol]

Yes, we should probably overhaul the way we manage and apply codegen backends.

[jieyouxu]

@rustbot author

[Kobzol]

@bors r=jieyouxu

[bors]

📌 Commit 7bf4a1a has been approved by jieyouxu

It is now in the queue for this repository.