Auto merge of #135709 - lqd:bring-back-len, r=compiler-errors

Temporarily bring back `Rvalue::Len`

r? `@compiler-errors` as requested in #135671 (comment)

> However, in the mean time, I'd rather we not crunch trying to find and more importantly validate the soundness of a solution 🤔

Agreed. To fix the IMO P-critical #135671 for which we somehow didn't have test coverage, this PR temporarily reverts:
- #133734
- its bugfix #134371
- #134330

cc `@scottmcm`

I added the few samples from that issue as a test, but we can add more in the future, in particular it seems `@steffahn` [will work on that](#135671 (comment)).

Fixes #135671. And if we want to land this, it should also be nominated for beta backport.

compiler/rustc_borrowck/src/lib.rs

@@ -829,6 +829,7 @@ use self::ReadOrWrite::{Activation, Read, Reservation, Write};
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 enum ArtificialField {
+    ArrayLength,
     FakeBorrow,
 }
 
@@ -1338,11 +1339,16 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                 );
             }
 
-            &Rvalue::Discriminant(place) => {
+            &(Rvalue::Len(place) | Rvalue::Discriminant(place)) => {
+                let af = match *rvalue {
+                    Rvalue::Len(..) => Some(ArtificialField::ArrayLength),
+                    Rvalue::Discriminant(..) => None,
+                    _ => unreachable!(),
+                };
                 self.access_place(
                     location,
                     (place, span),
-                    (Shallow(None), Read(ReadKind::Copy)),
+                    (Shallow(af), Read(ReadKind::Copy)),
                     LocalMutationIsAllowed::No,
                     state,
                 );

compiler/rustc_borrowck/src/places_conflict.rs

@@ -203,7 +203,8 @@ fn place_components_conflict<'tcx>(
             let base_ty = base.ty(body, tcx).ty;
 
             match (elem, base_ty.kind(), access) {
-                (_, _, Shallow(Some(ArtificialField::FakeBorrow))) => {
+                (_, _, Shallow(Some(ArtificialField::ArrayLength)))
+                | (_, _, Shallow(Some(ArtificialField::FakeBorrow))) => {
                     // The array length is like additional fields on the
                     // type; it does not overlap any existing data there.
                     // Furthermore, if cannot actually be a prefix of any

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

@@ -300,11 +300,16 @@ impl<'a, 'tcx> LoanInvalidationsGenerator<'a, 'tcx> {
                 self.consume_operand(location, op);
             }
 
-            &Rvalue::Discriminant(place) => {
+            &(Rvalue::Len(place) | Rvalue::Discriminant(place)) => {
+                let af = match rvalue {
+                    Rvalue::Len(..) => Some(ArtificialField::ArrayLength),
+                    Rvalue::Discriminant(..) => None,
+                    _ => unreachable!(),
+                };
                 self.access_place(
                     location,
                     place,
-                    (Shallow(None), Read(ReadKind::Copy)),
+                    (Shallow(af), Read(ReadKind::Copy)),
                     LocalMutationIsAllowed::No,
                 );
             }

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -2235,6 +2235,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
             Rvalue::RawPtr(..)
             | Rvalue::ThreadLocalRef(..)
+            | Rvalue::Len(..)
             | Rvalue::Discriminant(..)
             | Rvalue::NullaryOp(NullOp::OffsetOf(..), _) => {}
         }
@@ -2250,6 +2251,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             | Rvalue::Repeat(..)
             | Rvalue::Ref(..)
             | Rvalue::RawPtr(..)
+            | Rvalue::Len(..)
             | Rvalue::Cast(..)
             | Rvalue::ShallowInitBox(..)
             | Rvalue::BinaryOp(..)

