Don't allow upcasting to a supertype in the type of the match #23119
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@bors r+ |
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| struct S<'b>(&'b i32); | ||
| impl<'b> S<'b> { | ||
| fn bar<'a>(&'a mut self) -> &'a mut &'a i32 { | ||
| match self.0 { ref mut x => x } |
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Should this have an error message directive //~ ERROR?
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@bors r- I see a failure in run-pass, investigating. Also, what @jakub- said. |
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Actually, there is a more serious problem here. In particular, it's not covering the case of |
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@pnkfelix ok I pushed a more comprehensive fix. See the comment. |
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Definitely wants re-review. |
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| /// Checks if the pattern contains any patterns that bind something to |
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This comment seems wrong? It talks about by-value bindings.
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r? @pnkfelix |
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r? @nrc |
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lgtm, but I have no idea about match checking so probably better for Jakub to r? this |
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.
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rebased, addressed nit by @jakub- |
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…=pnkfelix Don't allow upcasting to a supertype in the type of the match discriminant. Fixes rust-lang#23116. This is a [breaking-change] in that it closes a type hole that previously existed. r? @pnkfelix
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Don't allow upcasting to a supertype in the type of the match discriminant. Fixes rust-lang#23116. This is a [breaking-change] in that it closes a type hole that previously existed. r? @pnkfelix
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- Successful merges: rust-lang#22954, rust-lang#23119, rust-lang#23509, rust-lang#23561, rust-lang#23590, rust-lang#23607, rust-lang#23608, rust-lang#23618, rust-lang#23622, rust-lang#23639, rust-lang#23641 - Failed merges: rust-lang#23401
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Don't allow upcasting to a supertype in the type of the match discriminant. Fixes #23116.
This is a [breaking-change] in that it closes a type hole that previously existed.
r? @pnkfelix