strip readonly/captures from MaybeDangling<&T>

Author: WaffleLapkin

compiler/rustc_abi/src/lib.rs

@@ -2153,18 +2153,14 @@ pub enum PointerKind {
 pub struct PointeeInfo {
     /// If this is `None`, then this is a raw pointer.
     pub safe: Option<PointerKind>,
-    /// If this is true, the pointer is wrapped in `MaybeDangling` and thus can't be assumed to
-    /// not alias. Having this as separate state from `safe: None` allows us to keep `readonly` and
-    /// `captures(address, read_provenance)` llvm attributes.
-    pub may_dangle: bool,
     /// If `size` is not zero, then the pointer is either null or dereferenceable for this many bytes
-    /// (independent of `safe` and `may_dangle`).
+    /// (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.
     pub size: Size,
-    /// The pointer is guaranteed to be aligned this much (independent of `safe` and `may_dangle`).
+    /// The pointer is guaranteed to be aligned this much (independent of `safe`).
     pub align: Align,
 }
 

compiler/rustc_middle/src/ty/layout.rs

@@ -1028,12 +1028,9 @@ where
         let optimize = tcx.sess.opts.optimize != OptLevel::No;
 
         let pointee_info = match *this.ty.kind() {
-            ty::RawPtr(_, _) | ty::FnPtr(..) if offset.bytes() == 0 => Some(PointeeInfo {
-                safe: None,
-                size: Size::ZERO,
-                align: Align::ONE,
-                may_dangle: false,
-            }),
+            ty::RawPtr(_, _) | ty::FnPtr(..) if offset.bytes() == 0 => {
+                Some(PointeeInfo { safe: None, size: Size::ZERO, align: Align::ONE })
+            }
             ty::Ref(_, ty, mt) if offset.bytes() == 0 => {
                 tcx.layout_of(typing_env.as_query_input(ty)).ok().map(|layout| {
                     let (size, kind);
@@ -1062,12 +1059,7 @@ where
                             kind = PointerKind::MutableRef { unpin };
                         }
                     };
-                    PointeeInfo {
-                        safe: Some(kind),
-                        size,
-                        align: layout.align.abi,
-                        may_dangle: false,
-                    }
+                    PointeeInfo { safe: Some(kind), size, align: layout.align.abi }
                 })
             }
 
@@ -1090,20 +1082,19 @@ where
                     size: Size::ZERO,
 
                     align: layout.align.abi,
-                    may_dangle: false,
                 })
             }
 
             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 possibly dangling
-                        // (thus removing noalias in case of llvm backend)
-                        may_dangle: true,
+                        // 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`.
-                        ..info
+                        align: info.align,
                     }
                 })
             }

compiler/rustc_ty_utils/src/abi.rs

@@ -351,12 +351,11 @@ fn arg_attrs_for_rust_scalar<'tcx>(
             // not return values.
             //
             // `&mut T` and `Box<T>` where `T: Unpin` are unique and hence `noalias`.
-            let no_alias = !pointee.may_dangle
-                && match kind {
-                    PointerKind::SharedRef { frozen } => frozen,
-                    PointerKind::MutableRef { unpin } => unpin && noalias_mut_ref,
-                    PointerKind::Box { unpin, global } => unpin && global && noalias_for_box,
-                };
+            let no_alias = match kind {
+                PointerKind::SharedRef { frozen } => frozen,
+                PointerKind::MutableRef { unpin } => unpin && noalias_mut_ref,
+                PointerKind::Box { unpin, global } => unpin && global && noalias_for_box,
+            };
             // We can never add `noalias` in return position; that LLVM attribute has some very surprising semantics
             // (see <https://github.com/rust-lang/unsafe-code-guidelines/issues/385#issuecomment-1368055745>).
             if no_alias && !is_return {

tests/codegen-llvm/maybe_dangling_refs.rs

@@ -15,7 +15,7 @@ pub fn f(x: MaybeDangling<Box<u8>>) -> MaybeDangling<Box<u8>> {
     x
 }
 
-// CHECK: define {{(dso_local )?}}noundef nonnull ptr @g(ptr noundef nonnull readonly captures(address, read_provenance) %x) unnamed_addr
+// CHECK: define {{(dso_local )?}}noundef nonnull ptr @g(ptr noundef nonnull %x) unnamed_addr
 #[no_mangle]
 pub fn g(x: MaybeDangling<&u8>) -> MaybeDangling<&u8> {
     x
@@ -33,7 +33,7 @@ pub fn i(x: MaybeDangling<Box<u32>>) -> MaybeDangling<Box<u32>> {
     x
 }
 
-// CHECK: define {{(dso_local )?}}noundef nonnull align 4 ptr @j(ptr noundef nonnull readonly align 4 captures(address, read_provenance) %x) unnamed_addr
+// CHECK: define {{(dso_local )?}}noundef nonnull align 4 ptr @j(ptr noundef nonnull align 4 %x) unnamed_addr
 #[no_mangle]
 pub fn j(x: MaybeDangling<&u32>) -> MaybeDangling<&u32> {
     x