compiler/rustc_codegen_cranelift/src/base.rs

@@ -828,6 +828,12 @@ fn codegen_stmt<'tcx>(
                         fx.bcx.ins().nop();
                     }
                 }
+                Rvalue::Len(place) => {
+                    let place = codegen_place(fx, place);
+                    let usize_layout = fx.layout_of(fx.tcx.types.usize);
+                    let len = codegen_array_len(fx, place);
+                    lval.write_cvalue(fx, CValue::by_val(len, usize_layout));
+                }
                 Rvalue::ShallowInitBox(ref operand, content_ty) => {
                     let content_ty = fx.monomorphize(content_ty);
                     let box_layout = fx.layout_of(Ty::new_box(fx.tcx, content_ty));

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -10,9 +10,9 @@ use rustc_session::config::OptLevel;
 use rustc_span::{DUMMY_SP, Span};
 use tracing::{debug, instrument};
 
-use super::FunctionCx;
 use super::operand::{OperandRef, OperandValue};
 use super::place::PlaceRef;
+use super::{FunctionCx, LocalRef};
 use crate::common::IntPredicate;
 use crate::traits::*;
 use crate::{MemFlags, base};
@@ -607,6 +607,14 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 self.codegen_place_to_pointer(bx, place, mk_ptr)
             }
 
+            mir::Rvalue::Len(place) => {
+                let size = self.evaluate_array_len(bx, place);
+                OperandRef {
+                    val: OperandValue::Immediate(size),
+                    layout: bx.cx().layout_of(bx.tcx().types.usize),
+                }
+            }
+
             mir::Rvalue::BinaryOp(op_with_overflow, box (ref lhs, ref rhs))
                 if let Some(op) = op_with_overflow.overflowing_to_wrapping() =>
             {
@@ -806,6 +814,24 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         }
     }
 
+    fn evaluate_array_len(&mut self, bx: &mut Bx, place: mir::Place<'tcx>) -> Bx::Value {
+        // ZST are passed as operands and require special handling
+        // because codegen_place() panics if Local is operand.
+        if let Some(index) = place.as_local() {
+            if let LocalRef::Operand(op) = self.locals[index] {
+                if let ty::Array(_, n) = op.layout.ty.kind() {
+                    let n = n
+                        .try_to_target_usize(bx.tcx())
+                        .expect("expected monomorphic const in codegen");
+                    return bx.cx().const_usize(n);
+                }
+            }
+        }
+        // use common size calculation for non zero-sized types
+        let cg_value = self.codegen_place(bx, place.as_ref());
+        cg_value.len(bx.cx())
+    }
+
     /// Codegen an `Rvalue::RawPtr` or `Rvalue::Ref`
     fn codegen_place_to_pointer(
         &mut self,
@@ -1077,6 +1103,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             mir::Rvalue::Ref(..) |
             mir::Rvalue::CopyForDeref(..) |
             mir::Rvalue::RawPtr(..) |
+            mir::Rvalue::Len(..) |
             mir::Rvalue::Cast(..) | // (*)
             mir::Rvalue::ShallowInitBox(..) | // (*)
             mir::Rvalue::BinaryOp(..) |

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -495,7 +495,8 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             Rvalue::Use(_)
             | Rvalue::CopyForDeref(..)
             | Rvalue::Repeat(..)
-            | Rvalue::Discriminant(..) => {}
+            | Rvalue::Discriminant(..)
+            | Rvalue::Len(_) => {}
 
             Rvalue::Aggregate(kind, ..) => {
                 if let AggregateKind::Coroutine(def_id, ..) = kind.as_ref()
@@ -579,27 +580,12 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             ) => {}
             Rvalue::ShallowInitBox(_, _) => {}
 
-            Rvalue::UnaryOp(op, operand) => {
+            Rvalue::UnaryOp(_, operand) => {
                 let ty = operand.ty(self.body, self.tcx);
-                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 => {
-                        if !ty.is_ref() && !ty.is_unsafe_ptr() {
-                            span_bug!(
-                                self.span,
-                                "non-pointer type in `Rvalue::UnaryOp({op:?})`: {ty:?}",
-                            );
-                        }
-                    }
+                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);
                 }
             }
 

compiler/rustc_const_eval/src/check_consts/qualifs.rs

@@ -230,7 +230,9 @@ where
             Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx))
         }
 
