Pass path or filename of built artifact to tests

Describe the problem you are trying to solve

I want to run compile test for procedural macros, but the test does not know how to look for the procedural library it should be testing.

Procedural macros are built to libmy_macro-xxxxxx.so in <target-dir>/<build-type>/deps. While it's possible to direct rustc to look at deps directory, there can be multiple instances of the library in the directory, making rustc unable to decide which one should be picked.

When cargo invokes rustc, however, it passes --extern my_macro=... to pick specific file for it, but tests have no way to get such information, mostly because it doesn't know the hash.

Describe the solution you'd like

It would be great if Cargo can pass the path or at least the file name of the built artifact to test via some new environment variable, for example CARGO_LIB_<name> or something like that, similar to the existing CARGO_BIN_EXE_<name>.

Notes

Start by tracing Cargo's existing CARGO_BIN_EXE_ handling and how test invocations receive build-artifact information. Determine where the procedural macro artifact path or filename can be exposed, then verify that a test can select the exact built library rather than an ambiguous file in target//deps.