rust/tests/ui/impl-trait/region-escape-via-bound-contravariant-closure.rs

// In contrast to `region-escape-via-bound-invariant`, in this case we
// *can* return a value of type `&'x u32`, even though `'x` does not
// appear in the bounds. This is because `&` is contravariant, and so
// we are *actually* returning a `&'y u32`.
//
// See https://github.com/rust-lang/rust/issues/46541 for more details.

//@ run-pass

#![allow(dead_code)]

fn foo<'x, 'y>(x: &'x u32) -> impl Fn() -> &'y u32
where 'x: 'y
{
    move || x
}

fn main() { }
use subtyping when we create a closure instead of for upvar types We used to make the upvar types in the closure `==` but that was stronger than we needed. Subtyping suffices, since we are copying the upvar value into the closure field. This in turn allows us to infer smaller lifetimes in captured values in some cases (like the example here), avoiding errors. 2018-03-15 14:49:05 +00:00			// In contrast to `region-escape-via-bound-invariant`, in this case we
			// can return a value of type `&'x u32`, even though `'x` does not
			// appear in the bounds. This is because `&` is contravariant, and so
			// we are actually returning a `&'y u32`.
			`//`
			`// See https://github.com/rust-lang/rust/issues/46541 for more details.`

			`//@ run-pass`

			`#![allow(dead_code)]`

Remove in-band lifetimes 2022-02-10 02:42:32 +00:00			`fn foo<'x, 'y>(x: &'x u32) -> impl Fn() -> &'y u32`
use subtyping when we create a closure instead of for upvar types We used to make the upvar types in the closure `==` but that was stronger than we needed. Subtyping suffices, since we are copying the upvar value into the closure field. This in turn allows us to infer smaller lifetimes in captured values in some cases (like the example here), avoiding errors. 2018-03-15 14:49:05 +00:00			`where 'x: 'y`
			`{`
			`move \|\| x`
			`}`

			`fn main() { }`