Rollup merge of rust-lang#77547 - RalfJung:stable-union-drop, r=matthewjasper

stabilize union with 'ManuallyDrop' fields and 'impl Drop for Union'

As [discussed by @SimonSapin and @withoutboats](rust-lang#55149 (comment)), this PR proposes to stabilize parts of the `untagged_union` feature gate:

* It will be possible to have a union with field type `ManuallyDrop<T>` for any `T`.
* While at it I propose we also stabilize `impl Drop for Union`; to my knowledge, there are no open concerns around this feature.

In the RFC discussion, we also talked about allowing `&mut T` as another non-`Copy` non-dropping type, but that felt to me like an overly specific exception so I figured we'd wait if there is actually any use for such a special case.

Some things remain unstable and still require the `untagged_union` feature gate:
* Union with fields that do not drop, are not `Copy`, and are not `ManuallyDrop<_>`. The reason to not stabilize this is to avoid semver concerns around libraries adding `Drop` implementations later. (This is already not fully semver compatible as, to my knowledge, the borrow checker will exploit the non-dropping nature of any type, but it seems prudent to avoid further increasing the amount of trouble adding an `impl Drop` can cause.)

Due to this, quite a few tests still need the `untagged_union` feature, but I think the ones where I could remove the feature flag provide good test coverage for the stable part.

Cc @rust-lang/lang

Author: JohnTitor

core/src/lib.rs

@@ -131,7 +131,7 @@
 #![feature(transparent_unions)]
 #![feature(unboxed_closures)]
 #![feature(unsized_locals)]
-#![feature(untagged_unions)]
+#![cfg_attr(bootstrap, feature(untagged_unions))]
 #![feature(unwind_attributes)]
 #![feature(variant_count)]
 #![feature(tbm_target_feature)]

core/src/ptr/mod.rs

@@ -229,6 +229,16 @@ pub(crate) struct FatPtr<T> {
     pub(crate) len: usize,
 }
 
+// Manual impl needed to avoid `T: Clone` bound.
+impl<T> Clone for FatPtr<T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+// Manual impl needed to avoid `T: Copy` bound.
+impl<T> Copy for FatPtr<T> {}
+
 /// Forms a raw slice from a pointer and a length.
 ///
 /// The `len` argument is the number of **elements**, not the number of bytes.

std/src/lib.rs

@@ -319,7 +319,7 @@
 #![feature(unsafe_block_in_unsafe_fn)]
 #![feature(unsafe_cell_get_mut)]
 #![feature(unsafe_cell_raw_get)]
-#![feature(untagged_unions)]
+#![cfg_attr(bootstrap, feature(untagged_unions))]
 #![feature(unwind_attributes)]
 #![feature(vec_into_raw_parts)]
 #![feature(wake_trait)]