Revert "Auto merge of #115105 - cjgillot:dest-prop-default, r=oli-obk"

This reverts commit cfb730450f, reversing
changes made to 91c0823ee6.

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -160,7 +160,7 @@ impl<'tcx> MirPass<'tcx> for DestinationPropagation {
         //  2. Despite being an overall perf improvement, this still causes a 30% regression in
         //     keccak. We can temporarily fix this by bounding function size, but in the long term
         //     we should fix this by being smarter about invalidating analysis results.
-        sess.mir_opt_level() >= 2
+        sess.mir_opt_level() >= 3
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {

compiler/rustc_mir_transform/src/lib.rs

@@ -88,6 +88,7 @@ mod match_branches;
 mod mentioned_items;
 mod multiple_return_terminators;
 mod normalize_array_len;
+mod nrvo;
 mod prettify;
 mod promote_consts;
 mod ref_prop;
@@ -607,12 +608,13 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &jump_threading::JumpThreading,
             &early_otherwise_branch::EarlyOtherwiseBranch,
             &simplify_comparison_integral::SimplifyComparisonIntegral,
+            &dest_prop::DestinationPropagation,
             &o1(simplify_branches::SimplifyConstCondition::Final),
             &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
             &o1(simplify::SimplifyCfg::Final),
             &copy_prop::CopyProp,
             &dead_store_elimination::DeadStoreElimination::Final,
-            &dest_prop::DestinationPropagation,
+            &nrvo::RenameReturnPlace,
             &simplify::SimplifyLocals::Final,
             &multiple_return_terminators::MultipleReturnTerminators,
             &deduplicate_blocks::DeduplicateBlocks,

