Type alias impl trait doesn’t properly enforce outlives relations.

[steffahn]

#![feature(min_type_alias_impl_trait)]

use std::marker::PhantomData;

trait ProofForConversion<'a, 'b> {
    fn convert<T: ?Sized>(_: PhantomData<Self>, r: &'a T) -> &'b T;
}

impl<'a, 'b> ProofForConversion<'a, 'b> for &'b &'a () {
    fn convert<T: ?Sized>(_: PhantomData<Self>, r: &'a T) -> &'b T {
        r
    }
}

type Converter<'a, 'b> = impl ProofForConversion<'a, 'b>;

// Even _defining_use with an explicit `'a: 'b` compiles fine, too.
fn _defining_use<'a, 'b>(x: &'b &'a ()) -> Converter<'a, 'b> {
    x
}

fn extend_lifetime<'a, 'b, T: ?Sized>(x: &'a T) -> &'b T {
    Converter::<'a, 'b>::convert(PhantomData, x)
}

fn main() {
    let d;
    {
        let x = "Hello World".to_string();
        d = extend_lifetime(&x);
    }
    println!("{}", d);
}

��C

(playground)

I would’ve expected rustc to complain about the _defining_use not matching the type Converter<'a, 'b> with a suggestion to turn it into type Converter<'a: 'b, 'a>.

@rustbot modify labels: A-typesystem, A-lifetimes, A-traits, A-impl-trait, F-type_alias_impl_trait, T-compiler, requires-nightly
and someone please add “I-unsound 💥” (and/or tell me if there is a way to do it myself).

[oli-obk]

One thing that is of note here is that we properly enforce type bounds, but not lifetime bounds, as the equivalent program with types fails to compile right where we expect that:

#![feature(type_alias_impl_trait)]

use std::marker::PhantomData;

trait Trait {
    fn foo<T, U>(t: T) -> U;
}

trait ProofForConversion<X> {
    fn convert<T, U>(_: PhantomData<Self>, r: T) -> U;
}

impl<X: Trait> ProofForConversion<X> for () {
    fn convert<T, U>(_: PhantomData<Self>, r: T) -> U {
        X::foo(r)
    }
}

type Converter<T> = impl ProofForConversion<T>;

fn _defining_use<T: Trait>() -> Converter<T> {
    ()
}

[oli-obk]

Aha! If we change

impl<'a, 'b> ProofForConversion<'a, 'b> for &'b &'a () {

to have an explicit 'a: 'b bound, then we get the right error:

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
  --> $DIR/outlives_bounds2.rs:15:26
   |
LL | type Converter<'a, 'b> = impl ProofForConversion<'a, 'b>;
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime `'a` as defined here...
  --> $DIR/outlives_bounds2.rs:15:16
   |
LL | type Converter<'a, 'b> = impl ProofForConversion<'a, 'b>;
   |                ^^
note: ...but the lifetime must also be valid for the lifetime `'b` as defined here...
  --> $DIR/outlives_bounds2.rs:15:20
   |
LL | type Converter<'a, 'b> = impl ProofForConversion<'a, 'b>;
   |                    ^^
note: ...so that the types are compatible
  --> $DIR/outlives_bounds2.rs:15:26
   |
LL | type Converter<'a, 'b> = impl ProofForConversion<'a, 'b>;
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: expected `ProofForConversion<'a, 'b>`
              found `ProofForConversion<'_, '_>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0495`.

so we're just missing a check of the implied bounds on the impl