Auto merge of #86525 - shamatar:array_len_opt, r=oli-obk

Array `.len()` MIR optimization pass

This pass kind-of works back the `[T; N].len()` call that at the moment is first coerced as `&[T; N]` -> `&[T]` and then uses `&[T].len()`. Depends on #86383

18 files changed, 979 insertions, 9 deletions

diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 0ca640cd7b1..9b11c8f0b24 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -58,6 +58,7 @@ mod lower_intrinsics;
 mod lower_slice_len;
 mod match_branches;
 mod multiple_return_terminators;
+mod normalize_array_len;
 mod nrvo;
 mod remove_noop_landing_pads;
 mod remove_storage_markers;
@@ -488,6 +489,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     // machine than on MIR with async primitives.
     let optimizations_with_generators: &[&dyn MirPass<'tcx>] = &[
         &lower_slice_len::LowerSliceLenCalls, // has to be done before inlining, otherwise actual call will be almost always inlined. Also simple, so can just do first
+        &normalize_array_len::NormalizeArrayLen, // has to run after `slice::len` lowering
         &unreachable_prop::UnreachablePropagation,
         &uninhabited_enum_branching::UninhabitedEnumBranching,
         &simplify::SimplifyCfg::new("after-uninhabited-enum-branching"),
diff --git a/compiler/rustc_mir_transform/src/normalize_array_len.rs b/compiler/rustc_mir_transform/src/normalize_array_len.rs
new file mode 100644
index 00000000000..60e71130cd1
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/normalize_array_len.rs
@@ -0,0 +1,287 @@
+//! This pass eliminates casting of arrays into slices when their length
+//! is taken using `.len()` method. Handy to preserve information in MIR for const prop
+
+use crate::MirPass;
+use rustc_data_structures::fx::FxIndexMap;
+use rustc_index::bit_set::BitSet;
+use rustc_index::vec::IndexVec;
+use rustc_middle::mir::*;
+use rustc_middle::ty::{self, TyCtxt};
+
+const MAX_NUM_BLOCKS: usize = 800;
+const MAX_NUM_LOCALS: usize = 3000;
+
+pub struct NormalizeArrayLen;
+
+impl<'tcx> MirPass<'tcx> for NormalizeArrayLen {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        if tcx.sess.mir_opt_level() < 4 {
+            return;
+        }
+
+        // early returns for edge cases of highly unrolled functions
+        if body.basic_blocks().len() > MAX_NUM_BLOCKS {
+            return;
+        }
+        if body.local_decls().len() > MAX_NUM_LOCALS {
+            return;
+        }
+        normalize_array_len_calls(tcx, body)
+    }
+}
+
+pub fn normalize_array_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+    let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
+
+    // do a preliminary analysis to see if we ever have locals of type `[T;N]` or `&[T;N]`
+    let mut interesting_locals = BitSet::new_empty(local_decls.len());
+    for (local, decl) in local_decls.iter_enumerated() {
+        match decl.ty.kind() {
+            ty::Array(..) => {
+                interesting_locals.insert(local);
+            }
+            ty::Ref(.., ty, Mutability::Not) => match ty.kind() {
+                ty::Array(..) => {
+                    interesting_locals.insert(local);
+                }
+                _ => {}
+            },
+            _ => {}
+        }
+    }
+    if interesting_locals.is_empty() {
+        // we have found nothing to analyze
+        return;
+    }
+    let num_intesting_locals = interesting_locals.count();
+    let mut state = FxIndexMap::with_capacity_and_hasher(num_intesting_locals, Default::default());
+    let mut patches_scratchpad =
+        FxIndexMap::with_capacity_and_hasher(num_intesting_locals, Default::default());
+    let mut replacements_scratchpad =
+        FxIndexMap::with_capacity_and_hasher(num_intesting_locals, Default::default());
+    for block in basic_blocks {
+        // make length calls for arrays [T; N] not to decay into length calls for &[T]
+        // that forbids constant propagation
+        normalize_array_len_call(
+            tcx,
+            block,
+            local_decls,
+            &interesting_locals,
+            &mut state,
+            &mut patches_scratchpad,
+            &mut replacements_scratchpad,
+        );
+        state.clear();
+        patches_scratchpad.clear();
+        replacements_scratchpad.clear();
+    }
+}
+
+struct Patcher<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    patches_scratchpad: &'a FxIndexMap<usize, usize>,
+    replacements_scratchpad: &'a mut FxIndexMap<usize, Local>,
+    local_decls: &'a mut IndexVec<Local, LocalDecl<'tcx>>,
+    statement_idx: usize,
+}
+
+impl<'a, 'tcx> Patcher<'a, 'tcx> {
+    fn patch_expand_statement(
+        &mut self,
+        statement: &mut Statement<'tcx>,
+    ) -> Option<std::vec::IntoIter<Statement<'tcx>>> {
+        let idx = self.statement_idx;
+        if let Some(len_statemnt_idx) = self.patches_scratchpad.get(&idx).copied() {
+            let mut statements = Vec::with_capacity(2);
+
+            // we are at statement that performs a cast. The only sound way is
+            // to create another local that performs a similar copy without a cast and then
+            // use this copy in the Len operation
+
+            match &statement.kind {
+                StatementKind::Assign(box (
+                    ..,
+                    Rvalue::Cast(
+                        CastKind::Pointer(ty::adjustment::PointerCast::Unsize),
+                        operand,
+                        _,
+                    ),
+                )) => {
+                    match operand {
+                        Operand::Copy(place) | Operand::Move(place) => {
+                            // create new local
+                            let ty = operand.ty(self.local_decls, self.tcx);
+                            let local_decl =
+                                LocalDecl::with_source_info(ty, statement.source_info.clone());
+                            let local = self.local_decls.push(local_decl);
+                            // make it live
+                            let mut make_live_statement = statement.clone();
+                            make_live_statement.kind = StatementKind::StorageLive(local);
+                            statements.push(make_live_statement);
+                            // copy into it
+
+                            let operand = Operand::Copy(*place);
+                            let mut make_copy_statement = statement.clone();
+                            let assign_to = Place::from(local);
+                            let rvalue = Rvalue::Use(operand);
+                            make_copy_statement.kind =
+                                StatementKind::Assign(box (assign_to, rvalue));
+                            statements.push(make_copy_statement);
+
+                            // to reorder we have to copy and make NOP
+                            statements.push(statement.clone());
+                            statement.make_nop();
+
+                            self.replacements_scratchpad.insert(len_statemnt_idx, local);
+                        }
+                        _ => {
+                            unreachable!("it's a bug in the implementation")
+                        }
+                    }
+                }
+                _ => {
+                    unreachable!("it's a bug in the implementation")
+                }
+            }
+
+            self.statement_idx += 1;
+
+            Some(statements.into_iter())
+        } else if let Some(local) = self.replacements_scratchpad.get(&idx).copied() {
+            let mut statements = Vec::with_capacity(2);
+
+            match &statement.kind {
+                StatementKind::Assign(box (into, Rvalue::Len(place))) => {
+                    let add_deref = if let Some(..) = place.as_local() {
+                        false
+                    } else if let Some(..) = place.local_or_deref_local() {
+                        true
+                    } else {
+                        unreachable!("it's a bug in the implementation")
+                    };
+                    // replace len statement
+                    let mut len_statement = statement.clone();
+                    let mut place = Place::from(local);
+                    if add_deref {
+                        place = self.tcx.mk_place_deref(place);
+                    }
+                    len_statement.kind = StatementKind::Assign(box (*into, Rvalue::Len(place)));
+                    statements.push(len_statement);
+
+                    // make temporary dead
+                    let mut make_dead_statement = statement.clone();
+                    make_dead_statement.kind = StatementKind::StorageDead(local);
