Higher-Rank Trait Bounds -

# Higher-Rank Trait Bounds (HRTBs)

Rust's `Fn` traits are a little bit magic. For instance, we can write the following code:

    struct Closure<F> {
        data: (u8, u16),
        func: F,
    }

    impl<F> Closure<F>
        where F: Fn(&(u8, u16)) -> &u8,
    {
        fn call(&self) -> &u8 {
            (self.func)(&self.data)
        }
    }

    fn do_it(data: &(u8, u16)) -> &u8 { &data.0 }

    fn main() {
        let clo = Closure { data: (0, 1), func: do_it };
        println!("{}", clo.call());
    }

If we try to naively desugar this code in the same way that we did in the lifetimes section, we run into some trouble:

    struct Closure<F> {
        data: (u8, u16),
        func: F,
    }

    impl<F> Closure<F>
        // where F: Fn(&'??? (u8, u16)) -> &'??? u8,
    {
        fn call<'a>(&'a self) -> &'a u8 {
            (self.func)(&self.data)
        }
    }

    fn do_it<'b>(data: &'b (u8, u16)) -> &'b u8 { &'b data.0 }

    fn main() {
        'x: {
            let clo = Closure { data: (0, 1), func: do_it };
            println!("{}", clo.call());
        }
    }

How on earth are we supposed to express the lifetimes on `F`'s trait bound? We need to provide some lifetime there, but the lifetime we care about can't be named until we enter the body of `call`! Also, that isn't some fixed lifetime; `call` works with any lifetime `&self` happens to have at that point.

This job requires The Magic of Higher-Rank Trait Bounds (HRTBs). The way we desugar this is as follows:

    where for<'a> F: Fn(&'a (u8, u16)) -> &'a u8,

(Where `Fn(a, b, c) -> d` is itself just sugar for the unstable real `Fn` trait)

`for<'a>` can be read as "for all choices of `'a`", and basically produces an infinite list of trait bounds that F must satisfy. Intense. There aren't many places outside of the `Fn` traits where we encounter HRTBs, and even for those we have a nice magic sugar for the common cases.