Basic use cases like that have always been possible in Rust, either via defining your own enum with zero variants (`enum Foo {}`), or by using a function return type of `!` which was hardcoded to work for certain things, e.g. infinite loops. What this release does is generalizes it so that `!` can participate in types in general, and the coolest part is that pattern matching is aware of this, so for example if you have a `Result<u8, !>` where the error variant is impossible, a `match` over this would allow you to omit the impossible error arm, and furthermore because this enum now effectively only has a single variant, it would allow you to use the pleasant irrefutable destructuring assignment syntax `let Ok(foo) = blah();`.