add semantics to MaybeDangling

Author: WaffleLapkin

compiler/rustc_hir/src/lang_items.rs

@@ -341,7 +341,8 @@ language_item_table! {
 
     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/layout.rs

@@ -1060,6 +1060,12 @@ where
                 })
             }
 
+            ty::Adt(adt_def, ..) if adt_def.is_maybe_dangling() => {
+                // FIXME: what is the exact effect of maybe dangling?
+                Self::ty_and_layout_pointee_info_at(this.field(cx, 0), cx, offset)
+                    .map(|info| PointeeInfo { safe: None, ..info })
+            }
+
             _ => {
                 let mut data_variant = match &this.variants {
                     // Within the discriminant field, only the niche itself is
@@ -1098,7 +1104,7 @@ where
                         }
                     }
                     Variants::Multiple { .. } => None,
-                    _ => Some(this),
+                    Variants::Empty | Variants::Single { .. } => Some(this),
                 };
 
                 if let Some(variant) = data_variant

compiler/rustc_span/src/symbol.rs

@@ -1445,6 +1445,7 @@ symbols! {
         maxnumf128,
         may_dangle,
         may_unwind,
+        maybe_dangling,
         maybe_uninit,
         maybe_uninit_uninit,
         maybe_uninit_zeroed,

library/core/src/mem/maybe_dangling.rs

@@ -4,10 +4,6 @@ use crate::{mem, ptr};
 
 /// Allows wrapped [references] and [boxes] to dangle.
 ///
-/// <section class="warning">
-/// This type is not properly implemented yet, and the documentation below is thus not accurate.
-/// </section>
-///
 /// That is, if a reference (or a `Box`) is wrapped in `MaybeDangling` (including when in a
 /// (nested) field of a compound type wrapped in `MaybeDangling`), it does not have to follow
 /// pointer aliasing rules or be dereferenceable.
@@ -73,6 +69,7 @@ use crate::{mem, ptr};
 #[repr(transparent)]
 #[rustc_pub_transparent]
 #[derive(Debug, Copy, Clone, Default)]
+#[lang = "maybe_dangling"]
 pub struct MaybeDangling<P: ?Sized>(P);
 
 impl<P: ?Sized> MaybeDangling<P> {

tests/codegen-llvm/manually_drop_refs.rs

@@ -3,19 +3,19 @@
 
 use std::mem::ManuallyDrop;
 
-// CHECK: define noundef nonnull align 1 ptr @f(ptr noalias noundef nonnull readnone returned align 1 {{(captures\(ret: address, provenance\))?}} %x) unnamed_addr
+// CHECK: define noundef nonnull ptr @f(ptr noundef nonnull readnone returned {{(captures\(ret: address, provenance\) )?}}%x) unnamed_addr
 #[no_mangle]
 pub fn f(x: ManuallyDrop<Box<u8>>) -> ManuallyDrop<Box<u8>> {
     x
 }
 
-// CHECK: define noundef nonnull align 1 dereferenceable(1) ptr @g(ptr noalias noundef readonly returned align 1 {{(captures\(ret: address, read_provenance\))?}} dereferenceable(1) %x) unnamed_addr
+// CHECK: define noundef nonnull ptr @g(ptr noundef nonnull readnone returned {{(captures\(ret: address, provenance\) )?}}%x) unnamed_addr
 #[no_mangle]
 pub fn g(x: ManuallyDrop<&u8>) -> ManuallyDrop<&u8> {
     x
 }
 
-// CHECK: define noundef nonnull align 1 dereferenceable(1) ptr @h(ptr noalias noundef readnone returned align 1 {{(captures\(ret: address, provenance\))?}} dereferenceable(1) %x) unnamed_addr
+// CHECK: define noundef nonnull ptr @h(ptr noundef nonnull readnone returned {{(captures\(ret: address, provenance\) )?}}%x) unnamed_addr
 #[no_mangle]
 pub fn h(x: ManuallyDrop<&mut u8>) -> ManuallyDrop<&mut u8> {
     x