Revert "Auto merge of #134330 - scottmcm:no-more-rvalue-len, r=matthewjasper"

This reverts commit e108481f74ff123ad98a63bd107a18d13035b275, reversing
changes made to 303e8bd768526a5812bb1776e798e829ddb7d3ca.

(cherry picked from commit ca1c17c88d1f625763859396ba7a50f36ac45cc0)

37 files changed, 272 insertions, 51 deletions

diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index e7aa827e680..38b1d391be7 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -828,6 +828,7 @@ use self::ReadOrWrite::{Activation, Read, Reservation, Write};
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 enum ArtificialField {
+    ArrayLength,
     FakeBorrow,
 }
 
@@ -1345,11 +1346,16 @@ 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,
                 );
diff --git a/compiler/rustc_borrowck/src/places_conflict.rs b/compiler/rustc_borrowck/src/places_conflict.rs
index 560b8c0349a..679e111caa9 100644
--- a/compiler/rustc_borrowck/src/places_conflict.rs
+++ b/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
diff --git a/compiler/rustc_borrowck/src/polonius/legacy/loan_invalidations.rs b/compiler/rustc_borrowck/src/polonius/legacy/loan_invalidations.rs
index bb6d593d0d8..bb73bc9fcbd 100644
--- a/compiler/rustc_borrowck/src/polonius/legacy/loan_invalidations.rs
+++ b/compiler/rustc_borrowck/src/polonius/legacy/loan_invalidations.rs
@@ -298,11 +298,16 @@ 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,
                 );
             }
diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 10fb8a399a2..084f122a210 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -2221,6 +2221,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
             Rvalue::RawPtr(..)
             | Rvalue::ThreadLocalRef(..)
+            | Rvalue::Len(..)
             | Rvalue::Discriminant(..)
             | Rvalue::NullaryOp(NullOp::OffsetOf(..), _) => {}
         }
@@ -2236,6 +2237,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             | Rvalue::Repeat(..)
             | Rvalue::Ref(..)
             | Rvalue::RawPtr(..)
+            | Rvalue::Len(..)
             | Rvalue::Cast(..)
             | Rvalue::ShallowInitBox(..)
             | Rvalue::BinaryOp(..)
diff --git a/compiler/rustc_codegen_cranelift/src/base.rs b/compiler/rustc_codegen_cranelift/src/base.rs
index 956a024fa4d..34066eb83fc 100644
--- a/compiler/rustc_codegen_cranelift/src/base.rs
+++ b/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));
diff --git a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
index 3b62148abb7..cf537392234 100644
--- a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
+++ b/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};
@@ -593,6 +593,14 @@ 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() =>
             {
@@ -792,6 +800,24 @@ 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,
@@ -1063,6 +1089,7 @@ 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(..) |
diff --git a/compiler/rustc_const_eval/src/check_consts/check.rs b/compiler/rustc_const_eval/src/check_consts/check.rs
index e895c44199b..f4257ad9671 100644
--- a/compiler/rustc_const_eval/src/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/check_consts/check.rs
@@ -488,7 +488,8 @@ 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()
diff --git a/compiler/rustc_const_eval/src/check_consts/qualifs.rs b/compiler/rustc_const_eval/src/check_consts/qualifs.rs
index b1b7fb406b1..e244b50a4b5 100644
--- a/compiler/rustc_const_eval/src/check_consts/qualifs.rs
+++ b/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()),
 
diff --git a/compiler/rustc_const_eval/src/check_consts/resolver.rs b/compiler/rustc_const_eval/src/check_consts/resolver.rs
index 5a6e7ab2bee..79df63a9e84 100644
--- a/compiler/rustc_const_eval/src/check_consts/resolver.rs
+++ b/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(..)
diff --git a/compiler/rustc_const_eval/src/interpret/step.rs b/compiler/rustc_const_eval/src/interpret/step.rs
index 32e77fe1024..a26c2eca107 100644
--- a/compiler/rustc_const_eval/src/interpret/step.rs
+++ b/compiler/rustc_const_eval/src/interpret/step.rs
@@ -15,7 +15,7 @@ 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;
 