-        Rvalue::Discriminant(place) => in_place::<Q, _>(cx, in_local, place.as_ref()),
+        Rvalue::Discriminant(place) | Rvalue::Len(place) => {
+            in_place::<Q, _>(cx, in_local, place.as_ref())
+        }
 
         Rvalue::CopyForDeref(place) => in_place::<Q, _>(cx, in_local, place.as_ref()),
 

compiler/rustc_const_eval/src/check_consts/resolver.rs

@@ -197,6 +197,7 @@ where
             | mir::Rvalue::CopyForDeref(..)
             | mir::Rvalue::ThreadLocalRef(..)
             | mir::Rvalue::Repeat(..)
+            | mir::Rvalue::Len(..)
             | mir::Rvalue::BinaryOp(..)
             | mir::Rvalue::NullaryOp(..)
             | mir::Rvalue::UnaryOp(..)

compiler/rustc_const_eval/src/interpret/step.rs

@@ -9,13 +9,12 @@ use rustc_middle::ty::layout::FnAbiOf;
 use rustc_middle::ty::{self, Instance, Ty};
 use rustc_middle::{bug, mir, span_bug};
 use rustc_span::source_map::Spanned;
-use rustc_span::{DesugaringKind, Span};
 use rustc_target::callconv::FnAbi;
 use tracing::{info, instrument, trace};
 
 use super::{
     FnArg, FnVal, ImmTy, Immediate, InterpCx, InterpResult, Machine, MemPlaceMeta, PlaceTy,
-    Projectable, interp_ok, throw_ub,
+    Projectable, Scalar, interp_ok, throw_ub,
 };
 use crate::util;
 
@@ -81,9 +80,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         use rustc_middle::mir::StatementKind::*;
 
         match &stmt.kind {
-            Assign(box (place, rvalue)) => {
-                self.eval_rvalue_into_place(rvalue, *place, stmt.source_info.span)?
-            }
+            Assign(box (place, rvalue)) => self.eval_rvalue_into_place(rvalue, *place)?,
 
             SetDiscriminant { place, variant_index } => {
                 let dest = self.eval_place(**place)?;
@@ -162,7 +159,6 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         &mut self,
         rvalue: &mir::Rvalue<'tcx>,
         place: mir::Place<'tcx>,
-        span: Span,
     ) -> InterpResult<'tcx> {
         let dest = self.eval_place(place)?;
         // FIXME: ensure some kind of non-aliasing between LHS and RHS?
@@ -218,6 +214,12 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 self.write_repeat(operand, &dest)?;
             }
 
+            Len(place) => {
+                let src = self.eval_place(place)?;
+                let len = src.len(self)?;
+                self.write_scalar(Scalar::from_target_usize(len, self), &dest)?;
+            }
+
             Ref(_, borrow_kind, place) => {
                 let src = self.eval_place(place)?;
                 let place = self.force_allocation(&src)?;
@@ -248,13 +250,8 @@ 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
-                    && span.desugaring_kind() != Some(DesugaringKind::IndexBoundsCheckReborrow)
-                {
+                if !place_base_raw {
                     // If this was not already raw, it needs retagging.
-                    // As a special hack, we exclude the desugared `PtrMetadata(&raw const *_n)`
-                    // from indexing. (Really we should not do any retag on `&raw` but that does not
-                    // currently work with Stacked Borrows.)
                     val = M::retag_ptr_value(self, mir::RetagKind::Raw, &val)?;
                 }
                 self.write_immediate(*val, &dest)?;

compiler/rustc_middle/src/mir/consts.rs

@@ -467,9 +467,6 @@ impl<'tcx> Const<'tcx> {
                 let const_val = tcx.valtree_to_const_val((ty, valtree));
                 Self::Val(const_val, ty)
             }
-            ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, args }) => {
-                Self::Unevaluated(UnevaluatedConst { def, args, promoted: None }, ty)
-            }
             _ => Self::Ty(ty, c),
         }
     }

