Rust error E0692: transparent struct cannot have other repr hints

The compiler team's explanation

A repr(transparent) type was also annotated with other, incompatible representation hints.

Erroneous code example:

#[repr(transparent, C)] // error: incompatible representation hints
struct Grams(f32);

A type annotated as repr(transparent) delegates all representation concerns to another type, so adding more representation hints is contradictory. Remove either the transparent hint or the other hints, like this:

#[repr(transparent)]
struct Grams(f32);

Alternatively, move the other attributes to the contained type:

#[repr(C)]
struct Foo {
    x: i32,
    // ...
}

#[repr(transparent)]
struct FooWrapper(Foo);

Note that introducing another struct just to have a place for the other attributes may have unintended side effects on the representation:

#[repr(transparent)]
struct Grams(f32);

#[repr(C)]
struct Float(f32);

#[repr(transparent)]
struct Grams2(Float); // this is not equivalent to `Grams` above

Here, Grams2 is a not equivalent to Grams -- the former transparently wraps a (non-transparent) struct containing a single float, while Grams is a transparent wrapper around a float. This can make a difference for the ABI.

From rustc --explain E0692 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.