Error code E0692

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

Erroneous code example:

#![allow(unused)] fn main() { #[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:

#![allow(unused)] fn main() { #[repr(transparent)] struct Grams(f32); }

Alternatively, move the other attributes to the contained type:

#![allow(unused)] fn main() { #[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:

#![allow(unused)] fn main() { #[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.