+                    statements.push(make_dead_statement);
+
+                    // make original statement NOP
+                    statement.make_nop();
+                }
+                _ => {
+                    unreachable!("it's a bug in the implementation")
+                }
+            }
+
+            self.statement_idx += 1;
+
+            Some(statements.into_iter())
+        } else {
+            self.statement_idx += 1;
+            None
+        }
+    }
+}
+
+fn normalize_array_len_call<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    block: &mut BasicBlockData<'tcx>,
+    local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
+    interesting_locals: &BitSet<Local>,
+    state: &mut FxIndexMap<Local, usize>,
+    patches_scratchpad: &mut FxIndexMap<usize, usize>,
+    replacements_scratchpad: &mut FxIndexMap<usize, Local>,
+) {
+    for (statement_idx, statement) in block.statements.iter_mut().enumerate() {
+        match &mut statement.kind {
+            StatementKind::Assign(box (place, rvalue)) => {
+                match rvalue {
+                    Rvalue::Cast(
+                        CastKind::Pointer(ty::adjustment::PointerCast::Unsize),
+                        operand,
+                        cast_ty,
+                    ) => {
+                        let local = if let Some(local) = place.as_local() { local } else { return };
+                        match operand {
+                            Operand::Copy(place) | Operand::Move(place) => {
+                                let operand_local =
+                                    if let Some(local) = place.local_or_deref_local() {
+                                        local
+                                    } else {
+                                        return;
+                                    };
+                                if !interesting_locals.contains(operand_local) {
+                                    return;
+                                }
+                                let operand_ty = local_decls[operand_local].ty;
+                                match (operand_ty.kind(), cast_ty.kind()) {
+                                    (ty::Array(of_ty_src, ..), ty::Slice(of_ty_dst)) => {
+                                        if of_ty_src == of_ty_dst {
+                                            // this is a cast from [T; N] into [T], so we are good
+                                            state.insert(local, statement_idx);
+                                        }
+                                    }
+                                    // current way of patching doesn't allow to work with `mut`
+                                    (
+                                        ty::Ref(
+                                            ty::RegionKind::ReErased,
+                                            operand_ty,
+                                            Mutability::Not,
+                                        ),
+                                        ty::Ref(ty::RegionKind::ReErased, cast_ty, Mutability::Not),
+                                    ) => {
+                                        match (operand_ty.kind(), cast_ty.kind()) {
+                                            // current way of patching doesn't allow to work with `mut`
+                                            (ty::Array(of_ty_src, ..), ty::Slice(of_ty_dst)) => {
+                                                if of_ty_src == of_ty_dst {
+                                                    // this is a cast from [T; N] into [T], so we are good
+                                                    state.insert(local, statement_idx);
+                                                }
+                                            }
+                                            _ => {}
+                                        }
+                                    }
+                                    _ => {}
+                                }
+                            }
+                            _ => {}
+                        }
+                    }
+                    Rvalue::Len(place) => {
+                        let local = if let Some(local) = place.local_or_deref_local() {
+                            local
+                        } else {
+                            return;
+                        };
+                        if let Some(cast_statement_idx) = state.get(&local).copied() {
+                            patches_scratchpad.insert(cast_statement_idx, statement_idx);
+                        }
+                    }
+                    _ => {
+                        // invalidate
+                        state.remove(&place.local);
+                    }
+                }
+            }
+            _ => {}
+        }
+    }
+
+    let mut patcher = Patcher {
+        tcx,
+        patches_scratchpad: &*patches_scratchpad,
+        replacements_scratchpad,
+        local_decls,
+        statement_idx: 0,
+    };
+
+    block.expand_statements(|st| patcher.patch_expand_statement(st));
+}
diff --git a/src/test/mir-opt/const_prop/slice_len.main.ConstProp.32bit.diff b/src/test/mir-opt/const_prop/slice_len.main.ConstProp.32bit.diff
index f305d602bbd..dfb5b98d5c9 100644
--- a/src/test/mir-opt/const_prop/slice_len.main.ConstProp.32bit.diff
+++ b/src/test/mir-opt/const_prop/slice_len.main.ConstProp.32bit.diff
@@ -12,6 +12,7 @@
       let mut _7: usize;                   // in scope 0 at $DIR/slice_len.rs:5:5: 5:33
       let mut _8: bool;                    // in scope 0 at $DIR/slice_len.rs:5:5: 5:33
       let mut _9: &[u32; 3];               // in scope 0 at $DIR/slice_len.rs:5:6: 5:19
+      let mut _10: &[u32; 3];              // in scope 0 at $DIR/slice_len.rs:5:6: 5:19
   
       bb0: {
           StorageLive(_1);                 // scope 0 at $DIR/slice_len.rs:5:5: 5:33
@@ -27,14 +28,16 @@
                                            // + literal: Const { ty: &[u32; 3], val: Unevaluated(Unevaluated { def: WithOptConstParam { did: DefId(0:3 ~ slice_len[6547]::main), const_param_did: None }, substs_: Some([]), promoted: Some(promoted[0]) }) }
           _4 = _9;                         // scope 0 at $DIR/slice_len.rs:5:6: 5:19
           _3 = _4;                         // scope 0 at $DIR/slice_len.rs:5:6: 5:19
+          StorageLive(_10);                // scope 0 at $DIR/slice_len.rs:5:6: 5:19
+          _10 = _3;                        // scope 0 at $DIR/slice_len.rs:5:6: 5:19
           _2 = move _3 as &[u32] (Pointer(Unsize)); // scope 0 at $DIR/slice_len.rs:5:6: 5:19
           StorageDead(_3);                 // scope 0 at $DIR/slice_len.rs:5:18: 5:19
           StorageLive(_6);                 // scope 0 at $DIR/slice_len.rs:5:31: 5:32
           _6 = const 1_usize;              // scope 0 at $DIR/slice_len.rs:5:31: 5:32
--         _7 = Len((*_2));                 // scope 0 at $DIR/slice_len.rs:5:5: 5:33
+          _7 = const 3_usize;              // scope 0 at $DIR/slice_len.rs:5:5: 5:33
+          StorageDead(_10);                // scope 0 at $DIR/slice_len.rs:5:5: 5:33
 -         _8 = Lt(_6, _7);                 // scope 0 at $DIR/slice_len.rs:5:5: 5:33
 -         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> bb1; // scope 0 at $DIR/slice_len.rs:5:5: 5:33
-+         _7 = const 3_usize;              // scope 0 at $DIR/slice_len.rs:5:5: 5:33
 +         _8 = const true;                 // scope 0 at $DIR/slice_len.rs:5:5: 5:33
 +         assert(const true, "index out of bounds: the length is {} but the index is {}", const 3_usize, const 1_usize) -> bb1; // scope 0 at $DIR/slice_len.rs:5:5: 5:33
       }
diff --git a/src/test/mir-opt/const_prop/slice_len.main.ConstProp.64bit.diff b/src/test/mir-opt/const_prop/slice_len.main.ConstProp.64bit.diff
index f305d602bbd..dfb5b98d5c9 100644
--- a/src/test/mir-opt/const_prop/slice_len.main.ConstProp.64bit.diff
+++ b/src/test/mir-opt/const_prop/slice_len.main.ConstProp.64bit.diff
@@ -12,6 +12,7 @@
       let mut _7: usize;                   // in scope 0 at $DIR/slice_len.rs:5:5: 5:33
       let mut _8: bool;                    // in scope 0 at $DIR/slice_len.rs:5:5: 5:33
       let mut _9: &[u32; 3];               // in scope 0 at $DIR/slice_len.rs:5:6: 5:19
+      let mut _10: &[u32; 3];              // in scope 0 at $DIR/slice_len.rs:5:6: 5:19
   
       bb0: {
           StorageLive(_1);                 // scope 0 at $DIR/slice_len.rs:5:5: 5:33
@@ -27,14 +28,16 @@
                                            // + literal: Const { ty: &[u32; 3], val: Unevaluated(Unevaluated { def: WithOptConstParam { did: DefId(0:3 ~ slice_len[6547]::main), const_param_did: None }, substs_: Some([]), promoted: Some(promoted[0]) }) }