@@ -218,6 +218,12 @@ 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)?;
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index 47522f00bb1..2d1a0e36f90 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/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:?})"))
             }
diff --git a/compiler/rustc_middle/src/mir/statement.rs b/compiler/rustc_middle/src/mir/statement.rs
index da3fa9e324a..1ce735cec63 100644
--- a/compiler/rustc_middle/src/mir/statement.rs
+++ b/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
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index bbbaffc5a35..0c17a2e0fe5 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/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.
diff --git a/compiler/rustc_middle/src/mir/tcx.rs b/compiler/rustc_middle/src/mir/tcx.rs
index cbb26b83c79..db77017310a 100644
--- a/compiler/rustc_middle/src/mir/tcx.rs
+++ b/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);
diff --git a/compiler/rustc_middle/src/mir/visit.rs b/compiler/rustc_middle/src/mir/visit.rs
index 12a024a219e..058acbd4024 100644
--- a/compiler/rustc_middle/src/mir/visit.rs
+++ b/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));
diff --git a/compiler/rustc_mir_build/src/builder/custom/parse/instruction.rs b/compiler/rustc_mir_build/src/builder/custom/parse/instruction.rs
index 3dd5de02230..59f440432eb 100644
--- a/compiler/rustc_mir_build/src/builder/custom/parse/instruction.rs
+++ b/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(
diff --git a/compiler/rustc_mir_build/src/builder/expr/as_place.rs b/compiler/rustc_mir_build/src/builder/expr/as_place.rs
index 89c7bb357ef..01aec70f437 100644
--- a/compiler/rustc_mir_build/src/builder/expr/as_place.rs
+++ b/compiler/rustc_mir_build/src/builder/expr/as_place.rs
@@ -635,7 +635,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     ///
     /// For arrays it'll be `Operand::Constant` with the actual length;
     /// For slices it'll be `Operand::Move` of a local using `PtrMetadata`.
-    pub(in crate::builder) fn len_of_slice_or_array(
+    fn len_of_slice_or_array(
         &mut self,
         block: BasicBlock,
         place: Place<'tcx>,
diff --git a/compiler/rustc_mir_build/src/builder/matches/test.rs b/compiler/rustc_mir_build/src/builder/matches/test.rs
index 0d36b7bb3ee..8cca84d7fcc 100644
--- a/compiler/rustc_mir_build/src/builder/matches/test.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/test.rs
@@ -243,8 +243,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
 
             TestKind::Len { len, op } => {
+                let usize_ty = self.tcx.types.usize;
+                let actual = self.temp(usize_ty, test.span);
+
                 // actual = len(place)
-                let actual = self.len_of_slice_or_array(block, place, test.span, source_info);
+                self.cfg.push_assign(block, source_info, actual, Rvalue::Len(place));
 
