compiler/rustc_error_codes/src/error_codes/E0692.md

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

Erroneous code example:

```compile_fail,E0692
#[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.