Check Rustdoc warnings for the compiler in PR CI

[Noratrieb]

Docs (and specially intra-doc-links) are easy to get wrong and rarely tested locally, causing many rollups to fail. We should check them in CI as well to catch mistakes early.

[albertlarsan68]

@rustbot claim

[the8472]

But ./x test linkchecker already runs in PR CI

[jyn514]

cc #106117

maybe linkchecker is missing some cases?

[Noratrieb]

The most recent case I've seen was this: #100036 (comment)
where broken docs broke a try build while PR CI was green (that wasn't an intra doc link though but still docs)

[jyn514]

Ah ok, this is specifically about rustdoc warnings, not broken links. That's definitely doable to do, we can shove x doc --stage 0 compiler into mingw-check :) it doesn't need to build the compiler at all, only check it.

[lukas-code]

But ./x test linkchecker already runs in PR CI

Apparently ./x test linkchecker doesn't actually build the compiler docs (or tool docs) when the build.compiler-docs flag isn't set in config.toml. I don't know if this is a bug or a feature, but we could just set the flag for CI here:

rust/src/ci/docker/host-x86_64/x86_64-gnu-llvm-13/Dockerfile

Lines 45 to 49 in 002eccc

	ENV RUST_CONFIGURE_ARGS \
	--build=x86_64-unknown-linux-gnu \
	--llvm-root=/usr/lib/llvm-13 \
	--enable-llvm-link-shared \
	--set rust.thin-lto-import-instr-limit=10