Implement MaybeDangling compiler support

compiler/rustc_abi/src/lib.rs

@@ -2145,21 +2145,22 @@ pub enum PointerKind {
 }
 
 /// Encodes extra information we have about a pointer.
+///
 /// Note that this information is advisory only, and backends are free to ignore it:
 /// if the information is wrong, that can cause UB, but if the information is absent,
 /// that must always be okay.
 #[derive(Copy, Clone, Debug)]
 pub struct PointeeInfo {
-    /// If this is `None`, then this is a raw pointer, so size and alignment are not guaranteed to
-    /// be reliable.
+    /// If this is `None`, then this is a raw pointer.
     pub safe: Option<PointerKind>,
-    /// If `safe` is `Some`, then the pointer is either null or dereferenceable for this many bytes.
+    /// If `size` is not zero, then the pointer is either null or dereferenceable for this many bytes
+    /// (independent of `safe`).
+    ///
     /// On a function argument, "dereferenceable" here means "dereferenceable for the entire duration
     /// of this function call", i.e. it is UB for the memory that this pointer points to be freed
     /// while this function is still running.
-    /// The size can be zero if the pointer is not dereferenceable.
     pub size: Size,
-    /// If `safe` is `Some`, then the pointer is aligned as indicated.
+    /// The pointer is guaranteed to be aligned this much (independent of `safe`).
     pub align: Align,
 }
 

compiler/rustc_codegen_cranelift/src/base.rs

@@ -10,7 +10,7 @@ use rustc_data_structures::profiling::SelfProfilerRef;
 use rustc_index::IndexVec;
 use rustc_middle::ty::TypeVisitableExt;
 use rustc_middle::ty::adjustment::PointerCoercion;
-use rustc_middle::ty::layout::FnAbiOf;
+use rustc_middle::ty::layout::{FnAbiOf, HasTypingEnv as _};
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_session::config::OutputFilenames;
 use rustc_span::Symbol;
@@ -924,19 +924,26 @@ fn codegen_stmt<'tcx>(fx: &mut FunctionCx<'_, '_, 'tcx>, cur_block: Block, stmt:
                 count,
             }) => {
                 let dst = codegen_operand(fx, dst);
-                let pointee = dst
-                    .layout()
-                    .pointee_info_at(fx, rustc_abi::Size::ZERO)
-                    .expect("Expected pointer");
+
+                let &ty::RawPtr(pointee, _) = dst.layout().ty.kind() else {
+                    bug!("expected pointer")
+                };
+                let pointee_layout = fx
+                    .tcx
+                    .layout_of(fx.typing_env().as_query_input(pointee))
+                    .expect("expected pointee to have a layout");
+                let elem_size: u64 = pointee_layout.layout.size().bytes();
+
                 let dst = dst.load_scalar(fx);
                 let src = codegen_operand(fx, src).load_scalar(fx);
                 let count = codegen_operand(fx, count).load_scalar(fx);
-                let elem_size: u64 = pointee.size.bytes();
+
                 let bytes = if elem_size != 1 {
                     fx.bcx.ins().imul_imm(count, elem_size as i64)
                 } else {
                     count
                 };
+
                 fx.bcx.call_memcpy(fx.target_config, dst, src, bytes);
             }
         },

compiler/rustc_codegen_gcc/src/type_of.rs

@@ -288,7 +288,9 @@ impl<'tcx> LayoutGccExt<'tcx> for TyAndLayout<'tcx> {
             Float(f) => cx.type_from_float(f),
             Pointer(address_space) => {
                 // If we know the alignment, pick something better than i8.
-                let pointee = if let Some(pointee) = self.pointee_info_at(cx, offset) {
+                let pointee = if let Some(pointee) = self.pointee_info_at(cx, offset)
+                    && pointee.align > rustc_abi::Align::ONE
+                {
                     cx.type_pointee_for_align(pointee.align)
                 } else {
                     cx.type_i8()

compiler/rustc_codegen_llvm/src/builder.rs

@@ -714,7 +714,7 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
                     }
 
                     if let Some(pointee) = layout.pointee_info_at(bx, offset)
-                        && let Some(_) = pointee.safe
+                        && pointee.align > Align::ONE
                     {
                         bx.align_metadata(load, pointee.align);
                     }

