linker search path of proc_macro deps are not propagated

[erenon]

Reproduction Requirements

I could trigger this on macOS only. Install:

$ brew install bazelisk postgresql@14 pkg-config

Then clone:

$ git clone https://github.com/erenon/proc-macro-link-flags.git

What happens there

rust_decimal is a regular library crate for decimal numbers. With the 'db-diesel2-postgres' feature,
it pulls in the pq-sys dependency, that in its build.rs potentially finds libpq using pkg-config:

cargo:rustc-link-search=native=/opt/homebrew/lib/postgresql@14
cargo:rustc-link-lib=pq

(To make pkg-config work with the homebrew setup, we are modifying some envvars, see .bazelrc)

In the demo repository, the target //:good depends on rust_decimal, and bazel can compile and link it.

The rust_decimal_macros is a proc-macro crate, that provides (surprise!) macros for the above crate.
It depends on rust_decimal, therefore it also pulls in pq-sys transitively.

In the demo repository, the target //:bad depends on rust_decimal_macros, and bazel cannot compile it:

$ bazel build bad
[...]
ld: library 'pq' not found
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Suspicion

aquery shows that //:good inherits a linker search path from pq-sys:

$ bazel aquery good | grep pq
[...]
bazel-out/darwin_arm64-fastbuild/bin/external/rules_rust~~crate~crates__pq-sys-0.6.3/_bs.linksearchpaths \

but //bad: does not. Therefore the suspicion, that proc macro crates do not pass forward their link search path files?

Interestingly, even if bad also depends on rust_decimal or even pq-sys, it doesn't link.

[erenon]

Refined suspicion: The link search path given by the build.rs file is not propagated to the linking of proc-macro itself, that is linked in host configuration. Ugly workaround:

build:macos --host_linkopt "-L/opt/homebrew/lib/postgresql@14"