Error code E0517
A #[repr(..)]
attribute was placed on an unsupported item.
Examples of erroneous code:
#![allow(unused)] fn main() { #[repr(C)] type Foo = u8; #[repr(packed)] enum Foo {Bar, Baz} #[repr(u8)] struct Foo {bar: bool, baz: bool} #[repr(C)] impl Foo { // ... } }
- The
#[repr(C)]
attribute can only be placed on structs and enums. - The
#[repr(packed)]
and#[repr(simd)]
attributes only work on structs. - The
#[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.