src/test/ui/impl-trait/multiple-lifetimes/inverse-bounds.rs

   1 // edition:2018
   2 // check-pass
   3
   4 trait Trait<'a, 'b> {}
   5 impl<T> Trait<'_, '_> for T {}
   6
   7 // `Invert<'a> <: Invert<'b>` if `'b: 'a`, unlike most types.
   8 //
   9 // I am purposefully avoiding the terms co- and contra-variant because
  10 // their application to regions depends on how you interpreted Rust
  11 // regions. -nikomatsakis
  12 struct Invert<'a>(fn(&'a u8));
  13
  14 fn upper_bounds<'a, 'b, 'c, 'd, 'e>(a: Invert<'a>, b: Invert<'b>) -> impl Trait<'d, 'e>
  15 where
  16     'c: 'a,
  17     'c: 'b,
  18     'd: 'c,
  19 {
  20     // Representing the where clauses as a graph, where `A: B` is an
  21     // edge `B -> A`:
  22     //
  23     // ```
  24     // 'a -> 'c -> 'd
  25     //        ^
  26     //        |
  27     //       'b
  28     // ```
  29     //
  30     // Meanwhile we return a value &'0 u8 where we have the constraints:
  31     //
  32     // ```
  33     // '0: 'a
  34     // '0: 'b
  35     // '0 in ['d, 'e]
  36     // ```
  37     //
  38     // Here, ignoring the "in" constraint, the minimal choice for `'0`
  39     // is `'c`, but that is not in the "in set". Still, that reduces
  40     // the range of options in the "in set" to just `'d` (`'e: 'c`
  41     // does not hold).
  42     let p = if condition() { a } else { b };
  43     p
  44 }
  45
  46 fn condition() -> bool {
  47     true
  48 }
  49
  50 fn main() {}