Right, so this should really be .orReturnErr , by analogy with .await . We just overload the dot symbol to mean both "method call" and "weird monadic stuff is happening here".
let a: Result<u8, u8> = /* whatever *;
let b = match a {
Ok(v) => Ok(foo(v)),
e => e,
};
let a: Result<u8, u8> = /* whatever *;
let b = a.map(foo);
await cannot be a macro, it has to be built into the language, and so it doesn't have the ! because it's not one.