Environment variable for Cargo Workspace for finding workspace-wide assets

T-cargo notes:

Potential use cases:

• Accessing workspace-wide assets (this issue)
• Looking up file! at test runtime, see #15885
• Finding a binary's path, see
  • CARGO_BIN_EXE_<name>
  • #9661
• Looking up the lockfile, this would be insufficient because the lockfile path can be overridden. A new issue for this would be needed.
• Dependencies looking up information in your workspace: side-banned communication like this is rejected by the cargo team

Updates

• https://github.com/rust-lang/cargo/issues/3946#issuecomment-1402464796
• https://github.com/rust-lang/cargo/issues/3946#issuecomment-3224923131

Hi, while working on using workspace in html5ever, I've ran into issue of needing the CARGO_WORKSPACE directory, and being unable, to find it. What I resorted to is essentially, &Path(cargo_manifest).join("..") which feels hacky.

Could CARGO_WORKSPACE be added as environment variable? I'm not sure what it should be when there is no workspace defined, I assume it should either return Err or default it to CARGO_MANIFEST_DIR.

Sidenote I'm willing to work on this issue, if I could get quick pointers, to what I need to do.