Won't compile on musl without libstd

[clarfonthey]

See the linked thread at rust-lang/rust#37406.

I tried compiling my loop-forever crate on musl to reduce binary size, but it failed to compile due to an error that appears to be a bug; the link_cfg feature should never be called outside of the std build.

[alexcrichton]

I think we just need to put the relevant attributes behind a #[cfg_attr]

[clarfonthey]

This also might be worth looking at the CI setup to see what platforms were missed during testing in addition to musl.

[alexcrichton]

Looks like that was diagnosed in #685

[Others]

This is currently blocking something I'm working on, and I'd be happy to work on a fix if I understood what that would look like. If the solution is #[cfg_attr(some_check, ...)], what would some_check look like?

My error for reference:

ubuntu@ubuntu-xenial:~/rust2cos/lib_composite$ RUSTFLAGS='-C relocation-model=dynamic-no-pic -C link-args=-no-pie -L /usr/lib/i386-linux-musl/' xargo build --target i686-unknown-linux-musl --release
   Compiling libc v0.2.27
error: is feature gated (see issue #37406)
   --> /home/ubuntu/.cargo/registry/src/github.com-1ecc6299db9ec823/libc-0.2.27/src/unix/mod.rs:234:9
    |
234 |         #[link(name = "c", kind = "static", cfg(target_feature = "crt-static"))]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: add #![feature(link_cfg)] to the crate attributes to enable

error: is feature gated (see issue #37406)
   --> /home/ubuntu/.cargo/registry/src/github.com-1ecc6299db9ec823/libc-0.2.27/src/unix/mod.rs:235:9
    |
235 |         #[link(name = "c", cfg(not(target_feature = "crt-static")))]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: add #![feature(link_cfg)] to the crate attributes to enable

error: aborting due to 2 previous errors

error: Could not compile `libc`.

To learn more, run the command again with --verbose.

[alexcrichton]

@Others yes these can all be #[cfg_attr(stdbuild, ...)]

[kristate]

Hi @clarcharr ! I am trying to clean-up issues: is this still an issue for you?

[gnzlbg]

This was fixed in #930

@alexcrichton can you close this?