b2eb88843d
the CFG for match statements. There were two bugs in issue #14684. One was simply that the borrow check didn't know about the correct CFG for match statements: the pattern must be a predecessor of the guard. This disallows the bad behavior if there are bindings in the pattern. But it isn't enough to prevent the memory safety problem, because of wildcards; thus, this patch introduces a more restrictive rule, which disallows assignments and mutable borrows inside guards outright. I discussed this with Niko and we decided this was the best plan of action. This breaks code that performs mutable borrows in pattern guards. Most commonly, the code looks like this: impl Foo { fn f(&mut self, ...) {} fn g(&mut self, ...) { match bar { Baz if self.f(...) => { ... } _ => { ... } } } } Change this code to not use a guard. For example: impl Foo { fn f(&mut self, ...) {} fn g(&mut self, ...) { match bar { Baz => { if self.f(...) { ... } else { ... } } _ => { ... } } } } Sometimes this can result in code duplication, but often it illustrates a hidden memory safety problem. Closes #14684. [breaking-change]
32 lines
973 B
Rust
32 lines
973 B
Rust
// Copyright 2012-2014 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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#![allow(unused_variable)]
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#![allow(dead_assignment)]
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fn separate_arms() {
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// Here both arms perform assignments, but only is illegal.
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let mut x = None;
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match x {
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None => {
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// It is ok to reassign x here, because there is in
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// fact no outstanding loan of x!
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x = Some(0i);
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}
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Some(ref _i) => {
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x = Some(1i); //~ ERROR cannot assign
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}
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}
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x.clone(); // just to prevent liveness warnings
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}
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fn main() {}
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