Static REPR_TRANSPARENT_NON_ZST_FIELDS 

pub static REPR_TRANSPARENT_NON_ZST_FIELDS: &Lint

The repr_transparent_non_zst_fields lint detects types marked #[repr(transparent)] that (transitively) contain a type that is not guaranteed to remain a ZST type under all configurations.

Example

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

#[repr(C)]
struct CZst([u8; 0]);

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

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 field contains `NonExhaustiveZst`, which is marked with `#[non_exhaustive]`, so it could become non-zero-sized in the future.

error: zero-sized fields in repr(transparent) cannot contain `#[repr(C)]` types
 --> src/main.rs:5:28
  |
5 | struct Baz(u32, CZst);
  |                 ^^^^
  = 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 field contains `CZst`, which is a `#[repr(C)]` type, so it is not guaranteed to be zero-sized on all targets.

Explanation

Previous, Rust accepted fields that contain external private zero-sized types, even though those types could gain a non-zero-sized field in a future, semver-compatible update.

Rust also accepted fields that contain repr(C) zero-sized types, even though those types are not guaranteed to be zero-sized on all targets, and even though those types can make a difference for the ABI (and therefore cannot be ignored by repr(transparent)).

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