"Renaming of the library C was specified" error only on nightly

[dylanede]

This appears to be caused by a change in #37403.

This appears to be causing a breaking change on nightly from stable. I'm compiling an (unreleased) FFI library that handles the linking to certain static libraries in a build script (so the extern "C" block is unannotated). It depends on the libc crate. Other than that it does nothing out of the ordinary. I'm compiling using the windows msvc x64 toolchain. On nightly, but not on stable, I get the error message:

error: renaming of the library `C` was specified, however this crate contains no #[link(...)] attributes referencing this library.

Is this library C referring to libc? My crate has an entirely unrelated name.

[dylanede]

cc @vadimcn

[vadimcn]

@dylanede, are you putting the full library path behind -l by any chance?
(i.e. -l C:\foo\bar\baz.lib)

[dylanede]

Ah, yes I am. The path is constructed using the CARGO_MANIFEST_DIR environment variable. The lib is stored in a crate subdirectory.

[vadimcn]

Hmm, I was under impression that -l takes just the library name.
@alexcrichton, how is -l supposed to work?

[alexcrichton]

@vadimcn that was the intention, yes, but it looks like it works with paths as well I guess?

[alexcrichton]

I believe the relevant PR (#37973) is on beta, so I'm going to tag this as a stable-to-beta regression.

[vadimcn]

@alexcrichton: We have code in rustc that assumes that it's given just the library name. If you invoke ld -l/foo/libbar.a, it won't find the library. The proper way is this: ld -L/foo -lbar.

The full path works with MSVC (if you omit the .lib extension), but only by accident.

[alexcrichton]

The tools team discussed this during triage today and the conclusion was that this interpretation of -l was an unintended consequence and the current interpretation is working as intended. As a result I'm going to close this, but @dylanede if you need help updating your code please just let me know and I'd be more than willing to help out!

[hone]

I'm not sure if this is related, but I'm seeing the same error message while playing around with neon. This only happens on Windows. I'm on rust 1.19.0. This is the error message I'm seeing.

renaming of the library `C` was specified, however this crate contains no #[link(...)]

Just running cargo build on this sample repo triggers it. Here's a link to an Appveyor build demonstrating it. I'm not sure how to work around the issue. Thanks for any help.