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