| // Copyright 2016 The Rust Project Developers. See the COPYRIGHT |
| // file at the top-level directory of this distribution and at |
| // http://rust-lang.org/COPYRIGHT. |
| // |
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your |
| // option. This file may not be copied, modified, or distributed |
| // except according to those terms. |
| |
| // Rather convoluted setup where we infer a relationship between two |
| // free regions in the closure signature (`'a` and `'b`) on the basis |
| // of a relationship between two bound regions (`'x` and `'y`). |
| // |
| // The idea is that, thanks to invoking `demand_y`, `'x: 'y` must |
| // hold, where `'x` and `'y` are bound regions. The closure can't |
| // prove that directly, and because `'x` and `'y` are bound it cannot |
| // ask the caller to prove it either. But it has bounds on `'x` and |
| // `'y` in terms of `'a` and `'b`, and it can propagate a relationship |
| // between `'a` and `'b` to the caller. |
| // |
| // Note: the use of `Cell` here is to introduce invariance. One less |
| // variable. |
| |
| // compile-flags:-Zborrowck=mir -Zverbose |
| |
| #![feature(rustc_attrs)] |
| |
| use std::cell::Cell; |
| |
| // Callee knows that: |
| // |
| // 'x: 'a |
| // 'b: 'y |
| // |
| // so if we are going to ensure that `'x: 'y`, then `'a: 'b` must |
| // hold. |
| fn establish_relationships<'a, 'b, F>(_cell_a: &Cell<&'a u32>, _cell_b: &Cell<&'b u32>, _closure: F) |
| where |
| F: for<'x, 'y> FnMut( |
| &Cell<&'a &'x u32>, // shows that 'x: 'a |
| &Cell<&'y &'b u32>, // shows that 'b: 'y |
| &Cell<&'x u32>, |
| &Cell<&'y u32>, |
| ), |
| { |
| } |
| |
| fn demand_y<'x, 'y>(_cell_x: &Cell<&'x u32>, _cell_y: &Cell<&'y u32>, _y: &'y u32) {} |
| |
| #[rustc_regions] |
| fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) { |
| establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| { |
| |
| // Only works if 'x: 'y: |
| demand_y(x, y, x.get()) |
| //~^ ERROR unsatisfied lifetime constraints |
| }); |
| } |
| |
| fn main() {} |