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 larger than 1A 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.