52 lines
1.2 KiB
Rust
52 lines
1.2 KiB
Rust
// edition:2018
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#![feature(arbitrary_self_types, async_await, await_macro, pin)]
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trait Trait<'a, 'b> {}
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impl<T> Trait<'_, '_> for T {}
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// `Invert<'a> <: Invert<'b>` if `'b: 'a`, unlike most types.
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//
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// I am purposefully avoiding the terms co- and contra-variant because
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// their application to regions depends on how you interpreted Rust
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// regions. -nikomatsakis
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struct Invert<'a>(fn(&'a u8));
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fn upper_bounds<'a, 'b, 'c, 'd, 'e>(a: Invert<'a>, b: Invert<'b>) -> impl Trait<'d, 'e>
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where
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'c: 'a,
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'c: 'b,
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'd: 'c,
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{
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// Representing the where clauses as a graph, where `A: B` is an
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// edge `B -> A`:
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//
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// ```
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// 'a -> 'c -> 'd
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// ^
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// |
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// 'b
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// ```
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//
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// Meanwhile we return a value &'0 u8 where we have the constraints:
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//
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// ```
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// '0: 'a
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// '0: 'b
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// '0 in ['d, 'e]
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// ```
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//
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// Here, ignoring the "in" constraint, the minimal choice for `'0`
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// is `'c`, but that is not in the "in set". Still, that reduces
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// the range of options in the "in set" to just `'d` (`'e: 'c`
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// does not hold).
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let p = if condition() { a } else { b };
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p
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}
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fn condition() -> bool {
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true
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}
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fn main() {}
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