rust-tropes | tropes around the community of Rust developers | Dictionary library

The core of the issue is that drop is not a single function, but rather a parameterized set of functions that each drop some particular type. To satisfy a higher-ranked trait bound (hereafter hrtb), you'd need a single function that can simultaneously take references to a type with any given lifetime.

We'll use drop as our typical example of a generic function, but all this applies more generally too. Here's the code for reference: fn drop(_: T) {}.

Conceptually, drop is not a single function, but rather one function for every possible type T. Any particular instance of drop takes only arguments of a single type. This is called monomorphization. If a different T is used with drop, a different version of drop is compiled. That's why you can't pass a generic function as an argument and use that function in full generality (see this question)

On the other hand, a function like fn pass(x: &i32) -> &i32 {x} satisfies the hrtb for<'a> Fn(&'a i32) -> &'a i32. Unlike drop, we have a single function that simultaneously satisfies Fn(&'a i32) -> &'a i32 for every lifetime 'a. This is reflected in how pass can be used.