This error indicates that a #[repr(..)]
attribute was placed on an unsupported item.
Examples of erroneous code:
#[repr(C)] type Foo = u8; #[repr(packed)] enum Foo {Bar, Baz} #[repr(u8)] struct Foo {bar: bool, baz: bool} #[repr(C)] impl Foo { // ... }
#[repr(C)]
attribute can only be placed on structs and enums.#[repr(packed)]
and #[repr(simd)]
attributes only work on structs.#[repr(u8)]
, #[repr(i16)]
, etc attributes only work on enums.These attributes do not work on typedefs, since typedefs are just aliases.
Representations like #[repr(u8)]
, #[repr(i64)]
are for selecting the discriminant size for enums with no data fields on any of the variants, e.g. enum Color {Red, Blue, Green}
, effectively setting the size of the enum to the size of the provided type. Such an enum can be cast to a value of the same type as well. In short, #[repr(u8)]
makes the enum behave like an integer with a constrained set of allowed values.
Only field-less enums can be cast to numerical primitives, so this attribute will not apply to structs.
#[repr(packed)]
reduces padding to make the struct size smaller. The representation of enums isn‘t strictly defined in Rust, and this attribute won’t work on enums.
#[repr(simd)]
will give a struct consisting of a homogeneous series of machine types (i.e., u8
, i32
, etc) a representation that permits vectorization via SIMD. This doesn‘t make much sense for enums since they don’t consist of a single list of data.