Rust error E0691: #### Note: this error code is no longer emitted by the compiler

The compiler team's explanation

Note: this error code is no longer emitted by the compiler.

A struct, enum, or union with the repr(transparent) representation hint contains a zero-sized field that requires non-trivial alignment.

Erroneous code example:

#![feature(repr_align)]

#[repr(align(32))]
struct ForceAlign32;

#[repr(transparent)]
struct Wrapper(f32, ForceAlign32); // error: zero-sized field in transparent
                                   //        struct has alignment of 32, which
                                   //        is larger than 1

A transparent struct, enum, or union is supposed to be represented exactly like the piece of data it contains. Zero-sized fields with different alignment requirements potentially conflict with this property. In the example above, Wrapper would have to be aligned to 32 bytes even though f32 has a smaller alignment requirement.

Consider removing the over-aligned zero-sized field:

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

Alternatively, PhantomData<T> has alignment 1 for all T, so you can use it if you need to keep the field for some reason:

#![feature(repr_align)]

use std::marker::PhantomData;

#[repr(align(32))]
struct ForceAlign32;

#[repr(transparent)]
struct Wrapper(f32, PhantomData<ForceAlign32>);

Note that empty arrays [T; 0] have the same alignment requirement as the element type T. Also note that the error is conservatively reported even when the alignment of the zero-sized type is less than or equal to the data field's alignment.

From rustc --explain E0691 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.