Rust error E0517: #### Note: this error code is no longer emitted by the compiler. This error code was replaced with the `attribute cannot be used on...` diagnostic that does not have an error code

The compiler team's explanation

Note: this error code is no longer emitted by the compiler.

This error code was replaced with the attribute cannot be used on... diagnostic that does not have an error code.

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 {
    // ...
}

• 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. For enums with no data fields on any of the variants, e.g. enum Color {Red, Blue, Green}, this effectively sets 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 a field-less enum behave like an integer with a constrained set of allowed values.

For a description of how #[repr(C)] and representations like #[repr(u8)] affect the layout of enums with data fields, see RFC 2195.

Only field-less enums can be cast to numerical primitives. Representations like #[repr(u8)] 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.

From rustc --explain E0517 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.