Add a test for struct case

Author: m-ysk

tests/ui/lint/dead-code/only-used-for-destructuring-struct.rs

@@ -0,0 +1,17 @@
+//@ check-pass
+
+// Make sure we don't have any false positives here.
+
+#![deny(dead_code)]
+
+struct Foo {
+    bar: usize,
+}
+
+fn get_thing<T>() -> T {
+    todo!()
+}
+
+pub fn main() {
+    let Foo { bar: _x } = get_thing();
+}