src/test/ui/nll/relate_tys/var-appears-twice.rs

   1 // Test that the NLL `relate_tys` code correctly deduces that a
   2 // function returning always its first argument can be upcast to one
   3 // that returns either first or second argument.
   4
   5 #![feature(nll)]
   6 #![allow(warnings)]
   7
   8 use std::cell::Cell;
   9
  10 type DoubleCell<A> = Cell<(A, A)>;
  11 type DoublePair<A> = (A, A);
  12
  13 fn make_cell<'b>(x: &'b u32) -> Cell<(&'static u32, &'b u32)> {
  14     panic!()
  15 }
  16
  17 fn main() {
  18     let a: &'static u32 = &22;
  19     let b = 44;
  20
  21     // Here we get an error because `DoubleCell<_>` requires the same type
  22     // on both parts of the `Cell`, and we can't have that.
  23     let x: DoubleCell<_> = make_cell(&b); //~ ERROR
  24
  25     // Here we do not get an error because `DoublePair<_>` permits
  26     // variance on the lifetimes involved.
  27     let y: DoublePair<_> = make_cell(&b).get();
  28 }