           _4 = _9;                         // scope 0 at $DIR/slice_len.rs:5:6: 5:19
           _3 = _4;                         // scope 0 at $DIR/slice_len.rs:5:6: 5:19
+          StorageLive(_10);                // scope 0 at $DIR/slice_len.rs:5:6: 5:19
+          _10 = _3;                        // scope 0 at $DIR/slice_len.rs:5:6: 5:19
           _2 = move _3 as &[u32] (Pointer(Unsize)); // scope 0 at $DIR/slice_len.rs:5:6: 5:19
           StorageDead(_3);                 // scope 0 at $DIR/slice_len.rs:5:18: 5:19
           StorageLive(_6);                 // scope 0 at $DIR/slice_len.rs:5:31: 5:32
           _6 = const 1_usize;              // scope 0 at $DIR/slice_len.rs:5:31: 5:32
--         _7 = Len((*_2));                 // scope 0 at $DIR/slice_len.rs:5:5: 5:33
+          _7 = const 3_usize;              // scope 0 at $DIR/slice_len.rs:5:5: 5:33
+          StorageDead(_10);                // scope 0 at $DIR/slice_len.rs:5:5: 5:33
 -         _8 = Lt(_6, _7);                 // scope 0 at $DIR/slice_len.rs:5:5: 5:33
 -         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> bb1; // scope 0 at $DIR/slice_len.rs:5:5: 5:33
-+         _7 = const 3_usize;              // scope 0 at $DIR/slice_len.rs:5:5: 5:33
 +         _8 = const true;                 // scope 0 at $DIR/slice_len.rs:5:5: 5:33
 +         assert(const true, "index out of bounds: the length is {} but the index is {}", const 3_usize, const 1_usize) -> bb1; // scope 0 at $DIR/slice_len.rs:5:5: 5:33
       }
diff --git a/src/test/mir-opt/issue_76432.test.SimplifyComparisonIntegral.diff b/src/test/mir-opt/issue_76432.test.SimplifyComparisonIntegral.diff
index 96adadbe030..97c549dc9c8 100644
--- a/src/test/mir-opt/issue_76432.test.SimplifyComparisonIntegral.diff
+++ b/src/test/mir-opt/issue_76432.test.SimplifyComparisonIntegral.diff
@@ -22,6 +22,7 @@
       let mut _20: *const T;               // in scope 0 at $DIR/issue_76432.rs:9:70: 9:84
       let mut _21: *const T;               // in scope 0 at $DIR/issue_76432.rs:9:70: 9:84
       let mut _22: !;                      // in scope 0 at $SRC_DIR/core/src/panic.rs:LL:COL
+      let mut _23: &[T; 3];                // in scope 0 at $DIR/issue_76432.rs:7:19: 7:29
       scope 1 {
           debug v => _2;                   // in scope 1 at $DIR/issue_76432.rs:7:9: 7:10
           let _13: &T;                     // in scope 1 at $DIR/issue_76432.rs:9:10: 9:16
@@ -51,16 +52,17 @@
           StorageDead(_6);                 // scope 0 at $DIR/issue_76432.rs:7:28: 7:29
           _4 = &_5;                        // scope 0 at $DIR/issue_76432.rs:7:19: 7:29
           _3 = _4;                         // scope 0 at $DIR/issue_76432.rs:7:19: 7:29
+          StorageLive(_23);                // scope 0 at $DIR/issue_76432.rs:7:19: 7:29
+          _23 = _3;                        // scope 0 at $DIR/issue_76432.rs:7:19: 7:29
           _2 = move _3 as &[T] (Pointer(Unsize)); // scope 0 at $DIR/issue_76432.rs:7:19: 7:29
           StorageDead(_3);                 // scope 0 at $DIR/issue_76432.rs:7:28: 7:29
           StorageDead(_4);                 // scope 0 at $DIR/issue_76432.rs:7:29: 7:30
           StorageLive(_9);                 // scope 1 at $DIR/issue_76432.rs:8:5: 11:6
-          _10 = Len((*_2));                // scope 1 at $DIR/issue_76432.rs:9:9: 9:33
+          _10 = const 3_usize;             // scope 1 at $DIR/issue_76432.rs:9:9: 9:33
+          StorageDead(_23);                // scope 1 at $DIR/issue_76432.rs:9:9: 9:33
           _11 = const 3_usize;             // scope 1 at $DIR/issue_76432.rs:9:9: 9:33
--         _12 = Eq(move _10, const 3_usize); // scope 1 at $DIR/issue_76432.rs:9:9: 9:33
--         switchInt(move _12) -> [false: bb1, otherwise: bb2]; // scope 1 at $DIR/issue_76432.rs:9:9: 9:33
-+         nop;                             // scope 1 at $DIR/issue_76432.rs:9:9: 9:33
-+         switchInt(move _10) -> [3_usize: bb2, otherwise: bb1]; // scope 1 at $DIR/issue_76432.rs:9:9: 9:33
+          _12 = const true;                // scope 1 at $DIR/issue_76432.rs:9:9: 9:33
+          goto -> bb2;                     // scope 1 at $DIR/issue_76432.rs:9:9: 9:33
       }
   
       bb1: {
diff --git a/src/test/mir-opt/lower_array_len.array_bound.InstCombine.diff b/src/test/mir-opt/lower_array_len.array_bound.InstCombine.diff
new file mode 100644
index 00000000000..c7226573d75
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_bound.InstCombine.diff
@@ -0,0 +1,66 @@
+- // MIR for `array_bound` before InstCombine
++ // MIR for `array_bound` after InstCombine
+  
+  fn array_bound(_1: usize, _2: &[u8; N]) -> u8 {
+      debug index => _1;                   // in scope 0 at $DIR/lower_array_len.rs:6:36: 6:41
+      debug slice => _2;                   // in scope 0 at $DIR/lower_array_len.rs:6:50: 6:55
+      let mut _0: u8;                      // return place in scope 0 at $DIR/lower_array_len.rs:6:70: 6:72
+      let mut _3: bool;                    // in scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+      let mut _4: usize;                   // in scope 0 at $DIR/lower_array_len.rs:7:8: 7:13
+      let mut _5: usize;                   // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+      let mut _6: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+      let mut _7: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+      let _8: usize;                       // in scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
+      let mut _9: usize;                   // in scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+      let mut _10: bool;                   // in scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+      let mut _11: &[u8; N];               // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+  
+      bb0: {
+          StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+          StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:7:8: 7:13
+          _4 = _1;                         // scope 0 at $DIR/lower_array_len.rs:7:8: 7:13
+          StorageLive(_5);                 // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          StorageLive(_6);                 // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          StorageLive(_7);                 // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-         _7 = &(*_2);                     // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
++         _7 = _2;                         // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          StorageLive(_11);                // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          _11 = _7;                        // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          _6 = move _7 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          StorageDead(_7);                 // scope 0 at $DIR/lower_array_len.rs:7:20: 7:21
+-         _5 = Len((*_11));                // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
++         _5 = const N;                    // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          StorageDead(_11);                // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          StorageDead(_6);                 // scope 0 at $DIR/lower_array_len.rs:7:26: 7:27
+          _3 = Lt(move _4, move _5);       // scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+          StorageDead(_5);                 // scope 0 at $DIR/lower_array_len.rs:7:26: 7:27
+          StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:7:26: 7:27
+          switchInt(move _3) -> [false: bb3, otherwise: bb1]; // scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+      }
+  
+      bb1: {
+          StorageLive(_8);                 // scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
+          _8 = _1;                         // scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
+-         _9 = Len((*_2));                 // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
++         _9 = const N;                    // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+          _10 = Lt(_8, _9);                // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+          assert(move _10, "index out of bounds: the length is {} but the index is {}", move _9, _8) -> bb2; // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+      }
+  
+      bb2: {
+          _0 = (*_2)[_8];                  // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+          StorageDead(_8);                 // scope 0 at $DIR/lower_array_len.rs:9:5: 9:6