                 // expected = <N>
                 let expected = self.push_usize(block, source_info, len);
@@ -259,7 +262,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     fail_block,
                     source_info,
                     op,
-                    actual,
+                    Operand::Move(actual),
                     Operand::Move(expected),
                 );
             }
diff --git a/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs b/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
index 217594b3238..568d8a5acaf 100644
--- a/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
@@ -91,6 +91,7 @@ where
             | Rvalue::Use(..)
             | Rvalue::ThreadLocalRef(..)
             | Rvalue::Repeat(..)
+            | Rvalue::Len(..)
             | Rvalue::BinaryOp(..)
             | Rvalue::NullaryOp(..)
             | Rvalue::UnaryOp(..)
diff --git a/compiler/rustc_mir_dataflow/src/move_paths/builder.rs b/compiler/rustc_mir_dataflow/src/move_paths/builder.rs
index 80875f32e4f..d1b3a389e9e 100644
--- a/compiler/rustc_mir_dataflow/src/move_paths/builder.rs
+++ b/compiler/rustc_mir_dataflow/src/move_paths/builder.rs
@@ -413,6 +413,7 @@ impl<'a, 'tcx, F: Fn(Ty<'tcx>) -> bool> MoveDataBuilder<'a, 'tcx, F> {
             Rvalue::Ref(..)
             | Rvalue::RawPtr(..)
             | Rvalue::Discriminant(..)
+            | Rvalue::Len(..)
             | Rvalue::NullaryOp(
                 NullOp::SizeOf | NullOp::AlignOf | NullOp::OffsetOf(..) | NullOp::UbChecks,
                 _,
diff --git a/compiler/rustc_mir_transform/src/dataflow_const_prop.rs b/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
index cc44114782c..51af77778af 100644
--- a/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
+++ b/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
@@ -408,6 +408,18 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
         state: &mut State<FlatSet<Scalar>>,
     ) -> ValueOrPlace<FlatSet<Scalar>> {
         let val = match rvalue {
+            Rvalue::Len(place) => {
+                let place_ty = place.ty(self.local_decls, self.tcx);
+                if let ty::Array(_, len) = place_ty.ty.kind() {
+                    Const::Ty(self.tcx.types.usize, *len)
+                        .try_eval_scalar(self.tcx, self.typing_env)
+                        .map_or(FlatSet::Top, FlatSet::Elem)
+                } else if let [ProjectionElem::Deref] = place.projection[..] {
+                    state.get_len(place.local.into(), &self.map)
+                } else {
+                    FlatSet::Top
+                }
+            }
             Rvalue::Cast(CastKind::IntToInt | CastKind::IntToFloat, operand, ty) => {
                 let Ok(layout) = self.tcx.layout_of(self.typing_env.as_query_input(*ty)) else {
                     return ValueOrPlace::Value(FlatSet::Top);
diff --git a/compiler/rustc_mir_transform/src/dest_prop.rs b/compiler/rustc_mir_transform/src/dest_prop.rs
index e99bee6a01f..8f977d2979e 100644
--- a/compiler/rustc_mir_transform/src/dest_prop.rs
+++ b/compiler/rustc_mir_transform/src/dest_prop.rs
@@ -574,6 +574,7 @@ impl WriteInfo {
                     | Rvalue::NullaryOp(_, _)
                     | Rvalue::Ref(_, _, _)
                     | Rvalue::RawPtr(_, _)
+                    | Rvalue::Len(_)
                     | Rvalue::Discriminant(_)
                     | Rvalue::CopyForDeref(_) => {}
                 }
diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
index 283ed94b615..d5a813ec8ec 100644
--- a/compiler/rustc_mir_transform/src/gvn.rs
+++ b/compiler/rustc_mir_transform/src/gvn.rs
@@ -223,6 +223,8 @@ enum Value<'tcx> {
     Projection(VnIndex, ProjectionElem<VnIndex, Ty<'tcx>>),
     /// Discriminant of the given value.
     Discriminant(VnIndex),
+    /// Length of an array or slice.
+    Len(VnIndex),
 
     // Operations.
     NullaryOp(NullOp<'tcx>, Ty<'tcx>),
@@ -511,6 +513,13 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     self.ecx.discriminant_for_variant(base.layout.ty, variant).discard_err()?;
                 discr_value.into()
             }
+            Len(slice) => {
+                let slice = self.evaluated[slice].as_ref()?;
+                let usize_layout = self.ecx.layout_of(self.tcx.types.usize).unwrap();
+                let len = slice.len(&self.ecx).discard_err()?;
+                let imm = ImmTy::from_uint(len, usize_layout);
+                imm.into()
+            }
             NullaryOp(null_op, ty) => {
                 let layout = self.ecx.layout_of(ty).ok()?;
                 if let NullOp::SizeOf | NullOp::AlignOf = null_op
@@ -854,6 +863,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             }
 
             // Operations.
+            Rvalue::Len(ref mut place) => return self.simplify_len(place, location),
             Rvalue::Cast(ref mut kind, ref mut value, to) => {
                 return self.simplify_cast(kind, value, to, location);
             }
@@ -1423,6 +1433,47 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
         Some(self.insert(Value::Cast { kind: *kind, value, from, to }))
     }
 
