unions are not always trivially dropable

Fixes #52786

src/test/ui/dropck/dropck-union.nll.stderr

@@ -0,0 +1,16 @@
+error[E0597]: `v` does not live long enough
+  --> $DIR/dropck-union.rs:39:18
+   |
+LL |     v.0.set(Some(&v)); //~ ERROR: `v` does not live long enough
+   |                  ^^ borrowed value does not live long enough
+LL | }
+   | -
+   | |
+   | `v` dropped here while still borrowed
+   | borrow later used here, when `v` is dropped
+   |
+   = note: values in a scope are dropped in the opposite order they are defined
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/dropck/dropck-union.rs

@@ -0,0 +1,40 @@
+#![feature(untagged_unions)]
+
+use std::cell::Cell;
+use std::ops::Deref;
+use std::mem::ManuallyDrop;
+
+union Wrap<T> { x: ManuallyDrop<T> }
+
+impl<T> Drop for Wrap<T>  {
+    fn drop(&mut self) {
+        unsafe { std::ptr::drop_in_place(&mut *self.x as *mut T); }
+    }
+}
+
+impl<T> Wrap<T> {
+    fn new(x: T) -> Self {
+        Wrap { x: ManuallyDrop::new(x) }
+    }
+}
+
+impl<T> Deref for Wrap<T> {
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        unsafe {
+            &self.x
+        }
+    }
+}
+
+struct C<'a>(Cell<Option<&'a C<'a>>>);
+
+impl<'a> Drop for C<'a> {
+    fn drop(&mut self) {}
+}
+
+fn main() {
+    let v : Wrap<C> = Wrap::new(C(Cell::new(None)));
+    v.0.set(Some(&v)); //~ ERROR: `v` does not live long enough
+}

src/test/ui/dropck/dropck-union.stderr

@@ -0,0 +1,13 @@
+error[E0597]: `v` does not live long enough
+  --> $DIR/dropck-union.rs:39:19
+   |
+LL |     v.0.set(Some(&v)); //~ ERROR: `v` does not live long enough
+   |                   ^ borrowed value does not live long enough
+LL | }
+   | - `v` dropped here while still borrowed
+   |
+   = note: values in a scope are dropped in the opposite order they are created
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.