Static rustc_lint_defs::builtin::REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS

pub static REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS: &'static Lint

The repr_transparent_external_private_fields lint detects types marked #[repr(transparent)] that (transitively) contain an external ZST type marked #[non_exhaustive] or containing private fields

Example

#![deny(repr_transparent_external_private_fields)]
use foo::NonExhaustiveZst;

#[repr(transparent)]
struct Bar(u32, ([u32; 0], NonExhaustiveZst));

This will produce:

error: zero-sized fields in repr(transparent) cannot contain external non-exhaustive types
 --> src/main.rs:5:28
  |
5 | struct Bar(u32, ([u32; 0], NonExhaustiveZst));
  |                            ^^^^^^^^^^^^^^^^
  |
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![deny(repr_transparent_external_private_fields)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
  = note: this struct contains `NonExhaustiveZst`, which is marked with `#[non_exhaustive]`, and makes it not a breaking change to become non-zero-sized in the future.

Explanation

Previous, Rust accepted fields that contain external private zero-sized types, even though it should not be a breaking change to add a non-zero-sized field to that private type.

This is a future-incompatible lint to transition this to a hard error in the future. See issue #78586 for more details.