+          goto -> bb4;                     // scope 0 at $DIR/lower_array_len.rs:7:5: 11:6
+      }
+  
+      bb3: {
+          _0 = const 42_u8;                // scope 0 at $DIR/lower_array_len.rs:10:9: 10:11
+          goto -> bb4;                     // scope 0 at $DIR/lower_array_len.rs:7:5: 11:6
+      }
+  
+      bb4: {
+          StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:11:5: 11:6
+          return;                          // scope 0 at $DIR/lower_array_len.rs:12:2: 12:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_bound.NormalizeArrayLen.diff b/src/test/mir-opt/lower_array_len.array_bound.NormalizeArrayLen.diff
new file mode 100644
index 00000000000..d6c1c92cd91
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_bound.NormalizeArrayLen.diff
@@ -0,0 +1,68 @@
+- // MIR for `array_bound` before NormalizeArrayLen
++ // MIR for `array_bound` after NormalizeArrayLen
+  
+  fn array_bound(_1: usize, _2: &[u8; N]) -> u8 {
+      debug index => _1;                   // in scope 0 at $DIR/lower_array_len.rs:6:36: 6:41
+      debug slice => _2;                   // in scope 0 at $DIR/lower_array_len.rs:6:50: 6:55
+      let mut _0: u8;                      // return place in scope 0 at $DIR/lower_array_len.rs:6:70: 6:72
+      let mut _3: bool;                    // in scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+      let mut _4: usize;                   // in scope 0 at $DIR/lower_array_len.rs:7:8: 7:13
+      let mut _5: usize;                   // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+      let mut _6: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+      let mut _7: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+      let _8: usize;                       // in scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
+      let mut _9: usize;                   // in scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+      let mut _10: bool;                   // in scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
++     let mut _11: &[u8; N];               // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+  
+      bb0: {
+          StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+          StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:7:8: 7:13
+          _4 = _1;                         // scope 0 at $DIR/lower_array_len.rs:7:8: 7:13
+          StorageLive(_5);                 // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          StorageLive(_6);                 // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          StorageLive(_7);                 // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          _7 = &(*_2);                     // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
++         StorageLive(_11);                // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
++         _11 = _7;                        // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          _6 = move _7 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          StorageDead(_7);                 // scope 0 at $DIR/lower_array_len.rs:7:20: 7:21
+-         _5 = Len((*_6));                 // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
++         _5 = Len((*_11));                // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
++         StorageDead(_11);                // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+          goto -> bb1;                     // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+      }
+  
+      bb1: {
+          StorageDead(_6);                 // scope 0 at $DIR/lower_array_len.rs:7:26: 7:27
+          _3 = Lt(move _4, move _5);       // scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+          StorageDead(_5);                 // scope 0 at $DIR/lower_array_len.rs:7:26: 7:27
+          StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:7:26: 7:27
+          switchInt(move _3) -> [false: bb4, otherwise: bb2]; // scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+      }
+  
+      bb2: {
+          StorageLive(_8);                 // scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
+          _8 = _1;                         // scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
+          _9 = Len((*_2));                 // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+          _10 = Lt(_8, _9);                // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+          assert(move _10, "index out of bounds: the length is {} but the index is {}", move _9, _8) -> bb3; // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+      }
+  
+      bb3: {
+          _0 = (*_2)[_8];                  // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+          StorageDead(_8);                 // scope 0 at $DIR/lower_array_len.rs:9:5: 9:6
+          goto -> bb5;                     // scope 0 at $DIR/lower_array_len.rs:7:5: 11:6
+      }
+  
+      bb4: {
+          _0 = const 42_u8;                // scope 0 at $DIR/lower_array_len.rs:10:9: 10:11
+          goto -> bb5;                     // scope 0 at $DIR/lower_array_len.rs:7:5: 11:6
+      }
+  
+      bb5: {
+          StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:11:5: 11:6
+          return;                          // scope 0 at $DIR/lower_array_len.rs:12:2: 12:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_bound.SimplifyLocals.diff b/src/test/mir-opt/lower_array_len.array_bound.SimplifyLocals.diff
new file mode 100644
index 00000000000..5cf3312cd64
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_bound.SimplifyLocals.diff
@@ -0,0 +1,73 @@
+- // MIR for `array_bound` before SimplifyLocals
++ // MIR for `array_bound` after SimplifyLocals
+  
+  fn array_bound(_1: usize, _2: &[u8; N]) -> u8 {
+      debug index => _1;                   // in scope 0 at $DIR/lower_array_len.rs:6:36: 6:41
+      debug slice => _2;                   // in scope 0 at $DIR/lower_array_len.rs:6:50: 6:55
+      let mut _0: u8;                      // return place in scope 0 at $DIR/lower_array_len.rs:6:70: 6:72
+      let mut _3: bool;                    // in scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+      let mut _4: usize;                   // in scope 0 at $DIR/lower_array_len.rs:7:8: 7:13
+      let mut _5: usize;                   // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-     let mut _6: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-     let mut _7: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-     let _8: usize;                       // in scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
+-     let mut _9: usize;                   // in scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+-     let mut _10: bool;                   // in scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+-     let mut _11: &[u8; N];               // in scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
++     let _6: usize;                       // in scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
++     let mut _7: usize;                   // in scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
++     let mut _8: bool;                    // in scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+  
+      bb0: {
+          StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+          StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:7:8: 7:13
+          _4 = _1;                         // scope 0 at $DIR/lower_array_len.rs:7:8: 7:13
+          StorageLive(_5);                 // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-         StorageLive(_6);                 // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-         StorageLive(_7);                 // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-         _7 = _2;                         // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-         StorageLive(_11);                // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-         _11 = _7;                        // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-         _6 = move _7 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-         StorageDead(_7);                 // scope 0 at $DIR/lower_array_len.rs:7:20: 7:21
+          _5 = const N;                    // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-         StorageDead(_11);                // scope 0 at $DIR/lower_array_len.rs:7:16: 7:27
+-         StorageDead(_6);                 // scope 0 at $DIR/lower_array_len.rs:7:26: 7:27
