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librustc: Disallow mutation and assignment in pattern guards, and modify

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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]
This commit is contained in:
Patrick Walton 2014-07-25 15:18:19 -07:00
parent 66a0b528a6
commit b2eb88843d
10 changed files with 384 additions and 272 deletions
Download patch file Download diff file Expand all files Collapse all files

41
src/librustc/middle/cfg/construct.rs
View file

@ -333,32 +333,37 @@ impl<'a> CFGBuilder<'a> {
// [discr]
// |
// v 2
// [guard1]
// [cond1]
// / \
// | \
// v 3 |
// [pat1] |
// |
// v 4 |
// [body1] v
// | [guard2]
// | / \
// | [body2] \
// | | ...
// | | |
// v 5 v v
// [....expr....]
// v 3 \
// [pat1] \
// | |
// v 4 |
// [guard1] |
// | |
// | |
// v 5 v
// [body1] [cond2]
// | / \
// | ... ...
// | | |
// v 6 v v
// [.....expr.....]
//
let discr_exit = self.expr(discr.clone(), pred); // 1
let expr_exit = self.add_node(expr.id, []);
let mut guard_exit = discr_exit;
let mut cond_exit = discr_exit;
for arm in arms.iter() {
guard_exit = self.opt_expr(arm.guard, guard_exit); // 2
cond_exit = self.add_dummy_node([cond_exit]); // 2
let pats_exit = self.pats_any(arm.pats.as_slice(),
guard_exit); // 3
let body_exit = self.expr(arm.body.clone(), pats_exit); // 4
self.add_contained_edge(body_exit, expr_exit); // 5
cond_exit); // 3
let guard_exit = self.opt_expr(arm.guard,
pats_exit); // 4
let body_exit = self.expr(arm.body.clone(),
guard_exit); // 5
self.add_contained_edge(body_exit, expr_exit); // 6
}
expr_exit
}