add an adt flag for MaybeDangling

compiler/rustc_hir/src/lang_items.rs

@@ -341,7 +341,8 @@ fn hash_stable(&self, _: &mut CTX, hasher: &mut StableHasher) {
 
     PhantomData,             sym::phantom_data,        phantom_data,               Target::Struct,         GenericRequirement::Exact(1);
 
-    ManuallyDrop,            sym::manually_drop,       manually_drop,              Target::Struct,         GenericRequirement::None;
+    ManuallyDrop,            sym::manually_drop,       manually_drop,              Target::Struct,         GenericRequirement::Exact(1);
+    MaybeDangling,           sym::maybe_dangling,      maybe_dangling,             Target::Struct,         GenericRequirement::Exact(1);
     BikeshedGuaranteedNoDrop, sym::bikeshed_guaranteed_no_drop, bikeshed_guaranteed_no_drop, Target::Trait, GenericRequirement::Exact(0);
 
     MaybeUninit,             sym::maybe_uninit,        maybe_uninit,               Target::Union,          GenericRequirement::None;

compiler/rustc_middle/src/ty/adt.rs

@@ -62,6 +62,8 @@ impl AdtFlags: u16 {
         const IS_PIN_PROJECT                = 1 << 12;
         /// Indicates whether the type is `FieldRepresentingType`.
         const IS_FIELD_REPRESENTING_TYPE    = 1 << 13;
+        /// Indicates whether the type is `MaybeDangling<_>`.
+        const IS_MAYBE_DANGLING             = 1 << 14;
     }
 }
 rustc_data_structures::external_bitflags_debug! { AdtFlags }
@@ -373,6 +375,9 @@ pub(super) fn new(
         if tcx.is_lang_item(did, LangItem::ManuallyDrop) {
             flags |= AdtFlags::IS_MANUALLY_DROP;
         }
+        if tcx.is_lang_item(did, LangItem::MaybeDangling) {
+            flags |= AdtFlags::IS_MAYBE_DANGLING;
+        }
         if tcx.is_lang_item(did, LangItem::UnsafeCell) {
             flags |= AdtFlags::IS_UNSAFE_CELL;
         }
@@ -500,6 +505,12 @@ pub fn is_manually_drop(self) -> bool {
         self.flags().contains(AdtFlags::IS_MANUALLY_DROP)
     }
 
+    /// Returns `true` if this is `MaybeDangling<T>`.
+    #[inline]
+    pub fn is_maybe_dangling(self) -> bool {
+        self.flags().contains(AdtFlags::IS_MAYBE_DANGLING)
+    }
+
     /// Returns `true` if this is `Pin<T>`.
     #[inline]
     pub fn is_pin(self) -> bool {

compiler/rustc_span/src/symbol.rs

@@ -1230,6 +1230,7 @@
         maxnumf128,
         may_dangle,
         may_unwind,
+        maybe_dangling,
         maybe_uninit,
         maybe_uninit_uninit,
         maybe_uninit_zeroed,

library/core/src/mem/maybe_dangling.rs

@@ -73,6 +73,7 @@
 #[repr(transparent)]
 #[rustc_pub_transparent]
 #[derive(Debug, Copy, Clone, Default)]
+#[lang = "maybe_dangling"]
 pub struct MaybeDangling<P: ?Sized>(P);
 
 impl<P: ?Sized> MaybeDangling<P> {