+          _3 = Lt(move _4, move _5);       // scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+          StorageDead(_5);                 // scope 0 at $DIR/lower_array_len.rs:7:26: 7:27
+          StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:7:26: 7:27
+          switchInt(move _3) -> [false: bb3, otherwise: bb1]; // scope 0 at $DIR/lower_array_len.rs:7:8: 7:27
+      }
+  
+      bb1: {
+-         StorageLive(_8);                 // scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
+-         _8 = _1;                         // scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
+-         _9 = const N;                    // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+-         _10 = Lt(_8, _9);                // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+-         assert(move _10, "index out of bounds: the length is {} but the index is {}", move _9, _8) -> bb2; // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
++         StorageLive(_6);                 // scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
++         _6 = _1;                         // scope 0 at $DIR/lower_array_len.rs:8:15: 8:20
++         _7 = const N;                    // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
++         _8 = Lt(_6, _7);                 // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
++         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> bb2; // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+      }
+  
+      bb2: {
+-         _0 = (*_2)[_8];                  // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
+-         StorageDead(_8);                 // scope 0 at $DIR/lower_array_len.rs:9:5: 9:6
++         _0 = (*_2)[_6];                  // scope 0 at $DIR/lower_array_len.rs:8:9: 8:21
++         StorageDead(_6);                 // scope 0 at $DIR/lower_array_len.rs:9:5: 9:6
+          goto -> bb4;                     // scope 0 at $DIR/lower_array_len.rs:7:5: 11:6
+      }
+  
+      bb3: {
+          _0 = const 42_u8;                // scope 0 at $DIR/lower_array_len.rs:10:9: 10:11
+          goto -> bb4;                     // scope 0 at $DIR/lower_array_len.rs:7:5: 11:6
+      }
+  
+      bb4: {
+          StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:11:5: 11:6
+          return;                          // scope 0 at $DIR/lower_array_len.rs:12:2: 12:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_bound_mut.InstCombine.diff b/src/test/mir-opt/lower_array_len.array_bound_mut.InstCombine.diff
new file mode 100644
index 00000000000..5622d484532
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_bound_mut.InstCombine.diff
@@ -0,0 +1,79 @@
+- // MIR for `array_bound_mut` before InstCombine
++ // MIR for `array_bound_mut` after InstCombine
+  
+  fn array_bound_mut(_1: usize, _2: &mut [u8; N]) -> u8 {
+      debug index => _1;                   // in scope 0 at $DIR/lower_array_len.rs:17:40: 17:45
+      debug slice => _2;                   // in scope 0 at $DIR/lower_array_len.rs:17:54: 17:59
+      let mut _0: u8;                      // return place in scope 0 at $DIR/lower_array_len.rs:17:78: 17:80
+      let mut _3: bool;                    // in scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+      let mut _4: usize;                   // in scope 0 at $DIR/lower_array_len.rs:18:8: 18:13
+      let mut _5: usize;                   // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+      let mut _6: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+      let mut _7: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+      let _8: usize;                       // in scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
+      let mut _9: usize;                   // in scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+      let mut _10: bool;                   // in scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+      let _11: usize;                      // in scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
+      let mut _12: usize;                  // in scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+      let mut _13: bool;                   // in scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+      let mut _14: &[u8; N];               // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+  
+      bb0: {
+          StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+          StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:18:8: 18:13
+          _4 = _1;                         // scope 0 at $DIR/lower_array_len.rs:18:8: 18:13
+          StorageLive(_5);                 // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          StorageLive(_6);                 // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          StorageLive(_7);                 // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          _7 = &(*_2);                     // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          StorageLive(_14);                // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          _14 = _7;                        // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          _6 = move _7 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          StorageDead(_7);                 // scope 0 at $DIR/lower_array_len.rs:18:20: 18:21
+-         _5 = Len((*_14));                // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
++         _5 = const N;                    // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          StorageDead(_14);                // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          StorageDead(_6);                 // scope 0 at $DIR/lower_array_len.rs:18:26: 18:27
+          _3 = Lt(move _4, move _5);       // scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+          StorageDead(_5);                 // scope 0 at $DIR/lower_array_len.rs:18:26: 18:27
+          StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:18:26: 18:27
+          switchInt(move _3) -> [false: bb3, otherwise: bb1]; // scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+      }
+  
+      bb1: {
+          StorageLive(_8);                 // scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
+          _8 = _1;                         // scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
+-         _9 = Len((*_2));                 // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
++         _9 = const N;                    // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+          _10 = Lt(_8, _9);                // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+          assert(move _10, "index out of bounds: the length is {} but the index is {}", move _9, _8) -> bb2; // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+      }
+  
+      bb2: {
+          _0 = (*_2)[_8];                  // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+          StorageDead(_8);                 // scope 0 at $DIR/lower_array_len.rs:20:5: 20:6
+          goto -> bb5;                     // scope 0 at $DIR/lower_array_len.rs:18:5: 24:6
+      }
+  
+      bb3: {
+          StorageLive(_11);                // scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
+          _11 = const 0_usize;             // scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
+-         _12 = Len((*_2));                // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
++         _12 = const N;                   // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+          _13 = Lt(_11, _12);              // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+          assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, _11) -> bb4; // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+      }
+  
+      bb4: {
+          (*_2)[_11] = const 42_u8;        // scope 0 at $DIR/lower_array_len.rs:21:9: 21:22
+          StorageDead(_11);                // scope 0 at $DIR/lower_array_len.rs:21:22: 21:23
+          _0 = const 42_u8;                // scope 0 at $DIR/lower_array_len.rs:23:9: 23:11
+          goto -> bb5;                     // scope 0 at $DIR/lower_array_len.rs:18:5: 24:6
+      }
+  
+      bb5: {
+          StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:24:5: 24:6
+          return;                          // scope 0 at $DIR/lower_array_len.rs:25:2: 25:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_bound_mut.NormalizeArrayLen.diff b/src/test/mir-opt/lower_array_len.array_bound_mut.NormalizeArrayLen.diff
new file mode 100644
index 00000000000..11fc20aa693
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_bound_mut.NormalizeArrayLen.diff
@@ -0,0 +1,81 @@
