Auto merge of #38600 - arielb1:dead-drop, r=eddyb

clear discriminant drop flag at the bottom of a drop ladder

Fixes #38437.

Beta-nominating because serious I-wrong.

r? @eddyb

src/test/run-pass/issue-38437.rs

@@ -0,0 +1,54 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Check that drop elaboration clears the "master" discriminant
+// drop flag even if it protects no fields.
+
+struct Good(usize);
+impl Drop for Good {
+    #[inline(never)]
+    fn drop(&mut self) {
+        println!("dropping Good({})", self.0);
+    }
+}
+
+struct Void;
+impl Drop for Void {
+    #[inline(never)]
+    fn drop(&mut self) {
+        panic!("Suddenly, a Void appears.");
+    }
+}
+
+enum E {
+    Never(Void),
+    Fine(Good)
+}
+
+fn main() {
+    let mut go = true;
+
+    loop {
+        let next;
+        match go {
+            true => next = E::Fine(Good(123)),
+            false => return,
+        }
+
+        match next {
+            E::Never(_) => return,
+            E::Fine(_good) => go = false,
+        }
+
+        // `next` is dropped and StorageDead'd here. We must reset the
+        // discriminant's drop flag to avoid random variants being
+        // dropped.
+    }
+}