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``

Author: matthiaskrgr

compiler/rustc_borrowck/src/lib.rs

@@ -1284,15 +1284,18 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                 );
             }
 
-            &Rvalue::RawPtr(mutability, place) => {
-                let access_kind = match mutability {
-                    Mutability::Mut => (
+            &Rvalue::RawPtr(kind, place) => {
+                let access_kind = match kind {
+                    RawPtrKind::Mut => (
                         Deep,
                         Write(WriteKind::MutableBorrow(BorrowKind::Mut {
                             kind: MutBorrowKind::Default,
                         })),
                     ),
-                    Mutability::Not => (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
+                    RawPtrKind::Const => (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
+                    RawPtrKind::FakeForPtrMetadata => {
+                        (Shallow(Some(ArtificialField::ArrayLength)), Read(ReadKind::Copy))
+                    }
                 };
 
                 self.access_place(

compiler/rustc_borrowck/src/polonius/legacy/loan_invalidations.rs

@@ -3,11 +3,7 @@ use std::ops::ControlFlow;
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_middle::bug;
 use rustc_middle::mir::visit::Visitor;
-use rustc_middle::mir::{
-    self, BasicBlock, Body, BorrowKind, FakeBorrowKind, InlineAsmOperand, Location, Mutability,
-    NonDivergingIntrinsic, Operand, Place, Rvalue, Statement, StatementKind, Terminator,
-    TerminatorKind,
-};
+use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 use tracing::debug;
 
@@ -60,7 +56,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LoanInvalidationsGenerator<'a, 'tcx> {
             StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(op)) => {
                 self.consume_operand(location, op);
             }
-            StatementKind::Intrinsic(box NonDivergingIntrinsic::CopyNonOverlapping(mir::CopyNonOverlapping {
+            StatementKind::Intrinsic(box NonDivergingIntrinsic::CopyNonOverlapping(CopyNonOverlapping {
                 src,
                 dst,
                 count,
@@ -273,15 +269,18 @@ impl<'a, 'tcx> LoanInvalidationsGenerator<'a, 'tcx> {
                 self.access_place(location, place, access_kind, LocalMutationIsAllowed::No);
             }
 
-            &Rvalue::RawPtr(mutability, place) => {
-                let access_kind = match mutability {
-                    Mutability::Mut => (
+            &Rvalue::RawPtr(kind, place) => {
+                let access_kind = match kind {
+                    RawPtrKind::Mut => (
                         Deep,
                         Write(WriteKind::MutableBorrow(BorrowKind::Mut {
-                            kind: mir::MutBorrowKind::Default,
+                            kind: MutBorrowKind::Default,
                         })),
                     ),
-                    Mutability::Not => (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
+                    RawPtrKind::Const => (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
+                    RawPtrKind::FakeForPtrMetadata => {
+                        (Shallow(Some(ArtificialField::ArrayLength)), Read(ReadKind::Copy))
+                    }
                 };
 
                 self.access_place(location, place, access_kind, LocalMutationIsAllowed::No);

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -349,8 +349,8 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
             let tcx = self.tcx();
             let maybe_uneval = match constant.const_ {
                 Const::Ty(_, ct) => match ct.kind() {
-                    ty::ConstKind::Unevaluated(_) => {
-                        bug!("should not encounter unevaluated Const::Ty here, got {:?}", ct)
+                    ty::ConstKind::Unevaluated(uv) => {
+                        Some(UnevaluatedConst { def: uv.def, args: uv.args, promoted: None })
                     }
                     _ => None,
                 },

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -612,9 +612,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             mir::Rvalue::CopyForDeref(place) => {
                 self.codegen_operand(bx, &mir::Operand::Copy(place))
             }
-            mir::Rvalue::RawPtr(mutability, place) => {
-                let mk_ptr =
-                    move |tcx: TyCtxt<'tcx>, ty: Ty<'tcx>| Ty::new_ptr(tcx, ty, mutability);
+            mir::Rvalue::RawPtr(kind, place) => {
+                let mk_ptr = move |tcx: TyCtxt<'tcx>, ty: Ty<'tcx>| {
+                    Ty::new_ptr(tcx, ty, kind.to_mutbl_lossy())
+                };
                 self.codegen_place_to_pointer(bx, place, mk_ptr)
             }
 

