There should be a way to make a type as "stably a 1-ZST" for repr(transparent) purposes

Currently, if one crate defines a 1-ZST with private fields, and then another crate tries to use that in a repr(transparent) type, that produces an error:

// Crate A
pub struct MyMarkerType((());

// Crate B

#[repr(transparent)]
struct Transparent(i32, A::MyMarkerType);

This warning is caused by https://github.com/rust-lang/rust/issues/78586. The intention of the warning is to guard against a semver hazard: if crate A adds non-zero-sized fields to MyMarkerType in the future, that would break compilation of crate B.

However, what if crate A actually wants to promise that it will never add such fields? Currently it has no way to express this, the lint will always fire for types with private fields in other crates. This is on track to becoming a hard error (https://github.com/rust-lang/rust/issues/78586).

See here for where @CAD97 originally brought up this issue: a type like this is defined by the generativity crate.

Start by reading issue #78586 and the linked discussion in pull request #155299, then examine how repr(transparent) handles private fields across crates. Define a way for a crate to promise that a type remains a 1-ZST without weakening the semver protection. Done means the proposal is specified and the compiler accepts the intended cross-crate use while still rejecting unpromised types.