Why "borrowed value does not live long enough" with Self but not specific type?

Question

The following code:

trait Passable: Sized { fn call<F: Fn(Self)>(f: F); } impl Passable for &[i32] { fn call<F: Fn(Self)>(f: F) { let vec = vec![1, 2, 3]; f(&vec); } }

gives the error

error[E0597]: `vec` does not live long enough --> src/main.rs:8:11 | 8 | f(&vec); | ^^^^ borrowed value does not live long enough 9 | } | - `vec` dropped here while still borrowed

By contrast, if I write the same thing but with a fixed type of &[i32] instead of Self, it compiles:

trait Passable: Sized { fn call<F: Fn(&[i32])>(f: F); } impl Passable for &[i32] { fn call<F: Fn(&[i32])>(f: F) { let vec = vec![1, 2, 3]; f(&vec); } }

Why does the first code fail but not the second? And can the first code be changed so that it does compile?

Jmb · Accepted Answer · 2021-11-15 09:45:31Z

Because of lifetime elision. Your first example (with Self) is equivalent to this code:

trait Passable: Sized { fn call<F: Fn(Self)>(f: F); } impl<'a> Passable for &'a [i32] { fn call<F: Fn(&'a [i32])>(f: F) { let vec = vec![1, 2, 3]; f(&vec); } }

Note that here f takes an argument with the same lifetime as Self but vec has a smaller lifetime.

On the other hand, your second example is equivalent to this code:

trait Passable: Sized { fn call<'b, F: Fn(&'b [i32])>(f: F); } impl<'a> Passable for &'a [i32] { fn call<'b, F: Fn(&'b [i32])>(f: F) { let vec = vec![1, 2, 3]; f(&vec); } }

which uses different lifetimes for Self and the argument of f.

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