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

   1 // Copyright 2016 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 // Rather convoluted setup where we infer a relationship between two
  12 // free regions in the closure signature (`'a` and `'b`) on the basis
  13 // of a relationship between two bound regions (`'x` and `'y`).
  14 //
  15 // The idea is that, thanks to invoking `demand_y`, `'x: 'y` must
  16 // hold, where `'x` and `'y` are bound regions. The closure can't
  17 // prove that directly, and because `'x` and `'y` are bound it cannot
  18 // ask the caller to prove it either. But it has bounds on `'x` and
  19 // `'y` in terms of `'a` and `'b`, and it can propagate a relationship
  20 // between `'a` and `'b` to the caller.
  21 //
  22 // Note: the use of `Cell` here is to introduce invariance. One less
  23 // variable.
  24
  25 // compile-flags:-Zborrowck=mir -Zverbose
  26
  27 #![feature(rustc_attrs)]
  28
  29 use std::cell::Cell;
  30
  31 // Callee knows that:
  32 //
  33 // 'x: 'a
  34 // 'b: 'y
  35 //
  36 // so if we are going to ensure that `'x: 'y`, then `'a: 'b` must
  37 // hold.
  38 fn establish_relationships<'a, 'b, F>(_cell_a: &Cell<&'a u32>, _cell_b: &Cell<&'b u32>, _closure: F)
  39 where
  40     F: for<'x, 'y> FnMut(
  41         &Cell<&'a &'x u32>, // shows that 'x: 'a
  42         &Cell<&'y &'b u32>, // shows that 'b: 'y
  43         &Cell<&'x u32>,
  44         &Cell<&'y u32>,
  45     ),
  46 {
  47 }
  48
  49 fn demand_y<'x, 'y>(_cell_x: &Cell<&'x u32>, _cell_y: &Cell<&'y u32>, _y: &'y u32) {}
  50
  51 #[rustc_regions]
  52 fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
  53     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
  54         //~^ ERROR lifetime mismatch
  55
  56         // Only works if 'x: 'y:
  57         demand_y(x, y, x.get()) //~ WARNING not reporting region error due to nll
  58     });
  59 }
  60
  61 fn main() {}