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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/E0690.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/E0690.md')
-rw-r--r-- | compiler/rustc_error_codes/src/error_codes/E0690.md | 33 |
1 files changed, 33 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0690.md b/compiler/rustc_error_codes/src/error_codes/E0690.md new file mode 100644 index 000000000..ba706ad2b --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0690.md @@ -0,0 +1,33 @@ +A struct with the representation hint `repr(transparent)` had two or more fields +that were not guaranteed to be zero-sized. + +Erroneous code example: + +```compile_fail,E0690 +#[repr(transparent)] +struct LengthWithUnit<U> { // error: transparent struct needs at most one + value: f32, // non-zero-sized field, but has 2 + unit: U, +} +``` + +Because transparent structs are represented exactly like one of their fields at +run time, said field must be uniquely determined. If there are multiple fields, +it is not clear how the struct should be represented. +Note that fields of zero-sized types (e.g., `PhantomData`) can also exist +alongside the field that contains the actual data, they do not count for this +error. When generic types are involved (as in the above example), an error is +reported because the type parameter could be non-zero-sized. + +To combine `repr(transparent)` with type parameters, `PhantomData` may be +useful: + +``` +use std::marker::PhantomData; + +#[repr(transparent)] +struct LengthWithUnit<U> { + value: f32, + unit: PhantomData<U>, +} +``` |