Use a path instead of a string for emit in rustc command-line arguments.

[matts1]

This can improve the cache hit rate, as it will work with path mapping.

Also generate the emit path unconditionally, as this works better with the binary_ext parameter better (as it guarantees that you won't get an error where the file names don't match).

[krasimirgg]

Looks good!

This can improve the cache hit rate, as it will work with path mapping.

Could you go into more detail what's path mapping?

[matts1]

Suppose you enter a transition from foo=False to foo=True, and foo doesn't affect a rust binary you're building.

Bazel will still have to rebuild, because when entering a transition it changes the output path from bazel-out/k8-fastbuild/... to bazel-out/<hash>. However, with path mapping, you can tell bazel that these are equivalent, and that one of these paths maps to another in the action key.

[krasimirgg]

Thank you! Looks good!