add test checking that 'if cond { .. }' where 'cond: &mut? bool' isn't accepted.

src/test/ui/if/if-no-match-bindings.rs

@@ -0,0 +1,22 @@
+// Checks for `if` expressions with respect to default match bindings.
+// Specifically, we do not accept `if cond { ... }` where `cond: &mut? bool`.
+// Meanwhile, `match cond { true => ..., _ => ... }` does accept that.
+
+// FIXME(@rust-lang/lang-team): consider relaxing this?
+
+fn b_ref<'a>() -> &'a bool { &true }
+fn b_mut_ref<'a>() -> &'a mut bool { &mut true }
+
+fn main() {
+    // This is OK:
+    match b_ref() { true => {}, _ => {} }
+    match b_mut_ref() { true => {}, _ => {} }
+    match &true { true => {}, _ => {} }
+    match &mut true { true => {}, _ => {} }
+
+    // This is NOT:
+    if b_ref() {} //~ ERROR mismatched types [E0308]
+    if b_mut_ref() {} //~ ERROR mismatched types [E0308]
+    if &true {} //~ ERROR mismatched types [E0308]
+    if &mut true {} //~ ERROR mismatched types [E0308]
+}

src/test/ui/if/if-no-match-bindings.stderr

@@ -0,0 +1,39 @@
+error[E0308]: mismatched types
+  --> $DIR/if-no-match-bindings.rs:18:8
+   |
+LL |     if b_ref() {}
+   |        ^^^^^^^ expected bool, found &bool
+   |
+   = note: expected type `bool`
+              found type `&bool`
+
+error[E0308]: mismatched types
+  --> $DIR/if-no-match-bindings.rs:19:8
+   |
+LL |     if b_mut_ref() {}
+   |        ^^^^^^^^^^^ expected bool, found &mut bool
+   |
+   = note: expected type `bool`
+              found type `&mut bool`
+
+error[E0308]: mismatched types
+  --> $DIR/if-no-match-bindings.rs:20:8
+   |
+LL |     if &true {}
+   |        ^^^^^ expected bool, found &bool
+   |
+   = note: expected type `bool`
+              found type `&bool`
+
+error[E0308]: mismatched types
+  --> $DIR/if-no-match-bindings.rs:21:8
+   |
+LL |     if &mut true {}
+   |        ^^^^^^^^^ expected bool, found &mut bool
+   |
+   = note: expected type `bool`
+              found type `&mut bool`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.