compiler/rustc_mir_transform/src/nrvo.rs

@@ -0,0 +1,235 @@
+//! See the docs for [`RenameReturnPlace`].
+
+use rustc_hir::Mutability;
+use rustc_index::bit_set::BitSet;
+use rustc_middle::bug;
+use rustc_middle::mir::visit::{MutVisitor, NonUseContext, PlaceContext, Visitor};
+use rustc_middle::mir::{self, BasicBlock, Local, Location};
+use rustc_middle::ty::TyCtxt;
+
+use crate::MirPass;
+
+/// This pass looks for MIR that always copies the same local into the return place and eliminates
+/// the copy by renaming all uses of that local to `_0`.
+///
+/// This allows LLVM to perform an optimization similar to the named return value optimization
+/// (NRVO) that is guaranteed in C++. This avoids a stack allocation and `memcpy` for the
+/// relatively common pattern of allocating a buffer on the stack, mutating it, and returning it by
+/// value like so:
+///
+/// ```rust
+/// fn foo(init: fn(&mut [u8; 1024])) -> [u8; 1024] {
+///     let mut buf = [0; 1024];
+///     init(&mut buf);
+///     buf
+/// }
+/// ```
+///
+/// For now, this pass is very simple and only capable of eliminating a single copy. A more general
+/// version of copy propagation, such as the one based on non-overlapping live ranges in [#47954] and
+/// [#71003], could yield even more benefits.
+///
+/// [#47954]: https://github.com/rust-lang/rust/pull/47954
+/// [#71003]: https://github.com/rust-lang/rust/pull/71003
+pub struct RenameReturnPlace;
+
+impl<'tcx> MirPass<'tcx> for RenameReturnPlace {
+    fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
+        // unsound: #111005
+        sess.mir_opt_level() > 0 && sess.opts.unstable_opts.unsound_mir_opts
+    }
+
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut mir::Body<'tcx>) {
+        let def_id = body.source.def_id();
+        let Some(returned_local) = local_eligible_for_nrvo(body) else {
+            debug!("`{:?}` was ineligible for NRVO", def_id);
+            return;
+        };
+
+        if !tcx.consider_optimizing(|| format!("RenameReturnPlace {def_id:?}")) {
+            return;
+        }
+
+        debug!(
+            "`{:?}` was eligible for NRVO, making {:?} the return place",
+            def_id, returned_local
+        );
+
+        RenameToReturnPlace { tcx, to_rename: returned_local }.visit_body_preserves_cfg(body);
+
+        // Clean up the `NOP`s we inserted for statements made useless by our renaming.
+        for block_data in body.basic_blocks.as_mut_preserves_cfg() {
+            block_data.statements.retain(|stmt| stmt.kind != mir::StatementKind::Nop);
+        }
+
+        // Overwrite the debuginfo of `_0` with that of the renamed local.
+        let (renamed_decl, ret_decl) =
+            body.local_decls.pick2_mut(returned_local, mir::RETURN_PLACE);
+
+        // Sometimes, the return place is assigned a local of a different but coercible type, for
+        // example `&mut T` instead of `&T`. Overwriting the `LocalInfo` for the return place means
+        // its type may no longer match the return type of its function. This doesn't cause a
+        // problem in codegen because these two types are layout-compatible, but may be unexpected.
+        debug!("_0: {:?} = {:?}: {:?}", ret_decl.ty, returned_local, renamed_decl.ty);
+        ret_decl.clone_from(renamed_decl);
+
+        // The return place is always mutable.
+        ret_decl.mutability = Mutability::Mut;
+    }
+}
+
+/// MIR that is eligible for the NRVO must fulfill two conditions:
+///   1. The return place must not be read prior to the `Return` terminator.
+///   2. A simple assignment of a whole local to the return place (e.g., `_0 = _1`) must be the
+///      only definition of the return place reaching the `Return` terminator.
+///
+/// If the MIR fulfills both these conditions, this function returns the `Local` that is assigned
+/// to the return place along all possible paths through the control-flow graph.
+fn local_eligible_for_nrvo(body: &mir::Body<'_>) -> Option<Local> {
+    if IsReturnPlaceRead::run(body) {
+        return None;
+    }
+
+    let mut copied_to_return_place = None;
+    for block in body.basic_blocks.indices() {
+        // Look for blocks with a `Return` terminator.
+        if !matches!(body[block].terminator().kind, mir::TerminatorKind::Return) {
+            continue;
+        }
+
+        // Look for an assignment of a single local to the return place prior to the `Return`.