+    fn simplify_len(&mut self, place: &mut Place<'tcx>, location: Location) -> Option<VnIndex> {
+        // Trivial case: we are fetching a statically known length.
+        let place_ty = place.ty(self.local_decls, self.tcx).ty;
+        if let ty::Array(_, len) = place_ty.kind() {
+            return self.insert_constant(Const::from_ty_const(
+                *len,
+                self.tcx.types.usize,
+                self.tcx,
+            ));
+        }
+
+        let mut inner = self.simplify_place_value(place, location)?;
+
+        // The length information is stored in the wide pointer.
+        // Reborrowing copies length information from one pointer to the other.
+        while let Value::Address { place: borrowed, .. } = self.get(inner)
+            && let [PlaceElem::Deref] = borrowed.projection[..]
+            && let Some(borrowed) = self.locals[borrowed.local]
+        {
+            inner = borrowed;
+        }
+
+        // We have an unsizing cast, which assigns the length to wide pointer metadata.
+        if let Value::Cast { kind, from, to, .. } = self.get(inner)
+            && let CastKind::PointerCoercion(ty::adjustment::PointerCoercion::Unsize, _) = kind
+            && let Some(from) = from.builtin_deref(true)
+            && let ty::Array(_, len) = from.kind()
+            && let Some(to) = to.builtin_deref(true)
+            && let ty::Slice(..) = to.kind()
+        {
+            return self.insert_constant(Const::from_ty_const(
+                *len,
+                self.tcx.types.usize,
+                self.tcx,
+            ));
+        }
+
+        // Fallback: a symbolic `Len`.
+        Some(self.insert(Value::Len(inner)))
+    }
+
     fn pointers_have_same_metadata(&self, left_ptr_ty: Ty<'tcx>, right_ptr_ty: Ty<'tcx>) -> bool {
         let left_meta_ty = left_ptr_ty.pointee_metadata_ty_or_projection(self.tcx);
         let right_meta_ty = right_ptr_ty.pointee_metadata_ty_or_projection(self.tcx);
diff --git a/compiler/rustc_mir_transform/src/known_panics_lint.rs b/compiler/rustc_mir_transform/src/known_panics_lint.rs
index f1705d0c831..acf3eb2b62c 100644
--- a/compiler/rustc_mir_transform/src/known_panics_lint.rs
+++ b/compiler/rustc_mir_transform/src/known_panics_lint.rs
@@ -440,6 +440,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             | Rvalue::Use(..)
             | Rvalue::CopyForDeref(..)
             | Rvalue::Repeat(..)
+            | Rvalue::Len(..)
             | Rvalue::Cast(..)
             | Rvalue::ShallowInitBox(..)
             | Rvalue::Discriminant(..)
@@ -599,6 +600,20 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                 return None;
             }
 
+            Len(place) => {
+                let len = if let ty::Array(_, n) = place.ty(self.local_decls(), self.tcx).ty.kind()
+                {
+                    n.try_to_target_usize(self.tcx)?
+                } else {
+                    match self.get_const(place)? {
+                        Value::Immediate(src) => src.len(&self.ecx).discard_err()?,
+                        Value::Aggregate { fields, .. } => fields.len() as u64,
+                        Value::Uninit => return None,
+                    }
+                };
+                ImmTy::from_scalar(Scalar::from_target_usize(len, self), layout).into()
+            }
+
             Ref(..) | RawPtr(..) => return None,
 
             NullaryOp(ref null_op, ty) => {
diff --git a/compiler/rustc_mir_transform/src/promote_consts.rs b/compiler/rustc_mir_transform/src/promote_consts.rs
index 7451f419304..6be95b1f0f1 100644
--- a/compiler/rustc_mir_transform/src/promote_consts.rs
+++ b/compiler/rustc_mir_transform/src/promote_consts.rs
@@ -430,7 +430,9 @@ impl<'tcx> Validator<'_, 'tcx> {
                 self.validate_operand(op)?
             }
 
-            Rvalue::Discriminant(place) => self.validate_place(place.as_ref())?,
+            Rvalue::Discriminant(place) | Rvalue::Len(place) => {
+                self.validate_place(place.as_ref())?
+            }
 
             Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),
 
diff --git a/compiler/rustc_mir_transform/src/validate.rs b/compiler/rustc_mir_transform/src/validate.rs
index a670da94fcc..bce015046e1 100644
--- a/compiler/rustc_mir_transform/src/validate.rs
+++ b/compiler/rustc_mir_transform/src/validate.rs
@@ -1009,6 +1009,14 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
             }
             Rvalue::Ref(..) => {}
+            Rvalue::Len(p) => {
+                let pty = p.ty(&self.body.local_decls, self.tcx).ty;
+                check_kinds!(
+                    pty,
+                    "Cannot compute length of non-array type {:?}",
+                    ty::Array(..) | ty::Slice(..)
+                );
+            }
             Rvalue::BinaryOp(op, vals) => {
                 use BinOp::*;
                 let a = vals.0.ty(&self.body.local_decls, self.tcx);
diff --git a/compiler/rustc_smir/src/rustc_smir/convert/mir.rs b/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
index de933952c6a..a5a17b4b573 100644
--- a/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
+++ b/compiler/rustc_smir/src/rustc_smir/convert/mir.rs
@@ -181,6 +181,7 @@ impl<'tcx> Stable<'tcx> for mir::Rvalue<'tcx> {
             RawPtr(mutability, place) => {
                 stable_mir::mir::Rvalue::AddressOf(mutability.stable(tables), place.stable(tables))
             }
+            Len(place) => stable_mir::mir::Rvalue::Len(place.stable(tables)),
             Cast(cast_kind, op, ty) => stable_mir::mir::Rvalue::Cast(
                 cast_kind.stable(tables),
                 op.stable(tables),
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 4ecc4201f89..e3f55221d70 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -1283,6 +1283,7 @@ symbols! {
         mir_drop,
         mir_field,
         mir_goto,
+        mir_len,
         mir_make_place,
         mir_move,
         mir_offset,
diff --git a/library/core/src/intrinsics/mir.rs b/library/core/src/intrinsics/mir.rs
index 834f44c7790..55dcf7cd47e 100644
--- a/library/core/src/intrinsics/mir.rs
+++ b/library/core/src/intrinsics/mir.rs
@@ -233,7 +233,7 @@
 //!
 //!  - Operands implicitly convert to `Use` rvalues.
 //!  - `&`, `&mut`, `addr_of!`, and `addr_of_mut!` all work to create their associated rvalue.
-//!  - [`Discriminant`] and [`CopyForDeref`] have associated functions.
+//!  - [`Discriminant`], [`Len`], and [`CopyForDeref`] have associated functions.
 //!  - Unary and binary operations use their normal Rust syntax - `a * b`, `!c`, etc.
 //!  - The binary operation `Offset` can be created via [`Offset`].
 //!  - Checked binary operations are represented by wrapping the associated binop in [`Checked`].
@@ -401,6 +401,7 @@ define!("mir_storage_dead", fn StorageDead<T>(local: T));
 define!("mir_assume", fn Assume(operand: bool));
 define!("mir_deinit", fn Deinit<T>(place: T));
 define!("mir_checked", fn Checked<T>(binop: T) -> (T, bool));
+define!("mir_len", fn Len<T>(place: T) -> usize);
 define!(
     "mir_ptr_metadata",
     fn PtrMetadata<P: ?Sized>(place: *const P) -> <P as ::core::ptr::Pointee>::Metadata
diff --git a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
index 428b40c5771..104ae154e36 100644
--- a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
+++ b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
@@ -109,7 +109,7 @@ fn check_rvalue<'tcx>(
 ) -> McfResult {
     match rvalue {
         Rvalue::ThreadLocalRef(_) => Err((span, "cannot access thread local storage in const fn".into())),
-        Rvalue::Discriminant(place) | Rvalue::Ref(_, _, place) | Rvalue::RawPtr(_, place) => {
+        Rvalue::Len(place) | Rvalue::Discriminant(place) | Rvalue::Ref(_, _, place) | Rvalue::RawPtr(_, place) => {
             check_place(tcx, *place, span, body, msrv)
         },
         Rvalue::CopyForDeref(place) => check_place(tcx, *place, span, body, msrv),
diff --git a/tests/mir-opt/building/custom/arrays.arrays.built.after.mir b/tests/mir-opt/building/custom/arrays.arrays.built.after.mir
new file mode 100644
index 00000000000..30d11e31e4d
--- /dev/null
+++ b/tests/mir-opt/building/custom/arrays.arrays.built.after.mir
@@ -0,0 +1,14 @@
+// MIR for `arrays` after built
+
+fn arrays() -> usize {
+    let mut _0: usize;
+    let mut _1: [i32; C];
+    let mut _2: usize;
+
+    bb0: {
+        _1 = [const 5_i32; C];
+        _2 = Len(_1);
+        _0 = copy _2;
+        return;
+    }
+}
diff --git a/tests/mir-opt/building/custom/arrays.rs b/tests/mir-opt/building/custom/arrays.rs
new file mode 100644
index 00000000000..4bd6f93e113
--- /dev/null
+++ b/tests/mir-opt/building/custom/arrays.rs
@@ -0,0 +1,22 @@
+// skip-filecheck
+#![feature(custom_mir, core_intrinsics)]
+
+extern crate core;
+use core::intrinsics::mir::*;
+
+// EMIT_MIR arrays.arrays.built.after.mir
+#[custom_mir(dialect = "built")]
+fn arrays<const C: usize>() -> usize {
+    mir! {
+        {
+            let x = [5_i32; C];
+            let c = Len(x);
+            RET = c;
+            Return()
+        }
+    }
+}
+
+fn main() {
+    assert_eq!(arrays::<20>(), 20);
+}
diff --git a/tests/mir-opt/deduplicate_blocks.is_line_doc_comment_2.DeduplicateBlocks.panic-abort.diff b/tests/mir-opt/deduplicate_blocks.is_line_doc_comment_2.DeduplicateBlocks.panic-abort.diff
index b4197c09ac9..60742ef0e9a 100644
--- a/tests/mir-opt/deduplicate_blocks.is_line_doc_comment_2.DeduplicateBlocks.panic-abort.diff
+++ b/tests/mir-opt/deduplicate_blocks.is_line_doc_comment_2.DeduplicateBlocks.panic-abort.diff
@@ -22,14 +22,14 @@
   
