mirror of
https://github.com/rust-lang/rust.git
synced 2024-11-23 07:14:28 +00:00
51 lines
1.1 KiB
Rust
51 lines
1.1 KiB
Rust
//@ edition:2018
|
|
//@ check-pass
|
|
|
|
trait Trait<'a, 'b> {}
|
|
impl<T> Trait<'_, '_> for T {}
|
|
|
|
// `Invert<'a> <: Invert<'b>` if `'b: 'a`, unlike most types.
|
|
//
|
|
// I am purposefully avoiding the terms co- and contra-variant because
|
|
// their application to regions depends on how you interpreted Rust
|
|
// regions. -nikomatsakis
|
|
struct Invert<'a>(fn(&'a u8));
|
|
|
|
fn upper_bounds<'a, 'b, 'c, 'd, 'e>(a: Invert<'a>, b: Invert<'b>) -> impl Trait<'d, 'e>
|
|
where
|
|
'c: 'a,
|
|
'c: 'b,
|
|
'd: 'c,
|
|
{
|
|
// Representing the where clauses as a graph, where `A: B` is an
|
|
// edge `B -> A`:
|
|
//
|
|
// ```
|
|
// 'a -> 'c -> 'd
|
|
// ^
|
|
// |
|
|
// 'b
|
|
// ```
|
|
//
|
|
// Meanwhile we return a value &'0 u8 where we have the constraints:
|
|
//
|
|
// ```
|
|
// '0: 'a
|
|
// '0: 'b
|
|
// '0 in ['d, 'e]
|
|
// ```
|
|
//
|
|
// Here, ignoring the "in" constraint, the minimal choice for `'0`
|
|
// is `'c`, but that is not in the "in set". Still, that reduces
|
|
// the range of options in the "in set" to just `'d` (`'e: 'c`
|
|
// does not hold).
|
|
let p = if condition() { a } else { b };
|
|
p
|
|
}
|
|
|
|
fn condition() -> bool {
|
|
true
|
|
}
|
|
|
|
fn main() {}
|