add test for https://github.com/rust-lang/rust/issues/109812

tests/crashes/109812.rs

@@ -0,0 +1,22 @@
+//@ known-bug: #109812
+
+#![warn(rust_2021_incompatible_closure_captures)]
+
+enum Either {
+    One(X),
+    Two(X),
+}
+
+struct X(Y);
+
+struct Y;
+
+fn move_into_fnmut() {
+    let x = X(Y);
+
+    consume_fnmut(|| {
+        let Either::Two(ref mut _t) = x;
+
+        let X(mut _t) = x;
+    });
+}