+        let returned_local = find_local_assigned_to_return_place(block, body)?;
+        match body.local_kind(returned_local) {
+            // FIXME: Can we do this for arguments as well?
+            mir::LocalKind::Arg => return None,
+
+            mir::LocalKind::ReturnPointer => bug!("Return place was assigned to itself?"),
+            mir::LocalKind::Temp => {}
+        }
+
+        // If multiple different locals are copied to the return place. We can't pick a
+        // single one to rename.
+        if copied_to_return_place.is_some_and(|old| old != returned_local) {
+            return None;
+        }
+
+        copied_to_return_place = Some(returned_local);
+    }
+
+    copied_to_return_place
+}
+
+fn find_local_assigned_to_return_place(start: BasicBlock, body: &mir::Body<'_>) -> Option<Local> {
+    let mut block = start;
+    let mut seen = BitSet::new_empty(body.basic_blocks.len());
+
+    // Iterate as long as `block` has exactly one predecessor that we have not yet visited.
+    while seen.insert(block) {
+        trace!("Looking for assignments to `_0` in {:?}", block);
+
+        let local = body[block].statements.iter().rev().find_map(as_local_assigned_to_return_place);
+        if local.is_some() {
+            return local;
+        }
+
+        match body.basic_blocks.predecessors()[block].as_slice() {
+            &[pred] => block = pred,
+            _ => return None,
+        }
+    }
+
+    None
+}
+
+// If this statement is an assignment of an unprojected local to the return place,
+// return that local.
+fn as_local_assigned_to_return_place(stmt: &mir::Statement<'_>) -> Option<Local> {
+    if let mir::StatementKind::Assign(box (lhs, rhs)) = &stmt.kind {
+        if lhs.as_local() == Some(mir::RETURN_PLACE) {
+            if let mir::Rvalue::Use(mir::Operand::Copy(rhs) | mir::Operand::Move(rhs)) = rhs {
+                return rhs.as_local();
+            }
+        }
+    }
+
+    None
+}
+
+struct RenameToReturnPlace<'tcx> {
+    to_rename: Local,
+    tcx: TyCtxt<'tcx>,
+}
+
+/// Replaces all uses of `self.to_rename` with `_0`.
+impl<'tcx> MutVisitor<'tcx> for RenameToReturnPlace<'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn visit_statement(&mut self, stmt: &mut mir::Statement<'tcx>, loc: Location) {
+        // Remove assignments of the local being replaced to the return place, since it is now the
+        // return place:
+        //     _0 = _1
+        if as_local_assigned_to_return_place(stmt) == Some(self.to_rename) {
+            stmt.kind = mir::StatementKind::Nop;
+            return;
+        }
+
+        // Remove storage annotations for the local being replaced:
+        //     StorageLive(_1)
+        if let mir::StatementKind::StorageLive(local) | mir::StatementKind::StorageDead(local) =
+            stmt.kind
+        {
+            if local == self.to_rename {
+                stmt.kind = mir::StatementKind::Nop;
+                return;
+            }
+        }
+
+        self.super_statement(stmt, loc)
+    }
+
+    fn visit_terminator(&mut self, terminator: &mut mir::Terminator<'tcx>, loc: Location) {
+        // Ignore the implicit "use" of the return place in a `Return` statement.
+        if let mir::TerminatorKind::Return = terminator.kind {
+            return;
+        }
+
+        self.super_terminator(terminator, loc);
+    }
+
+    fn visit_local(&mut self, l: &mut Local, ctxt: PlaceContext, _: Location) {
+        if *l == mir::RETURN_PLACE {
+            assert_eq!(ctxt, PlaceContext::NonUse(NonUseContext::VarDebugInfo));
+        } else if *l == self.to_rename {
+            *l = mir::RETURN_PLACE;
+        }
+    }
+}
+
+struct IsReturnPlaceRead(bool);
+
+impl IsReturnPlaceRead {
+    fn run(body: &mir::Body<'_>) -> bool {
+        let mut vis = IsReturnPlaceRead(false);
+        vis.visit_body(body);
+        vis.0
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for IsReturnPlaceRead {
+    fn visit_local(&mut self, l: Local, ctxt: PlaceContext, _: Location) {
+        if l == mir::RETURN_PLACE && ctxt.is_use() && !ctxt.is_place_assignment() {
+            self.0 = true;
+        }
+    }
+
+    fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, loc: Location) {
+        // Ignore the implicit "use" of the return place in a `Return` statement.
+        if let mir::TerminatorKind::Return = terminator.kind {
+            return;
+        }
+
+        self.super_terminator(terminator, loc);
+    }
+}

