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#![feature(type_alias_impl_trait)]
type WithLifetime<T> = impl Equals<SelfType = ()>;
fn _defining_use<T>() -> WithLifetime<T> {}
trait Convert<'a> {
type Witness;
fn convert<'b, T: ?Sized>(_proof: &'b Self::Witness, x: &'a T) -> &'b T;
}
impl<'a> Convert<'a> for () {
type Witness = WithLifetime<&'a ()>;
fn convert<'b, T: ?Sized>(_proof: &'b WithLifetime<&'a ()>, x: &'a T) -> &'b T {
// compiler used to think it gets to assume 'a: 'b here because
// of the `&'b WithLifetime<&'a ()>` argument
x
//~^ ERROR lifetime may not live long enough
}
}
fn extend_lifetime<'a, 'b, T: ?Sized>(x: &'a T) -> &'b T {
WithLifetime::<&'a ()>::convert_helper::<(), T>(&(), x)
}
trait Equals {
type SelfType;
fn convert_helper<'a, 'b, W: Convert<'a, Witness = Self>, T: ?Sized>(
proof: &'b Self::SelfType,
x: &'a T,
) -> &'b T;
}
impl<S> Equals for S {
type SelfType = Self;
fn convert_helper<'a, 'b, W: Convert<'a, Witness = Self>, T: ?Sized>(
proof: &'b Self,
x: &'a T,
) -> &'b T {
W::convert(proof, x)
}
}
fn main() {
let r;
{
let x = String::from("Hello World?");
r = extend_lifetime(&x);
}
println!("{}", r);
}
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