+- // MIR for `array_bound_mut` before NormalizeArrayLen
++ // MIR for `array_bound_mut` after NormalizeArrayLen
+  
+  fn array_bound_mut(_1: usize, _2: &mut [u8; N]) -> u8 {
+      debug index => _1;                   // in scope 0 at $DIR/lower_array_len.rs:17:40: 17:45
+      debug slice => _2;                   // in scope 0 at $DIR/lower_array_len.rs:17:54: 17:59
+      let mut _0: u8;                      // return place in scope 0 at $DIR/lower_array_len.rs:17:78: 17:80
+      let mut _3: bool;                    // in scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+      let mut _4: usize;                   // in scope 0 at $DIR/lower_array_len.rs:18:8: 18:13
+      let mut _5: usize;                   // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+      let mut _6: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+      let mut _7: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+      let _8: usize;                       // in scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
+      let mut _9: usize;                   // in scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+      let mut _10: bool;                   // in scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+      let _11: usize;                      // in scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
+      let mut _12: usize;                  // in scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+      let mut _13: bool;                   // in scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
++     let mut _14: &[u8; N];               // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+  
+      bb0: {
+          StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+          StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:18:8: 18:13
+          _4 = _1;                         // scope 0 at $DIR/lower_array_len.rs:18:8: 18:13
+          StorageLive(_5);                 // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          StorageLive(_6);                 // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          StorageLive(_7);                 // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          _7 = &(*_2);                     // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
++         StorageLive(_14);                // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
++         _14 = _7;                        // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          _6 = move _7 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          StorageDead(_7);                 // scope 0 at $DIR/lower_array_len.rs:18:20: 18:21
+-         _5 = Len((*_6));                 // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
++         _5 = Len((*_14));                // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
++         StorageDead(_14);                // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+          goto -> bb1;                     // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+      }
+  
+      bb1: {
+          StorageDead(_6);                 // scope 0 at $DIR/lower_array_len.rs:18:26: 18:27
+          _3 = Lt(move _4, move _5);       // scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+          StorageDead(_5);                 // scope 0 at $DIR/lower_array_len.rs:18:26: 18:27
+          StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:18:26: 18:27
+          switchInt(move _3) -> [false: bb4, otherwise: bb2]; // scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+      }
+  
+      bb2: {
+          StorageLive(_8);                 // scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
+          _8 = _1;                         // scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
+          _9 = Len((*_2));                 // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+          _10 = Lt(_8, _9);                // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+          assert(move _10, "index out of bounds: the length is {} but the index is {}", move _9, _8) -> bb3; // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+      }
+  
+      bb3: {
+          _0 = (*_2)[_8];                  // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+          StorageDead(_8);                 // scope 0 at $DIR/lower_array_len.rs:20:5: 20:6
+          goto -> bb6;                     // scope 0 at $DIR/lower_array_len.rs:18:5: 24:6
+      }
+  
+      bb4: {
+          StorageLive(_11);                // scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
+          _11 = const 0_usize;             // scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
+          _12 = Len((*_2));                // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+          _13 = Lt(_11, _12);              // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+          assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, _11) -> bb5; // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+      }
+  
+      bb5: {
+          (*_2)[_11] = const 42_u8;        // scope 0 at $DIR/lower_array_len.rs:21:9: 21:22
+          StorageDead(_11);                // scope 0 at $DIR/lower_array_len.rs:21:22: 21:23
+          _0 = const 42_u8;                // scope 0 at $DIR/lower_array_len.rs:23:9: 23:11
+          goto -> bb6;                     // scope 0 at $DIR/lower_array_len.rs:18:5: 24:6
+      }
+  
+      bb6: {
+          StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:24:5: 24:6
+          return;                          // scope 0 at $DIR/lower_array_len.rs:25:2: 25:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_bound_mut.SimplifyLocals.diff b/src/test/mir-opt/lower_array_len.array_bound_mut.SimplifyLocals.diff
new file mode 100644
index 00000000000..f72aee0e502
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_bound_mut.SimplifyLocals.diff
@@ -0,0 +1,96 @@
+- // MIR for `array_bound_mut` before SimplifyLocals
++ // MIR for `array_bound_mut` after SimplifyLocals
+  
+  fn array_bound_mut(_1: usize, _2: &mut [u8; N]) -> u8 {
+      debug index => _1;                   // in scope 0 at $DIR/lower_array_len.rs:17:40: 17:45
+      debug slice => _2;                   // in scope 0 at $DIR/lower_array_len.rs:17:54: 17:59
+      let mut _0: u8;                      // return place in scope 0 at $DIR/lower_array_len.rs:17:78: 17:80
+      let mut _3: bool;                    // in scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+      let mut _4: usize;                   // in scope 0 at $DIR/lower_array_len.rs:18:8: 18:13
+      let mut _5: usize;                   // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-     let mut _6: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-     let mut _7: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-     let _8: usize;                       // in scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
+-     let mut _9: usize;                   // in scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+-     let mut _10: bool;                   // in scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+-     let _11: usize;                      // in scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
+-     let mut _12: usize;                  // in scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+-     let mut _13: bool;                   // in scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+-     let mut _14: &[u8; N];               // in scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
++     let _6: usize;                       // in scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
++     let mut _7: usize;                   // in scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
++     let mut _8: bool;                    // in scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
++     let _9: usize;                       // in scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
++     let mut _10: usize;                  // in scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
++     let mut _11: bool;                   // in scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+  
+      bb0: {
+          StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+          StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:18:8: 18:13
+          _4 = _1;                         // scope 0 at $DIR/lower_array_len.rs:18:8: 18:13
