stop relying on feature(untagged_unions) in stdlib

Author: RalfJung

library/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)]

library/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.

library/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)]