Replace manually_drop lang item with attribute.

Co-authored-by: lcnr <rust@lcnr.de>

Author: ssbr

compiler/rustc_middle/src/ty/adt.rs

@@ -241,7 +241,7 @@ impl AdtDefData {
         if Some(did) == tcx.lang_items().owned_box() {
             flags |= AdtFlags::IS_BOX;
         }
-        if Some(did) == tcx.lang_items().manually_drop() || tcx.has_attr(did, sym::manually_drop) {
+        if tcx.has_attr(did, sym::manually_drop) {
             flags |= AdtFlags::IS_MANUALLY_DROP;
         }
         if Some(did) == tcx.lang_items().unsafe_cell_type() {

library/core/src/lib.rs

@@ -214,6 +214,7 @@
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]
 #![feature(macro_metavar_expr)]
+#![cfg_attr(not(bootstrap), feature(manually_drop_attr))]
 #![feature(min_specialization)]
 #![feature(must_not_suspend)]
 #![feature(negative_impls)]

library/core/src/mem/manually_drop.rs

@@ -44,7 +44,8 @@ use crate::ptr;
 /// [`mem::zeroed`]: crate::mem::zeroed
 /// [`MaybeUninit<T>`]: crate::mem::MaybeUninit
 #[stable(feature = "manually_drop", since = "1.20.0")]
-#[lang = "manually_drop"]
+#[cfg_attr(bootstrap, lang = "manually_drop")]
+#[cfg_attr(not(bootstrap), manually_drop)]
 #[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[repr(transparent)]
 pub struct ManuallyDrop<T: ?Sized> {