40 lines
1.1 KiB
Rust
40 lines
1.1 KiB
Rust
// edition:2018
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trait Trait<'a, 'b> {}
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impl<T> Trait<'_, '_> for T {}
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// `Ordinary<'a> <: Ordinary<'b>` if `'a: 'b`, as with 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 Ordinary<'a>(&'a u8);
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// Here we need something outlived by `'a` *and* outlived by `'b`, but
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// we can only name `'a` and `'b` (and neither suits). So we get an
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// error. Somewhat unfortunate, though, since the caller would have to
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// consider the loans for both `'a` and `'b` alive.
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fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b>
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{
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// We return a value:
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//
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// ```
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// 'a: '0
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// 'b: '1
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// '0 in ['a, 'b]
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// ```
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//
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// but we don't have it.
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//
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// We are forced to pick that '0 = 'e, because only 'e is outlived by *both* 'a and 'b.
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if condition() { a } else { b }
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//~^ ERROR hidden type for `impl Trait<'a, 'b>` captures lifetime that does not appear in bounds
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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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