tests/mir-opt/dest-prop/union.main.DestinationPropagation.panic-abort.diff

@@ -18,7 +18,7 @@
       bb0: {
           StorageLive(_1);
           StorageLive(_2);
-          nop;
+          _2 = const 1_u32;
           _1 = Un { us: const 1_u32 };
           StorageDead(_2);
           StorageLive(_3);

tests/mir-opt/dest-prop/union.main.DestinationPropagation.panic-unwind.diff

@@ -18,7 +18,7 @@
       bb0: {
           StorageLive(_1);
           StorageLive(_2);
-          nop;
+          _2 = const 1_u32;
           _1 = Un { us: const 1_u32 };
           StorageDead(_2);
           StorageLive(_3);

tests/mir-opt/nrvo_miscompile_111005.rs

@@ -1,13 +1,13 @@
 // skip-filecheck
 // This is a miscompilation, #111005 to track
 
-//@ test-mir-pass: DestinationPropagation
+//@ test-mir-pass: RenameReturnPlace
 
 #![feature(custom_mir, core_intrinsics)]
 extern crate core;
 use core::intrinsics::mir::*;
 
-// EMIT_MIR nrvo_miscompile_111005.wrong.DestinationPropagation.diff
+// EMIT_MIR nrvo_miscompile_111005.wrong.RenameReturnPlace.diff
 #[custom_mir(dialect = "runtime", phase = "initial")]
 pub fn wrong(arg: char) -> char {
     mir!({

tests/mir-opt/nrvo_miscompile_111005.wrong.RenameReturnPlace.diff

@@ -1,5 +1,5 @@
-- // MIR for `wrong` before DestinationPropagation
-+ // MIR for `wrong` after DestinationPropagation
+- // MIR for `wrong` before RenameReturnPlace
++ // MIR for `wrong` after RenameReturnPlace
   
   fn wrong(_1: char) -> char {
       let mut _0: char;
@@ -9,9 +9,8 @@
 -         _2 = _1;
 -         _0 = _2;
 -         _2 = const 'b';
-+         nop;
 +         _0 = _1;
-+         _1 = const 'b';
++         _0 = const 'b';
           return;
       }
   }

tests/mir-opt/nrvo_simple.nrvo.RenameReturnPlace.panic-abort.diff

@@ -1,5 +1,5 @@
-- // MIR for `nrvo` before DestinationPropagation
-+ // MIR for `nrvo` after DestinationPropagation
+- // MIR for `nrvo` before RenameReturnPlace
++ // MIR for `nrvo` after RenameReturnPlace
   
   fn nrvo(_1: for<'a> fn(&'a mut [u8; 1024])) -> [u8; 1024] {
       debug init => _1;
@@ -10,33 +10,32 @@
       let mut _5: &mut [u8; 1024];
       let mut _6: &mut [u8; 1024];
       scope 1 {
-          debug buf => _2;
+-         debug buf => _2;
++         debug buf => _0;
       }
   
       bb0: {
-          StorageLive(_2);
-          _2 = [const 0_u8; 1024];
+-         StorageLive(_2);
+-         _2 = [const 0_u8; 1024];
++         _0 = [const 0_u8; 1024];
           StorageLive(_3);
--         StorageLive(_4);
--         _4 = _1;
-+         nop;
-+         nop;
+          StorageLive(_4);
+          _4 = _1;
           StorageLive(_5);
           StorageLive(_6);
-          _6 = &mut _2;
+-         _6 = &mut _2;
++         _6 = &mut _0;
           _5 = &mut (*_6);
--         _3 = move _4(move _5) -> [return: bb1, unwind unreachable];
-+         _3 = move _1(move _5) -> [return: bb1, unwind unreachable];
+          _3 = move _4(move _5) -> [return: bb1, unwind unreachable];
       }
   
       bb1: {
           StorageDead(_5);
--         StorageDead(_4);
-+         nop;
+          StorageDead(_4);
           StorageDead(_6);
           StorageDead(_3);
-          _0 = _2;
-          StorageDead(_2);
+-         _0 = _2;
+-         StorageDead(_2);
           return;
       }
   }

tests/mir-opt/nrvo_simple.nrvo.RenameReturnPlace.panic-unwind.diff

@@ -1,5 +1,5 @@
-- // MIR for `nrvo` before DestinationPropagation
-+ // MIR for `nrvo` after DestinationPropagation
+- // MIR for `nrvo` before RenameReturnPlace
++ // MIR for `nrvo` after RenameReturnPlace
   
   fn nrvo(_1: for<'a> fn(&'a mut [u8; 1024])) -> [u8; 1024] {
       debug init => _1;
@@ -10,33 +10,32 @@
       let mut _5: &mut [u8; 1024];
       let mut _6: &mut [u8; 1024];
       scope 1 {
-          debug buf => _2;
+-         debug buf => _2;
++         debug buf => _0;
       }
   
