Tracking issue for rustc-cdylib-link-arg transitive warning

[ehuss]

In the 1.50 release, there was an unintended change that the cargo:rustc-cdylib-link-arg build script instruction started passing arguments to transitive cdylib dependencies. Previously this was ignored. In the 1.54 release, a warning was added to indicate that this may be rejected in the future.

If you see this warning, you are encouraged to change the way linker arguments are added. Only the build script of the package containing the cdylib should issue cargo:rustc-cdylib-link-arg in its build script. If there is common code for determining these flags that users may need, then an option is to have a build-dependency which provides that logic.

[thomcc]

It seems like if you have:

• crate A with crate-type=["cdylib", "rlib"], that emits println!("cargo:rustc-cdylib-link-arg=...") in its build.rs, and
• crate B of type cdylib that depends on crate A

We'll apply the link args from A to both cdylibs (expected, given the current workaround), but fail to emit the warning, even though the 2nd crate is getting the cdylib args transitively (unfortunate, since that would probably break if we started rejecting transitive cdylib-link-args).

I'm not sure the best way to handle this though, but I figured I'd mention it here.