This is a known limitation of the color coding. parse_operator is not followed by an open paren (it is followed by a '<' character). "Function" color is simply an identifier followed by a paren.
You can customize the color coding for Rust to handle this case. Document > Rust Options... > Color Coding > Tokens > Add Other
Type: Function
Start Delimeter: :v(#= *\<[^>]*\> *\() SlickEdit regex
This expression does not handle nested angle brackets. I leave that as an exercise for the reader.