compiler/rustc_codegen_ssa/src/mir/statement.rs

@@ -1,5 +1,5 @@
 use rustc_middle::mir::{self, NonDivergingIntrinsic, StmtDebugInfo};
-use rustc_middle::span_bug;
+use rustc_middle::{bug, span_bug, ty};
 use tracing::instrument;
 
 use super::{FunctionCx, LocalRef};
@@ -77,15 +77,21 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 let dst_val = self.codegen_operand(bx, dst);
                 let src_val = self.codegen_operand(bx, src);
                 let count = self.codegen_operand(bx, count).immediate();
-                let pointee_layout = dst_val
-                    .layout
-                    .pointee_info_at(bx, rustc_abi::Size::ZERO)
-                    .expect("Expected pointer");
-                let bytes = bx.mul(count, bx.const_usize(pointee_layout.size.bytes()));
 
-                let align = pointee_layout.align;
+                let &ty::RawPtr(pointee, _) = dst_val.layout.ty.kind() else {
+                    bug!("expected pointer")
+                };
+                let pointee_layout = bx
+                    .tcx()
+                    .layout_of(bx.typing_env().as_query_input(pointee))
+                    .expect("expected pointee to have a layout");
+                let elem_size = pointee_layout.layout.size().bytes();
+                let bytes = bx.mul(count, bx.const_usize(elem_size));
+
+                let align = pointee_layout.layout.align.abi;
                 let dst = dst_val.immediate();
                 let src = src_val.immediate();
+
                 bx.memcpy(dst, align, src, align, bytes, crate::MemFlags::empty(), None);
             }
             mir::StatementKind::FakeRead(..)

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/adt.rs

@@ -62,6 +62,8 @@ bitflags::bitflags! {
         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 @@ impl AdtDefData {
         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 @@ impl<'tcx> AdtDef<'tcx> {
         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_middle/src/ty/layout.rs

@@ -1022,41 +1022,80 @@ where
         let tcx = cx.tcx();
         let typing_env = cx.typing_env();
 
