How to elide inferable trait generics in rust

Question

Let's say I have the following traits, where I have a bunch of concrete implementations. For each concrete TokenParser there is only ever one concrete TokenHolder for it.

trait Token {}
trait TokenHolder<T: Token> {}
trait TokenParser<H: TokenHolder<T>, T: Token> {}

I have a bunch of code that consumes TokenParser and would like to cut down on the amount of boilerplate that I have. I feel like there should be a way of doing that like

trait TokenParser<H: TokenHolder<T: Token>> {}
// or
trait TokenParser<H: TokenHolder<T>> where T: Token {}

Answer 1

This can't really be achieved using generics but can be used with a type assigned within TokenHolder

trait TokenHolder {
    type Token: Token;
}

trait TokenParser<H: TokenHolder> {
    // This signature demonstrates using the type on the holder
    // in the signature for the parser.
    fn extract_token(holder: H) -> H::Token;
}