fn optional_type(&mut self) -> WeldResult<Type> {
        if *self.peek() == TColon {
            self.consume(TColon)?;
            self.type_()
        } else {
            Ok(Unknown)
        }
    }
Optionally parse a type annotation such as ": i32" and return the result as a Type;
gives Unknown if there is no type annotation at the current position.
weld/src/syntax/parser.rs