Add tests for issues #54966 and #52240

src/test/ui/issues/issue-52240.nll.stderr

@@ -0,0 +1,9 @@
+error[E0596]: cannot borrow data in a `&` reference as mutable
+  --> $DIR/issue-52240.rs:9:27
+   |
+LL |     if let (Some(Foo::Bar(ref mut val)), _) = (&arr.get(0), 0) {
+   |                           ^^^^^^^^^^^ cannot borrow as mutable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.

src/test/ui/issues/issue-52240.rs

@@ -0,0 +1,16 @@
+// issue-52240: Can turn immutable into mut with `ref mut`
+
+enum Foo {
+    Bar(i32),
+}
+
+fn main() {
+    let arr = vec!(Foo::Bar(0));
+    if let (Some(Foo::Bar(ref mut val)), _) = (&arr.get(0), 0) {
+        //~^ ERROR cannot borrow field of immutable binding as mutable
+        *val = 9001;
+    }
+    match arr[0] {
+        Foo::Bar(ref s) => println!("{}", s)
+    }
+}

src/test/ui/issues/issue-52240.stderr

@@ -0,0 +1,9 @@
+error[E0596]: cannot borrow field of immutable binding as mutable
+  --> $DIR/issue-52240.rs:9:27
+   |
+LL |     if let (Some(Foo::Bar(ref mut val)), _) = (&arr.get(0), 0) {
+   |                           ^^^^^^^^^^^ cannot mutably borrow field of immutable binding
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.

src/test/ui/issues/issue-54966.rs

@@ -0,0 +1,6 @@
+// issue-54966: ICE returning an unknown type with impl FnMut
+
+fn generate_duration() -> Oper<impl FnMut()> {}
+//~^ ERROR cannot find type `Oper` in this scope
+
+fn main() {}

src/test/ui/issues/issue-54966.stderr

@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `Oper` in this scope
+  --> $DIR/issue-54966.rs:3:27
+   |
+LL | fn generate_duration() -> Oper<impl FnMut()> {}
+   |                           ^^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0412`.