+          StorageLive(_5);                 // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-         StorageLive(_6);                 // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-         StorageLive(_7);                 // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-         _7 = &(*_2);                     // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-         StorageLive(_14);                // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-         _14 = _7;                        // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-         _6 = move _7 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-         StorageDead(_7);                 // scope 0 at $DIR/lower_array_len.rs:18:20: 18:21
+          _5 = const N;                    // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-         StorageDead(_14);                // scope 0 at $DIR/lower_array_len.rs:18:16: 18:27
+-         StorageDead(_6);                 // scope 0 at $DIR/lower_array_len.rs:18:26: 18:27
+          _3 = Lt(move _4, move _5);       // scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+          StorageDead(_5);                 // scope 0 at $DIR/lower_array_len.rs:18:26: 18:27
+          StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:18:26: 18:27
+          switchInt(move _3) -> [false: bb3, otherwise: bb1]; // scope 0 at $DIR/lower_array_len.rs:18:8: 18:27
+      }
+  
+      bb1: {
+-         StorageLive(_8);                 // scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
+-         _8 = _1;                         // scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
+-         _9 = const N;                    // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+-         _10 = Lt(_8, _9);                // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+-         assert(move _10, "index out of bounds: the length is {} but the index is {}", move _9, _8) -> bb2; // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
++         StorageLive(_6);                 // scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
++         _6 = _1;                         // scope 0 at $DIR/lower_array_len.rs:19:15: 19:20
++         _7 = const N;                    // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
++         _8 = Lt(_6, _7);                 // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
++         assert(move _8, "index out of bounds: the length is {} but the index is {}", move _7, _6) -> bb2; // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+      }
+  
+      bb2: {
+-         _0 = (*_2)[_8];                  // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
+-         StorageDead(_8);                 // scope 0 at $DIR/lower_array_len.rs:20:5: 20:6
++         _0 = (*_2)[_6];                  // scope 0 at $DIR/lower_array_len.rs:19:9: 19:21
++         StorageDead(_6);                 // scope 0 at $DIR/lower_array_len.rs:20:5: 20:6
+          goto -> bb5;                     // scope 0 at $DIR/lower_array_len.rs:18:5: 24:6
+      }
+  
+      bb3: {
+-         StorageLive(_11);                // scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
+-         _11 = const 0_usize;             // scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
+-         _12 = const N;                   // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+-         _13 = Lt(const 0_usize, _12);    // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+-         assert(move _13, "index out of bounds: the length is {} but the index is {}", move _12, const 0_usize) -> bb4; // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
++         StorageLive(_9);                 // scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
++         _9 = const 0_usize;              // scope 0 at $DIR/lower_array_len.rs:21:15: 21:16
++         _10 = const N;                   // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
++         _11 = Lt(const 0_usize, _10);    // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
++         assert(move _11, "index out of bounds: the length is {} but the index is {}", move _10, const 0_usize) -> bb4; // scope 0 at $DIR/lower_array_len.rs:21:9: 21:17
+      }
+  
+      bb4: {
+-         (*_2)[_11] = const 42_u8;        // scope 0 at $DIR/lower_array_len.rs:21:9: 21:22
+-         StorageDead(_11);                // scope 0 at $DIR/lower_array_len.rs:21:22: 21:23
++         (*_2)[_9] = const 42_u8;         // scope 0 at $DIR/lower_array_len.rs:21:9: 21:22
++         StorageDead(_9);                 // scope 0 at $DIR/lower_array_len.rs:21:22: 21:23
+          _0 = const 42_u8;                // scope 0 at $DIR/lower_array_len.rs:23:9: 23:11
+          goto -> bb5;                     // scope 0 at $DIR/lower_array_len.rs:18:5: 24:6
+      }
+  
+      bb5: {
+          StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:24:5: 24:6
+          return;                          // scope 0 at $DIR/lower_array_len.rs:25:2: 25:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_len.InstCombine.diff b/src/test/mir-opt/lower_array_len.array_len.InstCombine.diff
new file mode 100644
index 00000000000..a818de39bcc
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_len.InstCombine.diff
@@ -0,0 +1,27 @@
+- // MIR for `array_len` before InstCombine
++ // MIR for `array_len` after InstCombine
+  
+  fn array_len(_1: &[u8; N]) -> usize {
+      debug arr => _1;                     // in scope 0 at $DIR/lower_array_len.rs:30:34: 30:37
+      let mut _0: usize;                   // return place in scope 0 at $DIR/lower_array_len.rs:30:52: 30:57
+      let mut _2: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+      let mut _3: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+      let mut _4: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+  
+      bb0: {
+          StorageLive(_2);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         _3 = &(*_1);                     // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
++         _3 = _1;                         // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          _4 = _3;                         // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          _2 = move _3 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:31:7: 31:8
+-         _0 = Len((*_4));                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
++         _0 = const N;                    // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          StorageDead(_2);                 // scope 0 at $DIR/lower_array_len.rs:31:13: 31:14
+          return;                          // scope 0 at $DIR/lower_array_len.rs:32:2: 32:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_len.NormalizeArrayLen.diff b/src/test/mir-opt/lower_array_len.array_len.NormalizeArrayLen.diff
new file mode 100644
index 00000000000..892fdda818e
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_len.NormalizeArrayLen.diff
@@ -0,0 +1,30 @@
+- // MIR for `array_len` before NormalizeArrayLen
++ // MIR for `array_len` after NormalizeArrayLen
+  
+  fn array_len(_1: &[u8; N]) -> usize {
+      debug arr => _1;                     // in scope 0 at $DIR/lower_array_len.rs:30:34: 30:37
+      let mut _0: usize;                   // return place in scope 0 at $DIR/lower_array_len.rs:30:52: 30:57
+      let mut _2: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+      let mut _3: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
++     let mut _4: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+  
+      bb0: {
+          StorageLive(_2);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          _3 = &(*_1);                     // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
++         StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
++         _4 = _3;                         // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          _2 = move _3 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:31:7: 31:8
+-         _0 = Len((*_2));                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
++         _0 = Len((*_4));                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
++         StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+          goto -> bb1;                     // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+      }
+  
+      bb1: {
+          StorageDead(_2);                 // scope 0 at $DIR/lower_array_len.rs:31:13: 31:14
+          return;                          // scope 0 at $DIR/lower_array_len.rs:32:2: 32:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_len.SimplifyLocals.diff b/src/test/mir-opt/lower_array_len.array_len.SimplifyLocals.diff
new file mode 100644
index 00000000000..20e2685aba5
