Handle remapped paths correctly when generating "Source" links

[Urgau]

Fixes #150100.

This PR fixes a regression introduced by #149709, I was overzealous in my changes (https://github.com/rust-lang/rust/pull/149709/changes#diff-e1cf7ef2fb411d24980cd4cbea1e867cc36029e9496e1ceca64cfb6a0e3510f6) and accidentally changed the behavior of rustdoc in the presence of remapped, to simply reject them instead of handling them.

With this PR remapped paths are handled correctly, in a similar way as it was before.

I added a run-make test to make sure we don't regress it again, a simple rustdoc test in not sufficient as rustdoc is not called on the auxiliary crate. It's not pretty but it works. #150172 (comment)

rustdoc doesn't have any handling for --remap-path-scope, so I used the MACRO scope (it was already used elsewhere.

cf. https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/source.20links.20missing.20for.20nightly.20std.20docs

r? @GuillaumeGomez

[fmease]

a simple rustdoc test in not sufficient as rustdoc is not called on the auxiliary crate

//@ build-aux-docs enables doc building for auxiliaries.

[Urgau]

//@ build-aux-docs enables doc building for auxiliaries.

This almost work, except that the compile-flags of the auxiliary crate are also passed to rustdoc, and unfortunately it trips an assertion in rustdoc. EDIT: lol we trip the same assertion in another test.

Nevermind, the fix was incomplete. It works fine now.

Details

thread 'rustc' (52267) panicked at src/librustdoc/html/sources.rs:349:14:
only local crates should have sources emitted

[GuillaumeGomez]

Suggested change

	// even if it's paths are remapped.
	// even if its paths are remapped.

[GuillaumeGomez]

@Urgau told me they would be able to update the PR until tomorrow and told me to push if I wanted it to be merged sooner. So considering it's a big regression, I pushed a commit (to not appear as co-author of a previous one considering it's such a small change...). Will need to nominate it for beta-backport once merged as well.

In any case.

@bors r+ rollup

[bors]

📌 Commit c039edd has been approved by GuillaumeGomez

It is now in the queue for this repository.