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.