       bb0: {
-          StorageLive(_2);
-          _2 = [const 0_u8; 1024];
+-         StorageLive(_2);
+-         _2 = [const 0_u8; 1024];
++         _0 = [const 0_u8; 1024];
           StorageLive(_3);
--         StorageLive(_4);
--         _4 = _1;
-+         nop;
-+         nop;
+          StorageLive(_4);
+          _4 = _1;
           StorageLive(_5);
           StorageLive(_6);
-          _6 = &mut _2;
+-         _6 = &mut _2;
++         _6 = &mut _0;
           _5 = &mut (*_6);
--         _3 = move _4(move _5) -> [return: bb1, unwind continue];
-+         _3 = move _1(move _5) -> [return: bb1, unwind continue];
+          _3 = move _4(move _5) -> [return: bb1, unwind continue];
       }
   
       bb1: {
           StorageDead(_5);
--         StorageDead(_4);
-+         nop;
+          StorageDead(_4);
           StorageDead(_6);
           StorageDead(_3);
-          _0 = _2;
-          StorageDead(_2);
+-         _0 = _2;
+-         StorageDead(_2);
           return;
       }
   }

tests/mir-opt/nrvo_simple.rs

@@ -1,8 +1,8 @@
 // skip-filecheck
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
-//@ test-mir-pass: DestinationPropagation
+//@ test-mir-pass: RenameReturnPlace
 
-// EMIT_MIR nrvo_borrowed.nrvo.DestinationPropagation.diff
+// EMIT_MIR nrvo_simple.nrvo.RenameReturnPlace.diff
 fn nrvo(init: fn(&mut [u8; 1024])) -> [u8; 1024] {
     let mut buf = [0; 1024];
     init(&mut buf);

tests/mir-opt/pre-codegen/derived_ord.{impl#0}-partial_cmp.PreCodegen.after.mir

@@ -4,9 +4,10 @@ fn <impl at $DIR/derived_ord.rs:6:10: 6:20>::partial_cmp(_1: &MultiField, _2: &M
     debug self => _1;
     debug other => _2;
     let mut _0: std::option::Option<std::cmp::Ordering>;
-    let mut _6: i8;
+    let mut _6: std::option::Option<std::cmp::Ordering>;
+    let mut _7: i8;
     scope 1 {
-        debug cmp => _0;
+        debug cmp => _6;
     }
     scope 2 (inlined std::cmp::impls::<impl PartialOrd for char>::partial_cmp) {
         let mut _3: char;
@@ -14,9 +15,9 @@ fn <impl at $DIR/derived_ord.rs:6:10: 6:20>::partial_cmp(_1: &MultiField, _2: &M
         let mut _5: std::cmp::Ordering;
     }
     scope 3 (inlined std::cmp::impls::<impl PartialOrd for i16>::partial_cmp) {
-        let mut _7: i16;
         let mut _8: i16;
-        let mut _9: std::cmp::Ordering;
+        let mut _9: i16;
+        let mut _10: std::cmp::Ordering;
     }
 
     bb0: {
@@ -27,26 +28,27 @@ fn <impl at $DIR/derived_ord.rs:6:10: 6:20>::partial_cmp(_1: &MultiField, _2: &M
         _5 = Cmp(move _3, move _4);
         StorageDead(_4);
         StorageDead(_3);
-        _0 = Option::<std::cmp::Ordering>::Some(_5);
-        _6 = discriminant(_5);
-        switchInt(move _6) -> [0: bb1, otherwise: bb2];
+        _6 = Option::<std::cmp::Ordering>::Some(_5);
+        _7 = discriminant(_5);
+        switchInt(move _7) -> [0: bb1, otherwise: bb2];
     }
 
     bb1: {
-        StorageLive(_9);
-        StorageLive(_7);
-        _7 = ((*_1).1: i16);
+        StorageLive(_10);
         StorageLive(_8);
-        _8 = ((*_2).1: i16);
-        _9 = Cmp(move _7, move _8);
-        StorageDead(_8);
-        StorageDead(_7);
-        _0 = Option::<std::cmp::Ordering>::Some(move _9);
+        _8 = ((*_1).1: i16);
+        StorageLive(_9);
+        _9 = ((*_2).1: i16);
+        _10 = Cmp(move _8, move _9);
         StorageDead(_9);
+        StorageDead(_8);
+        _0 = Option::<std::cmp::Ordering>::Some(move _10);
+        StorageDead(_10);
         goto -> bb3;
     }
 
     bb2: {
+        _0 = _6;
         goto -> bb3;
     }
 

tests/mir-opt/pre-codegen/range_iter.forward_loop.PreCodegen.after.panic-abort.mir

