Cargo varies between absolute/relative path passing vendor dir to rustc

[nipunn1313]

Here's the setup with two workspaces

.cargo
   config
w1/
   Cargo.toml
   vendor/
w2/
   Cargo.toml

The .cargo/config points to

[source.vendored-sources]
directory = "./w1/vendor"

When compiling something in w1, the command line might look like this:

rustc --crate-name byteorder vendor/byteorder-1.1.0/src/lib.rs

When compiling something in w2 the command line might look like this

rustc --crate-name byteorder /Users/nipunn/src/client/rust/vendor/byteorder-1.1.0/src/lib.rs

Given that I share a CARGO_HOME across these two, and AFAICT, the cargo dep caching is based on rustc command line, if I alternate between compiling these two, it kicks off full recompiles.

If I move the vendor directory up to root, they both use absolute paths, and deps are cached across the two projects.

Proposed solution: cargo should always pass an absolute path to the vendor directory to rustc to work around this issue.

[dwijnand]

Similar (but distinct) to #5895, which is about relative paths in error messages.

[nipunn1313]

For anyone else running into this:
our workaround was to move the vendor directory outside of all of our workspaces (in our case the top level). This avoids dep recompiles when switching between workspaces that share a vendor dir.