alt Hacker NewsWithout looking up the Rust implementation I would hazard a guess that it is loose language to say "something to return when no return is actually possible" because if it is anything like the Bottom Type then there is no something that is being returned which is what this function signature indicates. Generally used for exception handling or intentional infinite loops (in both cases the function does not return from whence it came) … please correct me if I'm wrong here …
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();`.