Figure out an 390x build error

[sunfishcode]

In coreos/rpm-ostree#3839, they're seeing this error:

--> /builddir/build/BUILD/rpm-ostree-2022.10.110.gad3fecdc/vendor/rustix/src/imp/libc/fs/dir.rs:47:18
   |
47 | use libc_errno::{errno, set_errno, Errno};
   |                  ^^^^^  ^^^^^^^^^  ^^^^^ no `Errno` in the root
   |                  |      |
   |                  |      no `set_errno` in the root
   |                  no `errno` in the root

This is a surprising error message; it's not the "unresolved import libc_errno" error one might expect if we got the complex cfg wrong in Cargo.toml. Here, the build system seems to have correctly decided that we're using the libc dependencies, and the compiler appears to have found libc_errno. It appears it isn't finding the things inside of it 🤔 .

The errno crate doesn't have any way to disable errno, set_errno, and Errno; those are always unconditionally defined.

So I don't currently have any theories for what might be causing this.

[sunfishcode]

Ah, it's likely that this error is due to cargo-vendor-filterer, which truncates source files to zero length when it thinks they aren't in use. So this is likely just a case of incorrect dependencies, rather than being something mysterious. If anyone sees this error, please report it, but for now, I don't think there's anything immediate that needs to be done.