@@ -5,25 +5,27 @@ fn forward_loop(_1: u32, _2: u32, _3: impl Fn(u32)) -> () {
     debug end => _2;
     debug f => _3;
     let mut _0: ();
-    let mut _7: std::option::Option<u32>;
-    let mut _9: &impl Fn(u32);
-    let mut _10: (u32,);
-    let _11: ();
+    let mut _4: u32;
+    let mut _9: std::option::Option<u32>;
+    let mut _11: &impl Fn(u32);
+    let mut _12: (u32,);
+    let _13: ();
     scope 1 {
-        debug ((iter: std::ops::Range<u32>).0: u32) => _1;
+        debug ((iter: std::ops::Range<u32>).0: u32) => _4;
         debug ((iter: std::ops::Range<u32>).1: u32) => _2;
-        let _8: u32;
+        let _10: u32;
         scope 2 {
-            debug x => _8;
+            debug x => _10;
         }
         scope 4 (inlined iter::range::<impl Iterator for std::ops::Range<u32>>::next) {
             scope 5 (inlined <std::ops::Range<u32> as iter::range::RangeIteratorImpl>::spec_next) {
-                let mut _5: bool;
-                let _6: u32;
+                let mut _6: bool;
+                let _7: u32;
+                let mut _8: u32;
                 scope 6 {
                 }
                 scope 7 (inlined std::cmp::impls::<impl PartialOrd for u32>::lt) {
-                    let mut _4: u32;
+                    let mut _5: u32;
                 }
             }
         }
@@ -32,24 +34,27 @@ fn forward_loop(_1: u32, _2: u32, _3: impl Fn(u32)) -> () {
     }
 
     bb0: {
+        StorageLive(_4);
+        _4 = _1;
         goto -> bb1;
     }
 
     bb1: {
+        StorageLive(_9);
         StorageLive(_7);
         StorageLive(_6);
         StorageLive(_5);
-        StorageLive(_4);
-        _4 = _1;
-        _5 = Lt(move _4, _2);
-        StorageDead(_4);
-        switchInt(move _5) -> [0: bb2, otherwise: bb4];
+        _5 = _4;
+        _6 = Lt(move _5, _2);
+        StorageDead(_5);
+        switchInt(move _6) -> [0: bb2, otherwise: bb4];
     }
 
     bb2: {
-        StorageDead(_5);
         StorageDead(_6);
         StorageDead(_7);
+        StorageDead(_9);
+        StorageDead(_4);
         drop(_3) -> [return: bb3, unwind unreachable];
     }
 
@@ -58,26 +63,29 @@ fn forward_loop(_1: u32, _2: u32, _3: impl Fn(u32)) -> () {
     }
 
     bb4: {
-        _6 = _1;
-        _1 = <u32 as Step>::forward_unchecked(_6, const 1_usize) -> [return: bb5, unwind unreachable];
+        _7 = _4;
+        StorageLive(_8);
+        _8 = <u32 as Step>::forward_unchecked(_7, const 1_usize) -> [return: bb5, unwind unreachable];
     }
 
