mirror of https://github.com/rust-lang/rust
53 lines
1.3 KiB
Rust
53 lines
1.3 KiB
Rust
#![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 {
|
|
//~^ ERROR non-defining opaque type use
|
|
// 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);
|
|
}
|