+        // Use conservative pointer kind if not optimizing. This saves us the
+        // Freeze/Unpin queries, and can save time in the codegen backend (noalias
+        // attributes in LLVM have compile-time cost even in unoptimized builds).
+        let optimize = tcx.sess.opts.optimize != OptLevel::No;
+
         let pointee_info = match *this.ty.kind() {
-            ty::RawPtr(p_ty, _) if offset.bytes() == 0 => {
-                tcx.layout_of(typing_env.as_query_input(p_ty)).ok().map(|layout| PointeeInfo {
-                    size: layout.size,
-                    align: layout.align.abi,
-                    safe: None,
-                })
+            ty::RawPtr(_, _) | ty::FnPtr(..) if offset.bytes() == 0 => {
+                Some(PointeeInfo { safe: None, size: Size::ZERO, align: Align::ONE })
             }
-            ty::FnPtr(..) if offset.bytes() == 0 => {
-                tcx.layout_of(typing_env.as_query_input(this.ty)).ok().map(|layout| PointeeInfo {
-                    size: layout.size,
-                    align: layout.align.abi,
-                    safe: None,
+            ty::Ref(_, ty, mt) if offset.bytes() == 0 => {
+                tcx.layout_of(typing_env.as_query_input(ty)).ok().map(|layout| {
+                    let (size, kind);
+                    match mt {
+                        hir::Mutability::Not => {
+                            let frozen = optimize && ty.is_freeze(tcx, typing_env);
+
+                            // Non-frozen shared references are not necessarily dereferenceable for the entire duration of the function
+                            // (see <https://github.com/rust-lang/rust/pull/98017>)
+                            // (if we had "dereferenceable on entry", we could support this)
+                            size = if frozen { layout.size } else { Size::ZERO };
+
+                            kind = PointerKind::SharedRef { frozen };
+                        }
+                        hir::Mutability::Mut => {
+                            let unpin = optimize
+                                && ty.is_unpin(tcx, typing_env)
+                                && ty.is_unsafe_unpin(tcx, typing_env);
+
+                            // Mutable references to potentially self-referential types are not
+                            // necessarily dereferenceable for the entire duration of the function
+                            // (see <https://github.com/rust-lang/unsafe-code-guidelines/issues/381>)
+                            // (if we had "dereferenceable on entry", we could support this)
+                            size = if unpin { layout.size } else { Size::ZERO };
+
+                            kind = PointerKind::MutableRef { unpin };
+                        }
+                    };
+                    PointeeInfo { safe: Some(kind), size, align: layout.align.abi }
                 })
             }
-            ty::Ref(_, ty, mt) if offset.bytes() == 0 => {
-                // Use conservative pointer kind if not optimizing. This saves us the
-                // Freeze/Unpin queries, and can save time in the codegen backend (noalias
-                // attributes in LLVM have compile-time cost even in unoptimized builds).
-                let optimize = tcx.sess.opts.optimize != OptLevel::No;
-                let kind = match mt {
-                    hir::Mutability::Not => {
-                        PointerKind::SharedRef { frozen: optimize && ty.is_freeze(tcx, typing_env) }
-                    }
-                    hir::Mutability::Mut => PointerKind::MutableRef {
+
+            ty::Adt(..)
+                if offset.bytes() == 0
+                    && let Some(pointee) = this.ty.boxed_ty() =>
+            {
+                tcx.layout_of(typing_env.as_query_input(pointee)).ok().map(|layout| PointeeInfo {
+                    safe: Some(PointerKind::Box {
+                        // Same logic as for mutable references above.
                         unpin: optimize
-                            && ty.is_unpin(tcx, typing_env)
-                            && ty.is_unsafe_unpin(tcx, typing_env),
-                    },
-                };
+                            && pointee.is_unpin(tcx, typing_env)
+                            && pointee.is_unsafe_unpin(tcx, typing_env),
+                        global: this.ty.is_box_global(tcx),
+                    }),
+
+                    // `Box` are not necessarily dereferenceable for the entire duration of the function as
+                    // they can be deallocated at any time.
+                    // (if we had "dereferenceable on entry", we could support this)
+                    size: Size::ZERO,
 
-                tcx.layout_of(typing_env.as_query_input(ty)).ok().map(|layout| PointeeInfo {
-                    size: layout.size,
                     align: layout.align.abi,
-                    safe: Some(kind),
+                })
+            }
+
+            ty::Adt(adt_def, ..) if adt_def.is_maybe_dangling() => {
+                Self::ty_and_layout_pointee_info_at(this.field(cx, 0), cx, offset).map(|info| {
+                    PointeeInfo {
+                        // Mark the pointer as raw
+                        // (thus removing noalias/readonly/etc in case of the llvm backend)
+                        safe: None,
+                        // Make sure we don't assert dereferenceability of the pointer.
+                        size: Size::ZERO,
+                        // Preserve the alignment assertion! That is required even inside `MaybeDangling`.
+                        align: info.align,
+                    }
                 })
             }
 
@@ -1098,7 +1137,7 @@ where
                         }
                     }
                     Variants::Multiple { .. } => None,
-                    _ => Some(this),
+                    Variants::Empty | Variants::Single { .. } => Some(this),
                 };
 
                 if let Some(variant) = data_variant
@@ -1135,24 +1174,6 @@ where
                     }
                 }
 
-                // Fixup info for the first field of a `Box`. Recursive traversal will have found
-                // the raw pointer, so size and align are set to the boxed type, but `pointee.safe`
-                // will still be `None`.
-                if let Some(ref mut pointee) = result {
-                    if offset.bytes() == 0
-                        && let Some(boxed_ty) = this.ty.boxed_ty()
-                    {
-                        debug_assert!(pointee.safe.is_none());
-                        let optimize = tcx.sess.opts.optimize != OptLevel::No;
-                        pointee.safe = Some(PointerKind::Box {
-                            unpin: optimize
-                                && boxed_ty.is_unpin(tcx, typing_env)
-                                && boxed_ty.is_unsafe_unpin(tcx, typing_env),
-                            global: this.ty.is_box_global(tcx),
-                        });
-                    }
-                }
-
                 result
             }
         };

compiler/rustc_span/src/symbol.rs

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