[rustc.git] / src / doc / unstable-book / src / language-features / member-constraints.md

# `member_constraints`

The tracking issue for this feature is: [#61997]

[#61997]: https://github.com/rust-lang/rust/issues/61997

------------------------

The `member_constraints` feature gate lets you use `impl Trait` syntax with
multiple unrelated lifetime parameters.

A simple example is:

```rust
#![feature(member_constraints)]

trait Trait<'a, 'b> { }
impl<T> Trait<'_, '_> for T {}

fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> impl Trait<'a, 'b> {
  (x, y)
}

fn main() { }
```

Without the `member_constraints` feature gate, the above example is an
error because both `'a` and `'b` appear in the impl Trait bounds, but
neither outlives the other.
Commit	Line	Data
dc9dc135 XL	1	# `member_constraints`
dc9dc135 XL	2
dfeec247	3	The tracking issue for this feature is: [#61997]
dc9dc135	4
dfeec247	5	[#61997]: https://github.com/rust-lang/rust/issues/61997
dc9dc135 XL	6
	7	------------------------
	8
	9	The `member_constraints` feature gate lets you use `impl Trait` syntax with
	10	multiple unrelated lifetime parameters.
	11
	12	A simple example is:
	13
	14	```rust
	15	#![feature(member_constraints)]
	16
	17	trait Trait<'a, 'b> { }
	18	impl<T> Trait<'_, '_> for T {}
	19
	20	fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> impl Trait<'a, 'b> {
	21	(x, y)
	22	}
	23
	24	fn main() { }
	25	```
	26
	27	Without the `member_constraints` feature gate, the above example is an
	28	error because both `'a` and `'b` appear in the impl Trait bounds, but
	29	neither outlives the other.