预期的混凝土寿命，在结构中存储fn时找到约束寿命参数

Question

I'm trying to store a function in a struct:

trait T<'a> {}

struct A {}

struct B<'a> {
    a: &'a A
}

impl<'a> T<'a> for B<'a> {}

fn f1<'a, E: T<'a>>(a: &'a A) {}

struct D {
    f: fn(&A)
}

fn main() {
    let d = D { f: f1::<B> };
}

The compiler complains:

error[E0308]: mismatched types
  --> src/main.rs:18:20
   |
18 |     let d = D { f: f1::<B> };
   |                    ^^^^^^^ expected concrete lifetime, found bound lifetime parameter 
   |
   = note: expected type `fn(&A)`
   = note:    found type `fn(&A) {f1::<'_, B<'_>>}`

Answer 1

When you write f1::<B> , the compiler interprets that as f1::<B<'_>> , there '_ is a lifetime inferred by the compiler, because B is generic over a lifetime and you can only pass concrete types as type parameters.

But then, in D , the f field is expected to be a function that accepts references to A with any lifetime. f1::<B> does not fulfill that requirement, because the function has been instantiated with a specific lifetime.

Unfortunately, at the moment, there's no way to make this work. Rust would have to support either higher kinded types or associated type constructors . You could then define E in f1 to be a type constructor parameter, rather than a type parameter (though I'm wondering how the compiler would handle the 'a lifetime parameter).