Add tests for new match borrows

src/test/ui/issues/issue-27282-mutate-before-diverging-arm-3.rs

@@ -0,0 +1,30 @@
+// This is testing an attempt to corrupt the discriminant of the match
+// arm in a guard, followed by an attempt to continue matching on that
+// corrupted discriminant in the remaining match arms.
+//
+// Basically this is testing that our new NLL feature of emitting a
+// fake read on each match arm is catching cases like this.
+//
+// This case is interesting because a borrow of **x is untracked, because **x is
+// immutable. However, for matches we care that **x refers to the same value
+// until we have chosen a match arm.
+#![feature(nll)]
+struct ForceFnOnce;
+fn main() {
+    let mut x = &mut &Some(&2);
+    let force_fn_once = ForceFnOnce;
+    match **x {
+        None => panic!("unreachable"),
+        Some(&_) if {
+            // ForceFnOnce needed to exploit #27282
+            (|| { *x = &None; drop(force_fn_once); })();
+            //~^ ERROR cannot mutably borrow `x` in match guard [E0510]
+            false
+        } => {}
+        Some(&a) if { // this binds to garbage if we've corrupted discriminant
+            println!("{}", a);
+            panic!()
+        } => {}
+        _ => panic!("unreachable"),
+    }
+}

src/test/ui/issues/issue-27282-mutate-before-diverging-arm-3.stderr

@@ -0,0 +1,14 @@
+error[E0510]: cannot mutably borrow `x` in match guard
+  --> $DIR/issue-27282-mutate-before-diverging-arm-3.rs:20:14
+   |
+LL |     match **x {
+   |           --- value is immutable in match guard
+...
+LL |             (|| { *x = &None; drop(force_fn_once); })();
+   |              ^^    - borrow occurs due to use of `x` in closure
+   |              |
+   |              cannot mutably borrow
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0510`.