E0732 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0732

An `enum` with a discriminant must specify a `#[repr(inttype)]`.

Erroneous code example:

    enum Enum { // error!
        Unit = 1,
        Tuple() = 2,
        Struct{} = 3,
    }
    fn main() {}

A `#[repr(inttype)]` must be provided on an `enum` if it has a non-unit variant with a discriminant, or where there are both unit variants with discriminants and non-unit variants. This restriction ensures that there is a well-defined way to extract a variant’s discriminant from a value; for instance:

    #[repr(u8)]
    enum Enum {
        Unit = 3,
        Tuple(u16) = 2,
        Struct {
            a: u8,
            b: u16,
        } = 1,
    }

    fn discriminant(v : &Enum) -> u8 {
        unsafe { *(v as *const Enum as *const u8) }
    }

    fn main() {
        assert_eq!(3, discriminant(&Enum::Unit));
        assert_eq!(2, discriminant(&Enum::Tuple(5)));
        assert_eq!(1, discriminant(&Enum::Struct{a: 7, b: 11}));
    }