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_len.SimplifyLocals.diff
@@ -0,0 +1,25 @@
+- // MIR for `array_len` before SimplifyLocals
++ // MIR for `array_len` after SimplifyLocals
+  
+  fn array_len(_1: &[u8; N]) -> usize {
+      debug arr => _1;                     // in scope 0 at $DIR/lower_array_len.rs:30:34: 30:37
+      let mut _0: usize;                   // return place in scope 0 at $DIR/lower_array_len.rs:30:52: 30:57
+-     let mut _2: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-     let mut _3: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-     let mut _4: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+  
+      bb0: {
+-         StorageLive(_2);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         _3 = _1;                         // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         _4 = _3;                         // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         _2 = move _3 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:31:7: 31:8
+          _0 = const N;                    // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:31:5: 31:14
+-         StorageDead(_2);                 // scope 0 at $DIR/lower_array_len.rs:31:13: 31:14
+          return;                          // scope 0 at $DIR/lower_array_len.rs:32:2: 32:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_len_by_value.InstCombine.diff b/src/test/mir-opt/lower_array_len.array_len_by_value.InstCombine.diff
new file mode 100644
index 00000000000..ce12531e84b
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_len_by_value.InstCombine.diff
@@ -0,0 +1,26 @@
+- // MIR for `array_len_by_value` before InstCombine
++ // MIR for `array_len_by_value` after InstCombine
+  
+  fn array_len_by_value(_1: [u8; N]) -> usize {
+      debug arr => _1;                     // in scope 0 at $DIR/lower_array_len.rs:37:43: 37:46
+      let mut _0: usize;                   // return place in scope 0 at $DIR/lower_array_len.rs:37:60: 37:65
+      let mut _2: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+      let mut _3: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+      let mut _4: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+  
+      bb0: {
+          StorageLive(_2);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          _3 = &_1;                        // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          _4 = _3;                         // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          _2 = move _3 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:38:7: 38:8
+-         _0 = Len((*_4));                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
++         _0 = const N;                    // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          StorageDead(_2);                 // scope 0 at $DIR/lower_array_len.rs:38:13: 38:14
+          return;                          // scope 0 at $DIR/lower_array_len.rs:39:2: 39:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_len_by_value.NormalizeArrayLen.diff b/src/test/mir-opt/lower_array_len.array_len_by_value.NormalizeArrayLen.diff
new file mode 100644
index 00000000000..201fffbf0d4
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_len_by_value.NormalizeArrayLen.diff
@@ -0,0 +1,30 @@
+- // MIR for `array_len_by_value` before NormalizeArrayLen
++ // MIR for `array_len_by_value` after NormalizeArrayLen
+  
+  fn array_len_by_value(_1: [u8; N]) -> usize {
+      debug arr => _1;                     // in scope 0 at $DIR/lower_array_len.rs:37:43: 37:46
+      let mut _0: usize;                   // return place in scope 0 at $DIR/lower_array_len.rs:37:60: 37:65
+      let mut _2: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+      let mut _3: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
++     let mut _4: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+  
+      bb0: {
+          StorageLive(_2);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          _3 = &_1;                        // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
++         StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
++         _4 = _3;                         // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          _2 = move _3 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:38:7: 38:8
+-         _0 = Len((*_2));                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
++         _0 = Len((*_4));                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
++         StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+          goto -> bb1;                     // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+      }
+  
+      bb1: {
+          StorageDead(_2);                 // scope 0 at $DIR/lower_array_len.rs:38:13: 38:14
+          return;                          // scope 0 at $DIR/lower_array_len.rs:39:2: 39:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.array_len_by_value.SimplifyLocals.diff b/src/test/mir-opt/lower_array_len.array_len_by_value.SimplifyLocals.diff
new file mode 100644
index 00000000000..7e7b708145f
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.array_len_by_value.SimplifyLocals.diff
@@ -0,0 +1,25 @@
+- // MIR for `array_len_by_value` before SimplifyLocals
++ // MIR for `array_len_by_value` after SimplifyLocals
+  
+  fn array_len_by_value(_1: [u8; N]) -> usize {
+      debug arr => _1;                     // in scope 0 at $DIR/lower_array_len.rs:37:43: 37:46
+      let mut _0: usize;                   // return place in scope 0 at $DIR/lower_array_len.rs:37:60: 37:65
+-     let mut _2: &[u8];                   // in scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+-     let mut _3: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+-     let mut _4: &[u8; N];                // in scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+  
+      bb0: {
+-         StorageLive(_2);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+-         StorageLive(_3);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+-         _3 = &_1;                        // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+-         StorageLive(_4);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+-         _4 = _3;                         // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+-         _2 = move _3 as &[u8] (Pointer(Unsize)); // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+-         StorageDead(_3);                 // scope 0 at $DIR/lower_array_len.rs:38:7: 38:8
+          _0 = const N;                    // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+-         StorageDead(_4);                 // scope 0 at $DIR/lower_array_len.rs:38:5: 38:14
+-         StorageDead(_2);                 // scope 0 at $DIR/lower_array_len.rs:38:13: 38:14
+          return;                          // scope 0 at $DIR/lower_array_len.rs:39:2: 39:2
+      }
+  }
+  
diff --git a/src/test/mir-opt/lower_array_len.rs b/src/test/mir-opt/lower_array_len.rs
new file mode 100644
index 00000000000..fc12ee75fcf
--- /dev/null
+++ b/src/test/mir-opt/lower_array_len.rs
@@ -0,0 +1,47 @@
+// compile-flags: -Z mir-opt-level=4
+
+// EMIT_MIR lower_array_len.array_bound.NormalizeArrayLen.diff
+// EMIT_MIR lower_array_len.array_bound.SimplifyLocals.diff
+// EMIT_MIR lower_array_len.array_bound.InstCombine.diff
+pub fn array_bound<const N: usize>(index: usize, slice: &[u8; N]) -> u8 {
+    if index < slice.len() {
+        slice[index]
+    } else {
+        42
+    }
+}
+
+// EMIT_MIR lower_array_len.array_bound_mut.NormalizeArrayLen.diff
+// EMIT_MIR lower_array_len.array_bound_mut.SimplifyLocals.diff
+// EMIT_MIR lower_array_len.array_bound_mut.InstCombine.diff
+pub fn array_bound_mut<const N: usize>(index: usize, slice: &mut [u8; N]) -> u8 {
+    if index < slice.len() {
+        slice[index]
+    } else {
+        slice[0] = 42;
+
+        42
+    }
+}
+
+// EMIT_MIR lower_array_len.array_len.NormalizeArrayLen.diff
+// EMIT_MIR lower_array_len.array_len.SimplifyLocals.diff
+// EMIT_MIR lower_array_len.array_len.InstCombine.diff
+pub fn array_len<const N: usize>(arr: &[u8; N]) -> usize {
+    arr.len()
+}
+
+// EMIT_MIR lower_array_len.array_len_by_value.NormalizeArrayLen.diff
+// EMIT_MIR lower_array_len.array_len_by_value.SimplifyLocals.diff
+// EMIT_MIR lower_array_len.array_len_by_value.InstCombine.diff
+pub fn array_len_by_value<const N: usize>(arr: [u8; N]) -> usize {
+    arr.len()
+}
+
+fn main() {
+    let _ = array_bound(3, &[0, 1, 2, 3]);
+    let mut tmp = [0, 1, 2, 3, 4];
+    let _ = array_bound_mut(3, &mut [0, 1, 2, 3]);
+    let _ = array_len(&[0]);
+    let _ = array_len_by_value([0, 2]);
+}