     bb5: {
-        _7 = Option::<u32>::Some(_6);
-        StorageDead(_5);
+        _4 = move _8;
+        StorageDead(_8);
+        _9 = Option::<u32>::Some(_7);
         StorageDead(_6);
-        _8 = ((_7 as Some).0: u32);
-        StorageLive(_9);
-        _9 = &_3;
-        StorageLive(_10);
-        _10 = (_8,);
-        _11 = <impl Fn(u32) as Fn<(u32,)>>::call(move _9, move _10) -> [return: bb6, unwind unreachable];
+        StorageDead(_7);
+        _10 = ((_9 as Some).0: u32);
+        StorageLive(_11);
+        _11 = &_3;
+        StorageLive(_12);
+        _12 = (_10,);
+        _13 = <impl Fn(u32) as Fn<(u32,)>>::call(move _11, move _12) -> [return: bb6, unwind unreachable];
     }
 
     bb6: {
-        StorageDead(_10);
+        StorageDead(_12);
+        StorageDead(_11);
         StorageDead(_9);
-        StorageDead(_7);
         goto -> bb1;
     }
 }

tests/mir-opt/pre-codegen/range_iter.forward_loop.PreCodegen.after.panic-unwind.mir

@@ -5,25 +5,27 @@ fn forward_loop(_1: u32, _2: u32, _3: impl Fn(u32)) -> () {
     debug end => _2;
     debug f => _3;
     let mut _0: ();
-    let mut _7: std::option::Option<u32>;
-    let mut _9: &impl Fn(u32);
-    let mut _10: (u32,);
-    let _11: ();
+    let mut _4: u32;
+    let mut _9: std::option::Option<u32>;
+    let mut _11: &impl Fn(u32);
+    let mut _12: (u32,);
+    let _13: ();
     scope 1 {
-        debug ((iter: std::ops::Range<u32>).0: u32) => _1;
+        debug ((iter: std::ops::Range<u32>).0: u32) => _4;
         debug ((iter: std::ops::Range<u32>).1: u32) => _2;
-        let _8: u32;
+        let _10: u32;
         scope 2 {
-            debug x => _8;
+            debug x => _10;
         }
         scope 4 (inlined iter::range::<impl Iterator for std::ops::Range<u32>>::next) {
             scope 5 (inlined <std::ops::Range<u32> as iter::range::RangeIteratorImpl>::spec_next) {
-                let mut _5: bool;
-                let _6: u32;
+                let mut _6: bool;
+                let _7: u32;
+                let mut _8: u32;
                 scope 6 {
                 }
                 scope 7 (inlined std::cmp::impls::<impl PartialOrd for u32>::lt) {
-                    let mut _4: u32;
+                    let mut _5: u32;
                 }
             }
         }
@@ -32,24 +34,27 @@ fn forward_loop(_1: u32, _2: u32, _3: impl Fn(u32)) -> () {
     }
 
     bb0: {
+        StorageLive(_4);
+        _4 = _1;
         goto -> bb1;
     }
 
     bb1: {
+        StorageLive(_9);
         StorageLive(_7);
         StorageLive(_6);
         StorageLive(_5);
-        StorageLive(_4);
-        _4 = _1;
-        _5 = Lt(move _4, _2);
-        StorageDead(_4);
-        switchInt(move _5) -> [0: bb2, otherwise: bb4];
+        _5 = _4;
+        _6 = Lt(move _5, _2);
+        StorageDead(_5);
+        switchInt(move _6) -> [0: bb2, otherwise: bb4];
     }
 
     bb2: {
-        StorageDead(_5);
         StorageDead(_6);
         StorageDead(_7);
+        StorageDead(_9);
+        StorageDead(_4);
         drop(_3) -> [return: bb3, unwind continue];
     }
 
