rust/tests/ui/match/match-ref-mut-let-invariance.rs

// Check that when making a ref mut binding with type `&mut T`, the
// type `T` must match precisely the type `U` of the value being
// matched, and in particular cannot be some supertype of `U`. Issue
// #23116. This test focuses on a `let`.

#![allow(dead_code)]
struct S<'b>(&'b i32);
impl<'b> S<'b> {
    fn bar<'a>(&'a mut self) -> &'a mut &'a i32 {
        let ref mut x = self.0;
        x
        //~^ ERROR lifetime may not live long enough
    }
}

fn main() {}
When matching against a pattern (either via `match` or `let`) that contains ref-bindings, do not permit any upcasting from the type of the value being matched. Similarly, do not permit coercion in a `let`. This is a [breaking-change] in that it closes a type hole that previously existed, and in that coercion is not performed. You should be able to work around the latter by converting: ```rust let ref mut x: T = expr; ``` into ```rust let x: T = expr; let ref mut x = x; ``` Restricting coercion not to apply in the case of `let ref` or `let ref mut` is sort of unexciting to me, but seems the best solution: 1. Mixing coercion and `let ref` or `let ref mut` is a bit odd, because you are taking the address of a (coerced) temporary, but only sometimes. It's not syntactically evident, in other words, what's going on. When you're doing a coercion, you're kind of 2. Put another way, I would like to preserve the relationship that `equality <= subtyping <= coercion <= as-coercion`, where this is an indication of the number of `(T1,T2)` pairs that are accepted by the various relations. Trying to mix `let ref mut` and coercion would create another kind of relation that is like coercion, but acts differently in the case where a precise match is needed. 3. In any case, this is strictly more conservative than what we had before and we can undo it in the future if we find a way to make coercion mix with type equality. The change to match I feel ok about but similarly unthrilled. There is some subtle text already concerning whether to use eqtype or subtype for identifier bindings. The best fix I think would be to always have match use strict equality but use subtyping on identifier bindings, but the comment `(*)` explains why that's not working at the moment. As above, I think we can change this as we clean up the code there. 2015-03-06 09:14:12 -06:00			// Check that when making a ref mut binding with type `&mut T`, the
			// type `T` must match precisely the type `U` of the value being
			// matched, and in particular cannot be some supertype of `U`. Issue
			// #23116. This test focuses on a `let`.

			`#![allow(dead_code)]`
			`struct S<'b>(&'b i32);`
			`impl<'b> S<'b> {`
			`fn bar<'a>(&'a mut self) -> &'a mut &'a i32 {`
			`let ref mut x = self.0;`
Use revisions for NLL in const-generics and match 2022-05-21 13:27:58 -05:00			`x`
Fully stabilize NLL 2022-04-01 12:13:25 -05:00			`//~^ ERROR lifetime may not live long enough`
When matching against a pattern (either via `match` or `let`) that contains ref-bindings, do not permit any upcasting from the type of the value being matched. Similarly, do not permit coercion in a `let`. This is a [breaking-change] in that it closes a type hole that previously existed, and in that coercion is not performed. You should be able to work around the latter by converting: ```rust let ref mut x: T = expr; ``` into ```rust let x: T = expr; let ref mut x = x; ``` Restricting coercion not to apply in the case of `let ref` or `let ref mut` is sort of unexciting to me, but seems the best solution: 1. Mixing coercion and `let ref` or `let ref mut` is a bit odd, because you are taking the address of a (coerced) temporary, but only sometimes. It's not syntactically evident, in other words, what's going on. When you're doing a coercion, you're kind of 2. Put another way, I would like to preserve the relationship that `equality <= subtyping <= coercion <= as-coercion`, where this is an indication of the number of `(T1,T2)` pairs that are accepted by the various relations. Trying to mix `let ref mut` and coercion would create another kind of relation that is like coercion, but acts differently in the case where a precise match is needed. 3. In any case, this is strictly more conservative than what we had before and we can undo it in the future if we find a way to make coercion mix with type equality. The change to match I feel ok about but similarly unthrilled. There is some subtle text already concerning whether to use eqtype or subtype for identifier bindings. The best fix I think would be to always have match use strict equality but use subtyping on identifier bindings, but the comment `(*)` explains why that's not working at the moment. As above, I think we can change this as we clean up the code there. 2015-03-06 09:14:12 -06:00			`}`
			`}`

			`fn main() {}`