To learn more about how Higher-ranked trait bounds work in the _old_ trait
solver, see [this chapter][oldhrtb] of the rustc-guide.

To learn more about how they work in the _new_ trait solver, see [this
chapter][newhrtb].

[oldhrtb]: https://rust-lang.github.io/rustc-guide/traits/hrtb.html
[newhrtb]: https://rust-lang.github.io/rustc-guide/borrow_check/region_inference.html#placeholders-and-universes