       bb1: {
           StorageDead(_3);
-          _4 = PtrMetadata(copy _2);
+          _4 = Len((*_2));
           _5 = const 4_usize;
           _6 = Ge(move _4, move _5);
           switchInt(move _6) -> [0: bb2, otherwise: bb3];
       }
   
       bb2: {
-          _7 = PtrMetadata(copy _2);
+          _7 = Len((*_2));
           _8 = const 3_usize;
           _9 = Ge(move _7, move _8);
 -         switchInt(move _9) -> [0: bb7, otherwise: bb8];
diff --git a/tests/mir-opt/deduplicate_blocks.is_line_doc_comment_2.DeduplicateBlocks.panic-unwind.diff b/tests/mir-opt/deduplicate_blocks.is_line_doc_comment_2.DeduplicateBlocks.panic-unwind.diff
index 4bcb13ca49c..7337a32f525 100644
--- a/tests/mir-opt/deduplicate_blocks.is_line_doc_comment_2.DeduplicateBlocks.panic-unwind.diff
+++ b/tests/mir-opt/deduplicate_blocks.is_line_doc_comment_2.DeduplicateBlocks.panic-unwind.diff
@@ -22,14 +22,14 @@
   
       bb1: {
           StorageDead(_3);
-          _4 = PtrMetadata(copy _2);
+          _4 = Len((*_2));
           _5 = const 4_usize;
           _6 = Ge(move _4, move _5);
           switchInt(move _6) -> [0: bb2, otherwise: bb3];
       }
   
       bb2: {
-          _7 = PtrMetadata(copy _2);
+          _7 = Len((*_2));
           _8 = const 3_usize;
           _9 = Ge(move _7, move _8);
 -         switchInt(move _9) -> [0: bb7, otherwise: bb8];
diff --git a/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.panic-abort.diff b/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.panic-abort.diff
index 94ba7082c66..c02bab3524b 100644
--- a/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.panic-abort.diff
+++ b/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.panic-abort.diff
@@ -7,16 +7,18 @@
       let _2: &[T];
       let _3: &[T; 3];
       let _4: [T; 3];
-      let mut _8: !;
+      let mut _5: usize;
+      let mut _6: bool;
+      let mut _10: !;
       scope 1 {
           debug v => _2;
-          let _5: &T;
-          let _6: &T;
           let _7: &T;
+          let _8: &T;
+          let _9: &T;
           scope 2 {
-              debug v1 => _5;
-              debug v2 => _6;
-              debug v3 => _7;
+              debug v1 => _7;
+              debug v2 => _8;
+              debug v3 => _9;
           }
       }
   
