Support `cargo vendor` for a specific workspace package

[chbaker0]

Problem

When run in a workspace, cargo vendor will always vendor all packages for that workspace. This is true even when run from a package subdir (even though the vendor dir will be put in that subdir, confusingly).

It can be useful to vendor the dependencies needed to build some subset of workspace packages.

Example: for projects with their own toolchain build, vendoring the dependencies needed to build the standard library is often desirable. Especially when the standard library is built with an alternative build system. Chromium does this, building the standard library sysroot using its GN build system. cargo vendor will include many unnecessary dependencies right now.

Proposed Solution

Either:

• cargo vendor in a workspace package will vendor only that packages' dependencies.
• cargo vendor -p foo will vendor only foo's dependencies. This would match other subcommands' behavior.

Notes

No response

[weihanglo]

See also

• vendor only certain crates or only crates from certain sources #9234 for vendoring packages from certain sources.
• Support for vendoring on a single platform #7058 for vendoring packages on certain platforms.