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

   1 // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 // Test that the NLL `relate_tys` code correctly deduces that a
  12 // function returning always its first argument can be upcast to one
  13 // that returns either first or second argument.
  14
  15 #![feature(nll)]
  16 #![allow(warnings)]
  17
  18 use std::cell::Cell;
  19
  20 type DoubleCell<A> = Cell<(A, A)>;
  21 type DoublePair<A> = (A, A);
  22
  23 fn make_cell<'b>(x: &'b u32) -> Cell<(&'static u32, &'b u32)> {
  24     panic!()
  25 }
  26
  27 fn main() {
  28     let a: &'static u32 = &22;
  29     let b = 44;
  30
  31     // Here we get an error because `DoubleCell<_>` requires the same type
  32     // on both parts of the `Cell`, and we can't have that.
  33     let x: DoubleCell<_> = make_cell(&b); //~ ERROR
  34
  35     // Here we do not get an error because `DoublePair<_>` permits
  36     // variance on the lifetimes involved.
  37     let y: DoublePair<_> = make_cell(&b).get();
  38 }