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The ! type. In stable Rust. Since 1.14.0.

Better than an enum Never {} definition would be, since an instance of type ! automagically coerces to any type, whereas an instance of enum EmptyEnum {} needs an explicit match it {}.

That is, the following fails to compile:

let x: u32 = match <u32 as TryFrom<u8>>::try_from(42) {
    | Ok(it) => it,
    | Err(unreachable) => unreachable, // Error, expected `u32`, found `Infallible`

but the following doesn’t!

use ::never_say_never::Never;

let x: u32 = match Ok::<_, Never>(42) {
    | Ok(it) => it,
    | Err(unreachable) => unreachable,

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