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A function's return type is supposed to indicate the kind of data that it produces. Rust's "never" type, which is denoted by an exclamation mark (" ! "), is the type the language uses to mark a function that never returns and other places where a value can never occur. For a long time, the never type was used internally by the compiler, but was considered an unstable feature. On August 24 , after more than two years of work, Rust-compiler-contributor "waffle" finally managed to stabilize the type. It took so long, in part, because it involved a small breaking change to previous Rust editions, which the compiler maintainers needed to ensure did not impact much real code.

(Note: Rust also uses exclamation marks to indicate calls to macros.…