Rollup merge of #79177 - fanzier:drop-order-test, r=RalfJung

Test drop order for (destructuring) assignments

Add a test that checks whether the drop order of `let` bindings is consistent with the drop order of the corresponding destructuring assignments.

Thanks to ```@RalfJung``` for the suggesting this test ([here](https://github.com/rust-lang/rust/pull/79016#issuecomment-727608732)) and an implementation!

r? ```@RalfJung```

src/test/ui/destructuring-assignment/drop-order.rs

@@ -0,0 +1,44 @@
+// run-pass
+
+//! Test that let bindings and destructuring assignments have consistent drop orders
+
+#![feature(destructuring_assignment)]
+#![allow(unused_variables, unused_assignments)]
+
+use std::cell::RefCell;
+
+thread_local! {
+    static DROP_ORDER: RefCell<Vec<usize>> = RefCell::new(Vec::new());
+}
+
+struct DropRecorder(usize);
+impl Drop for DropRecorder {
+    fn drop(&mut self) {
+        DROP_ORDER.with(|d| d.borrow_mut().push(self.0));
+    }
+}
+
+fn main() {
+    let expected_drop_order = vec![1, 4, 5, 3, 2];
+    // Check the drop order for let bindings:
+    {
+        let _ = DropRecorder(1);
+        let _val = DropRecorder(2);
+        let (x, _) = (DropRecorder(3), DropRecorder(4));
+        drop(DropRecorder(5));
+    }
+    DROP_ORDER.with(|d| {
+        assert_eq!(&*d.borrow(), &expected_drop_order);
+        d.borrow_mut().clear();
+    });
+    // Check that the drop order for destructuring assignment is the same:
+    {
+        let _val;
+        let x;
+        _ = DropRecorder(1);
+        _val = DropRecorder(2);
+        (x, _) = (DropRecorder(3), DropRecorder(4));
+        drop(DropRecorder(5));
+    }
+    DROP_ORDER.with(|d| assert_eq!(&*d.borrow(), &expected_drop_order));
+}