Support custom remapping rules with trim-paths for `bootstrap`

[Urgau]

Problem

In rust-lang/rust we use a custom (and unique) remapping schema, it would be necessary for bootstrap and rust-lang/rust that Cargo supports using a custom remapping rule, otherwise we won't be able to use Cargo trim-paths feature.

https://github.com/rust-lang/rust/blob/1b29cfdbee8ee37333f4b24afb936350eb57e705/src/bootstrap/src/lib.rs#L362-L370

Proposed Solution

A potential solution could be to provide a nightly-only environment variable that bootstrap could set.

Notes

Related to #12137

[weihanglo]

I saw a few remap schemes there.

• Compiler — https://github.com/rust-lang/rust/blob/84b36a78a28a63f134171c670be1932ffa2485f8/src/bootstrap/src/core/builder/cargo.rs#L1114-L1147
• NonCompiler — https://github.com/rust-lang/rust/blob/84b36a78a28a63f134171c670be1932ffa2485f8/src/bootstrap/src/core/builder/cargo.rs#L1148-L1174
  • This also applies to cc-rs https://github.com/rust-lang/rust/blob/0c45ca314d60447c808f437c9df49ac81c0dc23d/src/bootstrap/src/lib.rs#L1324-L1333
• Vendor and crates-io deps — https://github.com/rust-lang/rust/blob/84b36a78a28a63f134171c670be1932ffa2485f8/src/bootstrap/src/core/builder/cargo.rs#L1178-L1194

However, I also saw this comment:

// `rustc` needs to know the remapping scheme, in order to know how to reverse it (unremap)
// later. Two env vars are set and made available to the compiler
//
// - `CFG_VIRTUAL_RUST_SOURCE_BASE_DIR`: `rust-src` remap scheme (`NonCompiler`)
// - `CFG_VIRTUAL_RUSTC_DEV_SOURCE_BASE_DIR`: `rustc-dev` remap scheme (`Compiler`)
//
// Keep this scheme in sync with `rustc_metadata::rmeta::decoder`'s
// `try_to_translate_virtual_to_real`.

I have a couple of questions:

• How does unremap work in rustc/bootstrap? If the remap is fully in Cargo, how can it unremap back?
• Probably related to the previous one, there are -Zsimulate-remapped-rust-src-base, -Ztranslate-remapped-path-to-local-path, -Zignore-directory-in-diagnostics-source-blocks. What are we going to deal with them?
• Why does OUT_DIR refer to here? (I am afraid of on Windows Cargo trim-paths doesn't deal with forward slahses correctly.)

[Urgau]

I completely forgot that we now set multiple remapping rules, that is quite recent and is due to the way Cargo is invoking rustc: relative paths for workspace dependencies and absolute paths for registry dependencies.

How does unremap work in rustc/bootstrap?

Unremapping is mostly a rustc thing, bootstrap only sets a environment variable at built-in so rustc knows which prefix to look for, once the given prefix is found rustc tries to find if rust-src/rustc-dev is installed and if so unremaps the paths.

This unremap is done so that if a user has rust-src (for std) or rustc-dev (for rustc, useful for Clippy devs) that we show the local paths to file when possible.

If the remap is fully in Cargo, how can it unremap back?

Same way, rustc only needs to know the prefix, as long as bootstrap keeps setting it (and that should still be the case), everything should continue to work.

Probably related to the previous one, there are -Zsimulate-remapped-rust-src-base, -Ztranslate-remapped-path-to-local-path, -Zignore-directory-in-diagnostics-source-blocks. What are we going to deal with them?

No need to worry about those CLI args, they are nightly-only and only here to "simulate" the remapping if it wasn't done, as is quite common when developing on rustc; nobody builds with remapping on, but the tests runs.

• Why does OUT_DIR refer to here?

If I remember correctly, I believe bootstrap wasn't using / (forwards-slashes) on Windows. See rust-lang/rust#156504.

This may be a bootstrap specific issue, the self.build.out path may be UNIX style even on Windows and as such not match Windows conventions.