@@ -25,23 +27,25 @@
           _4 = [copy _1, copy _1, copy _1];
           _3 = &_4;
           _2 = copy _3 as &[T] (PointerCoercion(Unsize, Implicit));
+          nop;
+          nop;
           goto -> bb2;
       }
   
       bb1: {
-          _8 = core::panicking::panic(const "internal error: entered unreachable code") -> unwind unreachable;
+          _10 = core::panicking::panic(const "internal error: entered unreachable code") -> unwind unreachable;
       }
   
       bb2: {
-          StorageLive(_5);
-          _5 = &(*_2)[0 of 3];
-          StorageLive(_6);
-          _6 = &(*_2)[1 of 3];
           StorageLive(_7);
-          _7 = &(*_2)[2 of 3];
+          _7 = &(*_2)[0 of 3];
+          StorageLive(_8);
+          _8 = &(*_2)[1 of 3];
+          StorageLive(_9);
+          _9 = &(*_2)[2 of 3];
+          StorageDead(_9);
+          StorageDead(_8);
           StorageDead(_7);
-          StorageDead(_6);
-          StorageDead(_5);
           StorageDead(_4);
           return;
       }
diff --git a/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.panic-unwind.diff b/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.panic-unwind.diff
index 0455b2c326e..49be042588c 100644
--- a/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.panic-unwind.diff
+++ b/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.panic-unwind.diff
@@ -7,16 +7,18 @@
       let _2: &[T];
       let _3: &[T; 3];
       let _4: [T; 3];
-      let mut _8: !;
+      let mut _5: usize;
+      let mut _6: bool;
+      let mut _10: !;
       scope 1 {
           debug v => _2;
-          let _5: &T;
-          let _6: &T;
           let _7: &T;
+          let _8: &T;
+          let _9: &T;
           scope 2 {
-              debug v1 => _5;
-              debug v2 => _6;
-              debug v3 => _7;
+              debug v1 => _7;
+              debug v2 => _8;
+              debug v3 => _9;
           }
       }
   
@@ -25,23 +27,25 @@
           _4 = [copy _1, copy _1, copy _1];
           _3 = &_4;
           _2 = copy _3 as &[T] (PointerCoercion(Unsize, Implicit));
+          nop;
+          nop;
           goto -> bb2;
       }
   
       bb1: {
-          _8 = core::panicking::panic(const "internal error: entered unreachable code") -> unwind continue;
+          _10 = core::panicking::panic(const "internal error: entered unreachable code") -> unwind continue;
       }
   
       bb2: {
-          StorageLive(_5);
-          _5 = &(*_2)[0 of 3];
-          StorageLive(_6);
-          _6 = &(*_2)[1 of 3];
           StorageLive(_7);
-          _7 = &(*_2)[2 of 3];
+          _7 = &(*_2)[0 of 3];
+          StorageLive(_8);
+          _8 = &(*_2)[1 of 3];
+          StorageLive(_9);
+          _9 = &(*_2)[2 of 3];
+          StorageDead(_9);
+          StorageDead(_8);
           StorageDead(_7);
-          StorageDead(_6);
-          StorageDead(_5);
           StorageDead(_4);
           return;
       }
diff --git a/tests/mir-opt/reference_prop.debuginfo.ReferencePropagation.diff b/tests/mir-opt/reference_prop.debuginfo.ReferencePropagation.diff
index e9eea69377f..05ad9dbf3cc 100644
--- a/tests/mir-opt/reference_prop.debuginfo.ReferencePropagation.diff
+++ b/tests/mir-opt/reference_prop.debuginfo.ReferencePropagation.diff
@@ -92,7 +92,7 @@
           StorageDead(_7);
 -         StorageDead(_6);
 -         StorageLive(_10);
-          StorageLive(_11);
+-         StorageLive(_11);
 -         StorageLive(_12);
           StorageLive(_13);
           _26 = const debuginfo::promoted[0];
@@ -105,8 +105,9 @@
       bb5: {
           StorageDead(_15);
           StorageDead(_13);
-          _11 = &(*_12);
-          _16 = PtrMetadata(copy _11);
+-         _11 = &(*_12);
+-         _16 = Len((*_11));
++         _16 = Len((*_12));
           _17 = const 3_usize;
           _18 = Ge(move _16, move _17);
           switchInt(move _18) -> [0: bb7, otherwise: bb6];
@@ -136,7 +137,7 @@
   
       bb8: {
 -         StorageDead(_12);
-          StorageDead(_11);
+-         StorageDead(_11);
 -         StorageDead(_10);
           StorageLive(_22);
           StorageLive(_23);