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_mir_build/src/builder/custom/parse/instruction.rs

@@ -253,7 +253,7 @@ impl<'a, 'tcx> ParseCtxt<'a, 'tcx> {
                 Rvalue::Ref(self.tcx.lifetimes.re_erased, *borrow_kind, self.parse_place(*arg)?)
             ),
             ExprKind::RawBorrow { mutability, arg } => Ok(
-                Rvalue::RawPtr(*mutability, self.parse_place(*arg)?)
+                Rvalue::RawPtr((*mutability).into(), self.parse_place(*arg)?)
             ),
             ExprKind::Binary { op, lhs, rhs } =>  Ok(
                 Rvalue::BinaryOp(*op, Box::new((self.parse_operand(*lhs)?, self.parse_operand(*rhs)?)))

compiler/rustc_mir_build/src/builder/expr/as_place.rs

@@ -630,6 +630,69 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         block.and(base_place.index(idx))
     }
 
+    /// Given a place that's either an array or a slice, returns an operand
+    /// with the length of the array/slice.
+    ///
+    /// For arrays it'll be `Operand::Constant` with the actual length;
+    /// For slices it'll be `Operand::Move` of a local using `PtrMetadata`.
+    fn len_of_slice_or_array(
+        &mut self,
+        block: BasicBlock,
+        place: Place<'tcx>,
+        span: Span,
+        source_info: SourceInfo,
+    ) -> Operand<'tcx> {
+        let place_ty = place.ty(&self.local_decls, self.tcx).ty;
+        match place_ty.kind() {
+            ty::Array(_elem_ty, len_const) => {
+                // We know how long an array is, so just use that as a constant
+                // directly -- no locals needed. We do need one statement so
+                // that borrow- and initialization-checking consider it used,
+                // though. FIXME: Do we really *need* to count this as a use?
+                // Could partial array tracking work off something else instead?
+                self.cfg.push_fake_read(block, source_info, FakeReadCause::ForIndex, place);
+                let const_ = Const::Ty(self.tcx.types.usize, *len_const);
+                Operand::Constant(Box::new(ConstOperand { span, user_ty: None, const_ }))
+            }
+            ty::Slice(_elem_ty) => {
+                let ptr_or_ref = if let [PlaceElem::Deref] = place.projection[..]
+                    && let local_ty = self.local_decls[place.local].ty
+                    && local_ty.is_trivially_pure_clone_copy()
+                {
+                    // It's extremely common that we have something that can be
+                    // directly passed to `PtrMetadata`, so avoid an unnecessary
+                    // temporary and statement in those cases. Note that we can
+                    // only do that for `Copy` types -- not `&mut [_]` -- because
+                    // the MIR we're building here needs to pass NLL later.
+                    Operand::Copy(Place::from(place.local))
+                } else {
+                    let ptr_ty = Ty::new_imm_ptr(self.tcx, place_ty);
+                    let slice_ptr = self.temp(ptr_ty, span);
+                    self.cfg.push_assign(
+                        block,
+                        source_info,
+                        slice_ptr,
+                        Rvalue::RawPtr(RawPtrKind::FakeForPtrMetadata, place),
+                    );
+                    Operand::Move(slice_ptr)
+                };
+
+                let len = self.temp(self.tcx.types.usize, span);
+                self.cfg.push_assign(
+                    block,
+                    source_info,
+                    len,
+                    Rvalue::UnaryOp(UnOp::PtrMetadata, ptr_or_ref),
+                );
+
+                Operand::Move(len)
+            }
+            _ => {
+                span_bug!(span, "len called on place of type {place_ty:?}")
+            }
+        }
+    }
+
     fn bounds_check(
         &mut self,
         block: BasicBlock,
@@ -638,25 +701,25 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         expr_span: Span,
         source_info: SourceInfo,
     ) -> BasicBlock {
-        let usize_ty = self.tcx.types.usize;
-        let bool_ty = self.tcx.types.bool;
-        // bounds check:
-        let len = self.temp(usize_ty, expr_span);
-        let lt = self.temp(bool_ty, expr_span);
+        let slice = slice.to_place(self);
 
         // len = len(slice)
-        self.cfg.push_assign(block, source_info, len, Rvalue::Len(slice.to_place(self)));
+        let len = self.len_of_slice_or_array(block, slice, expr_span, source_info);
+
         // lt = idx < len
+        let bool_ty = self.tcx.types.bool;
+        let lt = self.temp(bool_ty, expr_span);
         self.cfg.push_assign(
             block,
             source_info,
             lt,
             Rvalue::BinaryOp(
                 BinOp::Lt,
-                Box::new((Operand::Copy(Place::from(index)), Operand::Copy(len))),
+                Box::new((Operand::Copy(Place::from(index)), len.to_copy())),
             ),
         );
-        let msg = BoundsCheck { len: Operand::Move(len), index: Operand::Copy(Place::from(index)) };
+        let msg = BoundsCheck { len, index: Operand::Copy(Place::from(index)) };
+
         // assert!(lt, "...")
         self.assert(block, Operand::Move(lt), true, msg, expr_span)
     }

compiler/rustc_mir_build/src/builder/expr/into.rs

@@ -303,7 +303,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     hir::Mutability::Not => this.as_read_only_place(block, arg),
                     hir::Mutability::Mut => this.as_place(block, arg),
                 };
-                let address_of = Rvalue::RawPtr(mutability, unpack!(block = place));
+                let address_of = Rvalue::RawPtr(mutability.into(), unpack!(block = place));
                 this.cfg.push_assign(block, source_info, destination, address_of);
                 block.unit()
             }