45fae88256
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.
26 lines
913 B
Rust
26 lines
913 B
Rust
// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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// Check that when making a ref mut binding with type `&mut T`, the
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// type `T` must match precisely the type `U` of the value being
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// matched, and in particular cannot be some supertype of `U`. Issue
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// #23116. This test focuses on a `let`.
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#![allow(dead_code)]
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struct S<'b>(&'b i32);
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impl<'b> S<'b> {
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fn bar<'a>(&'a mut self) -> &'a mut &'a i32 {
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let ref mut x = self.0;
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x //~ ERROR mismatched types
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}
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}
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fn main() {}
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