Could not find module `Ident(mask_impl)`

This relatively recent code in the standard library is problematic for the cargo-unstable-api tool:

https://github.com/rust-lang/rust/blob/1.63.0/library/portable-simd/crates/core_simd/src/masks.rs#L5-L13

It fails with:

Error: could not find module `Ident(mask_impl)` in any of [
"library/core/src/../../portable-simd/crates/core_simd/src/masks/mask_impl.rs",
"library/core/src/../../portable-simd/crates/core_simd/src/masks/mask_impl/mod.rs",
] (this is a bug)

One of the cfg_attrs is a not of the other one, so at least one should definitely apply — that is, the tool should be loading either core_simd/src/masks/full_masks.rs or core_simd/src/masks/bitmask.rs (if the intention is to display the API for a single configuration only) or both (if the intention is to display the API of all configurations), but never core_simd/src/masks/mask_impl.rs or core_simd/src/masks/mask_impl/mod.rs.

The syn-inline-mod crate already implements loading of modules behind cfg attributes, so it shouldn't be necessary to reimplement all this logic in cargo-unstable-api.

Start in the cargo-unstable-api module-loading path and reproduce the failure using library/portable-simd/crates/core_simd/src/masks.rs. Compare the existing cfg-attribute handling with syn-inline-mod, then verify that loading selects full_masks.rs or bitmask.rs rather than mask_impl.rs; done means the reported module error no longer occurs.