internal: Forbid canonicalization of paths and normalize all rust-project.json paths

[Veykril]

Closes #14728
cc #14430

• Removes canonicalization (and forbids it from being used in a sense, clippy could help here again with its lint in the future)
• Normalizes all paths in rust-project.json which we weren't doing in some cases

[Veykril]

r? @matklad

(the stackoverflow post you linked outlines that clang also accepts relative paths to base, but I think given for this file we expect it to be generated anyways there is little use for allowing that)

[Veykril]

Note to self: Comment on #4887 regarding the changes from this PR

[matklad]

Tbh, I’d keep relative paths, resolving relative to the location of the file seems like a common idiom, and it’s convenient to, eg, have several checkouts in different dirs using git worktree, or to check the file into VCS.

What we should avoid is cwd-relative paths, but that’s why the abspath infra exists.

[matklad]

just

data.sysroot.map(|it| base.join(it)),

should do the trick. That's the semantics of Rust's join: if the path is absolute, base is ignored. It is a bit surprising, perhaps we can benefit from

impl AbsPath {
    pub fn absolutize(&self, path: impl AsRef<Path>) -> AbsPathBuf;
    pub fn join(&self, path: &RelPath) -> AbsPathBuf;
}
impl RelPath {
    pub fn join(&self, path: &RelPath) -> RelPathBuf;
}

[Veykril]

@bors r+

[bors]

📌 Commit f47caa6 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit f47caa6 with merge a7944a9...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing a7944a9 to master...