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

[pcwalton]

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]

r? @pnkfelix

[pcwalton]

Note that the code in the parser that I had to change was actually quite dangerous—it was matching on self.token and mutating self.token in the guard! This to me is evidence that this change is worthwhile.

[pnkfelix]

@pcwalton Am I right in thinking that in principle, we could later (backward compatibly) loosen the restrictions imposed here so that one could again mutate, assign, and mutably-borrow bindings that came from outside the current match expression that the guard occurs on?

[pcwalton]

Yes.