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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:02:58 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:02:58 +0000 |
commit | 698f8c2f01ea549d77d7dc3338a12e04c11057b9 (patch) | |
tree | 173a775858bd501c378080a10dca74132f05bc50 /compiler/rustc_error_codes/src/error_codes/E0691.md | |
parent | Initial commit. (diff) | |
download | rustc-698f8c2f01ea549d77d7dc3338a12e04c11057b9.tar.xz rustc-698f8c2f01ea549d77d7dc3338a12e04c11057b9.zip |
Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'compiler/rustc_error_codes/src/error_codes/E0691.md')
-rw-r--r-- | compiler/rustc_error_codes/src/error_codes/E0691.md | 48 |
1 files changed, 48 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0691.md b/compiler/rustc_error_codes/src/error_codes/E0691.md new file mode 100644 index 000000000..60060cacb --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0691.md @@ -0,0 +1,48 @@ +A struct, enum, or union with the `repr(transparent)` representation hint +contains a zero-sized field that requires non-trivial alignment. + +Erroneous code example: + +```compile_fail,E0691 +#![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 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. |