Commits on Jul 25, 2014

1. librustc: Disallow mutation and assignment in pattern guards, and modify …

   the CFG for match statements.
   
   There were two bugs in issue rust-lang#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 rust-lang#14684.
   
   [breaking-change]

   

   pcwalton committed Jul 25, 2014
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