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -518,7 +518,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             }
 
             Rvalue::Ref(_, BorrowKind::Mut { .. }, place)
-            | Rvalue::RawPtr(Mutability::Mut, place) => {
+            | Rvalue::RawPtr(RawPtrKind::Mut, place) => {
                 // Inside mutable statics, we allow arbitrary mutable references.
                 // We've allowed `static mut FOO = &mut [elements];` for a long time (the exact
                 // reasons why are lost to history), and there is no reason to restrict that to
@@ -536,7 +536,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             }
 
             Rvalue::Ref(_, BorrowKind::Shared | BorrowKind::Fake(_), place)
-            | Rvalue::RawPtr(Mutability::Not, place) => {
+            | Rvalue::RawPtr(RawPtrKind::Const, place) => {
                 let borrowed_place_has_mut_interior = qualifs::in_place::<HasMutInterior, _>(
                     self.ccx,
                     &mut |local| self.qualifs.has_mut_interior(self.ccx, local, location),
@@ -548,6 +548,12 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 }
             }
 
+            Rvalue::RawPtr(RawPtrKind::FakeForPtrMetadata, place) => {
+                // These are only inserted for slice length, so the place must already be indirect.
+                // This implies we do not have to worry about whether the borrow escapes.
+                assert!(place.is_indirect(), "fake borrows are always indirect");
+            }
+
             Rvalue::Cast(
                 CastKind::PointerCoercion(
                     PointerCoercion::MutToConstPointer
@@ -586,12 +592,23 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             ) => {}
             Rvalue::ShallowInitBox(_, _) => {}
 
-            Rvalue::UnaryOp(_, operand) => {
+            Rvalue::UnaryOp(op, operand) => {
                 let ty = operand.ty(self.body, self.tcx);
-                if is_int_bool_float_or_char(ty) {
-                    // Int, bool, float, and char operations are fine.
-                } else {
-                    span_bug!(self.span, "non-primitive type in `Rvalue::UnaryOp`: {:?}", ty);
+                match op {
+                    UnOp::Not | UnOp::Neg => {
+                        if is_int_bool_float_or_char(ty) {
+                            // Int, bool, float, and char operations are fine.
+                        } else {
+                            span_bug!(
+                                self.span,
+                                "non-primitive type in `Rvalue::UnaryOp{op:?}`: {ty:?}",
+                            );
+                        }
+                    }
+                    UnOp::PtrMetadata => {
+                        // Getting the metadata from a pointer is always const.
+                        // We already validated the type is valid in the validator.
+                    }
                 }
             }
 

compiler/rustc_const_eval/src/interpret/step.rs

@@ -237,7 +237,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 self.write_immediate(*val, &dest)?;
             }
 
-            RawPtr(_, place) => {
+            RawPtr(kind, place) => {
                 // Figure out whether this is an addr_of of an already raw place.
                 let place_base_raw = if place.is_indirect_first_projection() {
                     let ty = self.frame().body.local_decls[place.local].ty;
@@ -250,8 +250,9 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 let src = self.eval_place(place)?;
                 let place = self.force_allocation(&src)?;
                 let mut val = ImmTy::from_immediate(place.to_ref(self), dest.layout);
-                if !place_base_raw {
-                    // If this was not already raw, it needs retagging.
+                if !place_base_raw && !kind.is_fake() {
+                    // If this was not already raw, it needs retagging -- except for "fake"
+                    // raw borrows whose defining property is that they do not get retagged.
                     val = M::retag_ptr_value(self, mir::RetagKind::Raw, &val)?;
                 }
                 self.write_immediate(*val, &dest)?;

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);
                     }