Make unconditional_recursion warning detect recursive drops

tests/ui/lint/lint-unconditional-drop-recursion.rs

@@ -0,0 +1,38 @@
+// Because drop recursion can only be detected after drop elaboration which
+// happens for codegen:
+// build-fail
+
+#![deny(unconditional_recursion)]
+#![allow(dead_code)]
+
+pub struct RecursiveDrop;
+
+impl Drop for RecursiveDrop {
+    fn drop(&mut self) { //~ ERROR function cannot return without recursing
+        let _ = RecursiveDrop;
+    }
+}
+
+#[derive(Default)]
+struct NotRecursiveDrop1;
+
+impl Drop for NotRecursiveDrop1 {
+    fn drop(&mut self) {
+        // Before drop elaboration, the MIR can look like a recursive drop will
+        // occur. But it will not, since forget() prevents drop() from running.
+        let taken = std::mem::take(self);
+        std::mem::forget(taken);
+    }
+}
+
+struct NotRecursiveDrop2;
+
+impl Drop for NotRecursiveDrop2 {
+    fn drop(&mut self) {
+        // Before drop elaboration, the MIR can look like a recursive drop will
+        // occur. But it will not, since this will panic.
+        std::panic::panic_any(NotRecursiveDrop2);
+    }
+}
+
+fn main() {}