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ac922245f0
The SCCs of the region graph are not a reliable heuristic to use for blaming an interesting constraint for diagnostics. For region errors, if the outlived region is `'static`, or the involved types are invariant in their lifetiems, there will be cycles in the constraint graph containing both the target region and the most interesting constraints to blame. To get better diagnostics in these cases, this commit removes that heuristic.
52 lines
1.5 KiB
Rust
52 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-internals
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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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//~^ ERROR lifetime may not live long enough
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// Only works if 'x: 'y:
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demand_y(x, y, x.get())
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});
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}
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fn main() {}
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