Tracking Issue for path-bases

[dpaoliello]

Summary

RFC: #3529
Original issue: #9855
Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#path-bases
Implementation:

• Implement path-bases (RFC 3529) 1/n: path dep and patch support #14360
• Implement path-bases (RFC 3529) 2/n: cargo [add|remove|update] support #14427
• 3/n [patch] table in config support

Introduce shared base directories in Cargo configuration files that in turn enable base-relative path dependencies.

Changes from RFC

• base naming was switched to package / registry naming rules, meaning we support unicode instead of just ASCII (Implement path-bases (RFC 3529) 1/n: path dep and patch support #14360)

Unresolved Issues

• How should each layer (config, eventually manifest) layer? Section-wide, per-field? We should have tests to confirm the approach we have
• Can we switch our existing "make dep paths absolute" to build on top of this? (Implement path-bases (RFC 3529) 1/n: path dep and patch support #14360)
• What exact names we should use for the table (path-bases) and field names (base)?
• What other built-in base paths could be useful?
  • package or current-dir for the directory of the current project?
  • home or user_home for the user's home directory?
  • sysroot for the current rustc sysroot?
• Do we actually want to support the [patch] table? Adding support for [patch] in config files greatly complicates the implementation
• Should base = "workspace" work with implicit workspaces?

Future Extensions

• Add support for declaring path bases in the manifest
  • Motivating example: You have a Git dependency that has a path dependency to one of its submodules. It would be nice if the Git dependency could use a path base to point to the submodule, but currently this is broken as the configuration file in that git dependency is never loaded. See https://blog.rust-lang.org/inside-rust/2024/08/15/this-development-cycle-in-cargo-1.81.html#path-bases
• Path bases relative to other path bases
• Path dependency with just a base
• Git dependencies

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