compiler/rustc_middle/src/mir/pretty.rs

@@ -1068,6 +1068,7 @@ impl<'tcx> Debug for Rvalue<'tcx> {
                 pretty_print_const(b, fmt, false)?;
                 write!(fmt, "]")
             }
+            Len(ref a) => write!(fmt, "Len({a:?})"),
             Cast(ref kind, ref place, ref ty) => {
                 with_no_trimmed_paths!(write!(fmt, "{place:?} as {ty} ({kind:?})"))
             }

compiler/rustc_middle/src/mir/statement.rs

@@ -424,6 +424,7 @@ impl<'tcx> Rvalue<'tcx> {
             | Rvalue::Ref(_, _, _)
             | Rvalue::ThreadLocalRef(_)
             | Rvalue::RawPtr(_, _)
+            | Rvalue::Len(_)
             | Rvalue::Cast(
                 CastKind::IntToInt
                 | CastKind::FloatToInt

compiler/rustc_middle/src/mir/syntax.rs

@@ -1351,6 +1351,16 @@ pub enum Rvalue<'tcx> {
     /// model.
     RawPtr(Mutability, Place<'tcx>),
 
+    /// Yields the length of the place, as a `usize`.
+    ///
+    /// If the type of the place is an array, this is the array length. For slices (`[T]`, not
+    /// `&[T]`) this accesses the place's metadata to determine the length. This rvalue is
+    /// ill-formed for places of other types.
+    ///
+    /// This cannot be a `UnOp(PtrMetadata, _)` because that expects a value, and we only
+    /// have a place, and `UnOp(PtrMetadata, RawPtr(place))` is not a thing.
+    Len(Place<'tcx>),
+
     /// Performs essentially all of the casts that can be performed via `as`.
     ///
     /// This allows for casts from/to a variety of types.

compiler/rustc_middle/src/mir/tcx.rs

@@ -210,6 +210,7 @@ impl<'tcx> Rvalue<'tcx> {
                 let place_ty = place.ty(local_decls, tcx).ty;
                 Ty::new_ptr(tcx, place_ty, mutability)
             }
+            Rvalue::Len(..) => tcx.types.usize,
             Rvalue::Cast(.., ty) => ty,
             Rvalue::BinaryOp(op, box (ref lhs, ref rhs)) => {
                 let lhs_ty = lhs.ty(local_decls, tcx);

compiler/rustc_middle/src/mir/visit.rs

@@ -695,6 +695,14 @@ macro_rules! make_mir_visitor {
                         self.visit_place(path, ctx, location);
                     }
 
+                    Rvalue::Len(path) => {
+                        self.visit_place(
+                            path,
+                            PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect),
+                            location
+                        );
+                    }
+
                     Rvalue::Cast(_cast_kind, operand, ty) => {
                         self.visit_operand(operand, location);
                         self.visit_ty($(& $mutability)? *ty, TyContext::Location(location));

compiler/rustc_mir_build/src/builder/custom/parse/instruction.rs

@@ -246,6 +246,7 @@ impl<'a, 'tcx> ParseCtxt<'a, 'tcx> {
                 let offset = self.parse_operand(args[1])?;
                 Ok(Rvalue::BinaryOp(BinOp::Offset, Box::new((ptr, offset))))
             },
+            @call(mir_len, args) => Ok(Rvalue::Len(self.parse_place(args[0])?)),
             @call(mir_ptr_metadata, args) => Ok(Rvalue::UnaryOp(UnOp::PtrMetadata, self.parse_operand(args[0])?)),
             @call(mir_copy_for_deref, args) => Ok(Rvalue::CopyForDeref(self.parse_place(args[0])?)),
             ExprKind::Borrow { borrow_kind, arg } => Ok(