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39 lines
768 B
Rust
39 lines
768 B
Rust
#![allow(dead_code)]
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// Get<T> is covariant in T
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trait Get<T> {
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fn get(&self) -> T;
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}
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struct Cloner<T:Clone> {
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t: T
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}
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impl<T:Clone> Get<T> for Cloner<T> {
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fn get(&self) -> T {
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self.t.clone()
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}
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}
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fn get<'a, G>(get: &G) -> i32
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where G : Get<&'a i32>
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{
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// This fails to type-check because, without variance, we can't
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// use `G : Get<&'a i32>` as evidence that `G : Get<&'b i32>`,
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// even if `'a : 'b`.
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pick(get, &22) //~ ERROR explicit lifetime required in the type of `get` [E0621]
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}
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fn pick<'b, G>(get: &'b G, if_odd: &'b i32) -> i32
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where G : Get<&'b i32>
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{
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let v = *get.get();
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if v % 2 != 0 { v } else { *if_odd }
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}
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fn main() {
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let x = Cloner { t: &23 };
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let y = get(&x);
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assert_eq!(y, 23);
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}
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