Understanding Rust Error E0692: Incompatible Repr Hints

Introduction to Rust Error E0692

Error E0692 in Rust occurs when a type with repr(transparent) is also annotated with other, incompatible representation hints. The purpose of repr(transparent) is to delegate all representation concerns to another type, so adding more representation hints creates a contradiction in the code.

Illustrating Error E0692

Here is an example of code that would trigger Rust Error E0692:

#![allow(unused)]
fn main() {
#[repr(transparent, C)] // error: incompatible representation hints
struct Grams(f32);
}

The code above combines repr(transparent) with repr(C), which results in an error. To fix this error, either remove the transparent hint or the other hints.

Fixing the Error

It is possible to resolve Rust Error E0692 by following these methods:

1. Remove the other hints (for example, repr(C)) and keep repr(transparent):

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

2. 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 to accommodate the additional attributes might cause unintended side effects on the representation.

Consider the following example:

#![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
}

In this example, Grams2 is not equivalent to Grams. Grams2 transparently wraps a non-transparent struct containing a single float, whereas Grams is a transparent wrapper around a float. This difference can impact the Application Binary Interface (ABI).