fix #7563

[12101111]

Fix building proc-macro crates on musl host when --target is specified.

[rust-highfive]

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[alexcrichton]

Thanks for the PR!

I'm not sure that Cargo has a great way to fix this though. Cargo in general doesn't want to configure and/or encode platform-specific settings, but rather provide the knobs for the configuration to be passed down to Cargo. Cargo doesn't have a great way to specify flags for "only host compilations" today, and that's a known bug in Cargo.

[ehuss]

I'm going to close this due to inactivity.

I think figuring out how to handle proc-macro and musl should be discussed on rust-lang/rust. rust-lang/rust#40174 and rust-lang/rust#59302 seem like the correct places to work out what to do. If I had to take a wild guess, I would say rust-lang/rust#40174 (comment) sounds promising.