src/test/ui/nll/closure-requirements/propagate-approximated-ref.rs

   1 // Rather convoluted setup where we infer a relationship between two
   2 // free regions in the closure signature (`'a` and `'b`) on the basis
   3 // of a relationship between two bound regions (`'x` and `'y`).
   4 //
   5 // The idea is that, thanks to invoking `demand_y`, `'x: 'y` must
   6 // hold, where `'x` and `'y` are bound regions. The closure can't
   7 // prove that directly, and because `'x` and `'y` are bound it cannot
   8 // ask the caller to prove it either. But it has bounds on `'x` and
   9 // `'y` in terms of `'a` and `'b`, and it can propagate a relationship
  10 // between `'a` and `'b` to the caller.
  11 //
  12 // Note: the use of `Cell` here is to introduce invariance. One less
  13 // variable.
  14
  15 // compile-flags:-Zverbose
  16
  17 #![feature(rustc_attrs)]
  18
  19 use std::cell::Cell;
  20
  21 // Callee knows that:
  22 //
  23 // 'x: 'a
  24 // 'b: 'y
  25 //
  26 // so if we are going to ensure that `'x: 'y`, then `'a: 'b` must
  27 // hold.
  28 fn establish_relationships<'a, 'b, F>(_cell_a: &Cell<&'a u32>, _cell_b: &Cell<&'b u32>, _closure: F)
  29 where
  30     F: for<'x, 'y> FnMut(
  31         &Cell<&'a &'x u32>, // shows that 'x: 'a
  32         &Cell<&'y &'b u32>, // shows that 'b: 'y
  33         &Cell<&'x u32>,
  34         &Cell<&'y u32>,
  35     ),
  36 {
  37 }
  38
  39 fn demand_y<'x, 'y>(_cell_x: &Cell<&'x u32>, _cell_y: &Cell<&'y u32>, _y: &'y u32) {}
  40
  41 #[rustc_regions]
  42 fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
  43     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
  44         // Only works if 'x: 'y:
  45         demand_y(x, y, x.get())
  46         //~^ ERROR lifetime may not live long enough
  47     });
  48 }
  49
  50 fn main() {}