[anderskaseorg]

In the language,

    fn foo<'a, 'b, T>(_: &'a &'b (), v: &'b T) -> &'a T { v }

should be equivalent to

    fn foo<'a, 'b, T>(_: &'a &'b (), v: &'b T) -> &'a T where 'b: 'a { v }

because the type &'a &'b is only well-formed if 'b: 'a. However, in the implementation, only the first form where the constraint is left implicit is subject to the bug: the implicit constraint is incorrectly lost when 'static is substituted for 'b. This is clearly an implementation bug, not a language bug (insofar as there is a distinction at all—ideally there would be a written formal specification that we could point to, but I don’t think there’s any disagreement in principle about what it should say about this issue).

[nicklecompte]

See my other comment above: https://news.ycombinator.com/item?id=39448909

The point is that rustc does not even implicitly have the "where 'b: 'a { v }" clause. The programmer knows "where 'b: 'a { v }" is true because otherwise &'a &'b would be self-contradictory, and in most cases that's more than enough. But in certain edge cases this runs into problems with the contravariance requirement since we can substitute either one of the 'bs with a type that outlives 'b (i.e. 'static), and the design of contravariance lets us do this pretty arbitrarily.

[anderskaseorg]

We could choose to design the language as I wrote where the constraint is implicitly present and variance consequently ought to preserve it, or as you wrote where it is not present at all and variance is unsound. But we want Rust to be sound, so we should choose the former design. We would need a very compelling reason to prefer the latter—for example, some evidence that “it would break too much code” or “it’s impossible to implement”—but I don’t see anyone claiming to actually have such evidence. The only reason the implementation has not been updated to align with the sound design is that it takes work and that work is still ongoing.