The tracking issue for this feature is: #61997

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

A simple example is:


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.