51 lines
1.5 KiB
Rust
51 lines
1.5 KiB
Rust
// Rather convoluted setup where we infer a relationship between two
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// free regions in the closure signature (`'a` and `'b`) on the basis
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// of a relationship between two bound regions (`'x` and `'y`).
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//
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// The idea is that, thanks to invoking `demand_y`, `'x: 'y` must
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// hold, where `'x` and `'y` are bound regions. The closure can't
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// prove that directly, and because `'x` and `'y` are bound it cannot
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// ask the caller to prove it either. But it has bounds on `'x` and
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// `'y` in terms of `'a` and `'b`, and it can propagate a relationship
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// between `'a` and `'b` to the caller.
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//
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// Note: the use of `Cell` here is to introduce invariance. One less
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// variable.
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// compile-flags:-Zverbose
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#![feature(rustc_attrs)]
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use std::cell::Cell;
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// Callee knows that:
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//
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// 'x: 'a
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// 'b: 'y
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//
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// so if we are going to ensure that `'x: 'y`, then `'a: 'b` must
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// hold.
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fn establish_relationships<'a, 'b, F>(_cell_a: &Cell<&'a u32>, _cell_b: &Cell<&'b u32>, _closure: F)
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where
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F: for<'x, 'y> FnMut(
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&Cell<&'a &'x u32>, // shows that 'x: 'a
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&Cell<&'y &'b u32>, // shows that 'b: 'y
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&Cell<&'x u32>,
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&Cell<&'y u32>,
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),
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{
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}
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fn demand_y<'x, 'y>(_cell_x: &Cell<&'x u32>, _cell_y: &Cell<&'y u32>, _y: &'y u32) {}
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#[rustc_regions]
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fn supply<'a, 'b>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>) {
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establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
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// Only works if 'x: 'y:
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demand_y(x, y, x.get())
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//~^ ERROR lifetime may not live long enough
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});
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}
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fn main() {}
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