Rollup merge of #135748 - compiler-errors:len-2, r=RalfJung,oli-obk

Lower index bounds checking to `PtrMetadata`, this time with the right fake borrow semantics 😸

Change `Rvalue::RawRef` to take a `RawRefKind` instead of just a `Mutability`. Then introduce `RawRefKind::FakeForPtrMetadata` and use that for lowering index bounds checking to a `PtrMetadata`. This new `RawRefKind::FakeForPtrMetadata` acts like a shallow fake borrow in borrowck, which mimics the semantics of the old `Rvalue::Len` operation we're replacing.

We can then use this `RawRefKind` instead of using a span desugaring hack in CTFE.

cc ``@scottmcm`` ``@RalfJung``

compiler/rustc_middle/src/mir/syntax.rs

@@ -180,6 +180,59 @@ pub enum BorrowKind {
     Mut { kind: MutBorrowKind },
 }
 
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, TyEncodable, TyDecodable)]
+#[derive(Hash, HashStable)]
+pub enum RawPtrKind {
+    Mut,
+    Const,
+    /// Creates a raw pointer to a place that will only be used to access its metadata,
+    /// not the data behind the pointer. Note that this limitation is *not* enforced
+    /// by the validator.
+    ///
+    /// The borrow checker allows overlap of these raw pointers with references to the
+    /// data. This is sound even if the pointer is "misused" since any such use is anyway
+    /// unsafe. In terms of the operational semantics (i.e., Miri), this is equivalent
+    /// to `RawPtrKind::Mut`, but will never incur a retag.
+    FakeForPtrMetadata,
+}
+
+impl From<Mutability> for RawPtrKind {
+    fn from(other: Mutability) -> Self {
+        match other {
+            Mutability::Mut => RawPtrKind::Mut,
+            Mutability::Not => RawPtrKind::Const,
+        }
+    }
+}
+
+impl RawPtrKind {
+    pub fn is_fake(self) -> bool {
+        match self {
+            RawPtrKind::Mut | RawPtrKind::Const => false,
+            RawPtrKind::FakeForPtrMetadata => true,
+        }
+    }
+
+    pub fn to_mutbl_lossy(self) -> Mutability {
+        match self {
+            RawPtrKind::Mut => Mutability::Mut,
+            RawPtrKind::Const => Mutability::Not,
+
+            // We have no type corresponding to a fake borrow, so use
+            // `*const` as an approximation.
+            RawPtrKind::FakeForPtrMetadata => Mutability::Not,
+        }
+    }
+
+    pub fn ptr_str(self) -> &'static str {
+        match self {
+            RawPtrKind::Mut => "mut",
+            RawPtrKind::Const => "const",
+            RawPtrKind::FakeForPtrMetadata => "const (fake)",
+        }
+    }
+}
+
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, TyEncodable, TyDecodable)]
 #[derive(Hash, HashStable)]
 pub enum MutBorrowKind {
@@ -1356,7 +1409,7 @@ pub enum Rvalue<'tcx> {
     ///
     /// Like with references, the semantics of this operation are heavily dependent on the aliasing
     /// model.
-    RawPtr(Mutability, Place<'tcx>),
+    RawPtr(RawPtrKind, Place<'tcx>),
 
     /// Yields the length of the place, as a `usize`.
     ///

compiler/rustc_middle/src/mir/tcx.rs

@@ -206,9 +206,9 @@ impl<'tcx> Rvalue<'tcx> {
                 let place_ty = place.ty(local_decls, tcx).ty;
                 Ty::new_ref(tcx, reg, place_ty, bk.to_mutbl_lossy())
             }
-            Rvalue::RawPtr(mutability, ref place) => {
+            Rvalue::RawPtr(kind, ref place) => {
                 let place_ty = place.ty(local_decls, tcx).ty;
-                Ty::new_ptr(tcx, place_ty, mutability)
+                Ty::new_ptr(tcx, place_ty, kind.to_mutbl_lossy())
             }
             Rvalue::Len(..) => tcx.types.usize,
             Rvalue::Cast(.., ty) => ty,

compiler/rustc_middle/src/mir/type_foldable.rs

@@ -15,6 +15,7 @@ TrivialTypeTraversalImpls! {
     SourceScopeLocalData,
     UserTypeAnnotationIndex,
     BorrowKind,
+    RawPtrKind,
     CastKind,
     BasicBlock,
     SwitchTargets,

compiler/rustc_middle/src/mir/visit.rs

@@ -685,12 +685,15 @@ macro_rules! make_mir_visitor {
 
                     Rvalue::RawPtr(m, path) => {
                         let ctx = match m {
-                            Mutability::Mut => PlaceContext::MutatingUse(
+                            RawPtrKind::Mut => PlaceContext::MutatingUse(
                                 MutatingUseContext::RawBorrow
                             ),
-                            Mutability::Not => PlaceContext::NonMutatingUse(
+                            RawPtrKind::Const => PlaceContext::NonMutatingUse(
                                 NonMutatingUseContext::RawBorrow
                             ),
+                            RawPtrKind::FakeForPtrMetadata => PlaceContext::NonMutatingUse(
+                                NonMutatingUseContext::Inspect
+                            ),
                         };
                         self.visit_place(path, ctx, location);
                     }