@@ -58,26 +63,29 @@ fn forward_loop(_1: u32, _2: u32, _3: impl Fn(u32)) -> () {
     }
 
     bb4: {
-        _6 = _1;
-        _1 = <u32 as Step>::forward_unchecked(_6, const 1_usize) -> [return: bb5, unwind: bb7];
+        _7 = _4;
+        StorageLive(_8);
+        _8 = <u32 as Step>::forward_unchecked(_7, const 1_usize) -> [return: bb5, unwind: bb7];
     }
 
     bb5: {
-        _7 = Option::<u32>::Some(_6);
-        StorageDead(_5);
+        _4 = move _8;
+        StorageDead(_8);
+        _9 = Option::<u32>::Some(_7);
         StorageDead(_6);
-        _8 = ((_7 as Some).0: u32);
-        StorageLive(_9);
-        _9 = &_3;
-        StorageLive(_10);
-        _10 = (_8,);
-        _11 = <impl Fn(u32) as Fn<(u32,)>>::call(move _9, move _10) -> [return: bb6, unwind: bb7];
+        StorageDead(_7);
+        _10 = ((_9 as Some).0: u32);
+        StorageLive(_11);
+        _11 = &_3;
+        StorageLive(_12);
+        _12 = (_10,);
+        _13 = <impl Fn(u32) as Fn<(u32,)>>::call(move _11, move _12) -> [return: bb6, unwind: bb7];
     }
 
     bb6: {
-        StorageDead(_10);
+        StorageDead(_12);
+        StorageDead(_11);
         StorageDead(_9);
-        StorageDead(_7);
         goto -> bb1;
     }
 

tests/mir-opt/pre-codegen/slice_iter.enumerated_loop.PreCodegen.after.panic-abort.mir

@@ -4,22 +4,22 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     debug slice => _1;
     debug f => _2;
     let mut _0: ();
-    let mut _11: std::slice::Iter<'_, T>;
-    let mut _12: std::iter::Enumerate<std::slice::Iter<'_, T>>;
+    let mut _12: std::slice::Iter<'_, T>;
     let mut _13: std::iter::Enumerate<std::slice::Iter<'_, T>>;
-    let mut _14: &mut std::iter::Enumerate<std::slice::Iter<'_, T>>;
-    let mut _15: std::option::Option<(usize, &T)>;
-    let mut _16: isize;
-    let mut _19: &impl Fn(usize, &T);
-    let mut _20: (usize, &T);
-    let _21: ();
+    let mut _14: std::iter::Enumerate<std::slice::Iter<'_, T>>;
+    let mut _15: &mut std::iter::Enumerate<std::slice::Iter<'_, T>>;
+    let mut _16: std::option::Option<(usize, &T)>;
+    let mut _17: isize;
+    let mut _20: &impl Fn(usize, &T);
+    let mut _21: (usize, &T);
+    let _22: ();
     scope 1 {
-        debug iter => _13;
-        let _17: usize;
-        let _18: &T;
+        debug iter => _14;
+        let _18: usize;
+        let _19: &T;
         scope 2 {
-            debug i => _17;
-            debug x => _18;
+            debug i => _18;
+            debug x => _19;
         }
     }
     scope 3 (inlined core::slice::<impl [T]>::iter) {
@@ -28,10 +28,11 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
             let mut _7: bool;
             let mut _8: *mut T;
             let mut _9: *mut T;
-            let mut _10: *const T;
+            let mut _11: *const T;
             scope 5 {
                 let _6: std::ptr::NonNull<T>;
                 scope 6 {
+                    let _10: *const T;
                     scope 7 {
                     }
                     scope 11 (inlined without_provenance::<T>) {
@@ -60,16 +61,16 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     }
 
     bb0: {
-        StorageLive(_11);
+        StorageLive(_12);
         StorageLive(_3);
         StorageLive(_6);
-        StorageLive(_10);
         StorageLive(_4);
         StorageLive(_5);
         _3 = Len((*_1));
         _4 = &raw const (*_1);
         _5 = _4 as *const T (PtrToPtr);
         _6 = NonNull::<T> { pointer: _5 };
+        StorageLive(_10);
         StorageLive(_7);
         _7 = const <T as std::mem::SizedTypeProperties>::IS_ZST;
         switchInt(move _7) -> [0: bb1, otherwise: bb2];
@@ -93,35 +94,38 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
 
     bb3: {
         StorageDead(_7);
-        _11 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _10, _marker: const ZeroSized: PhantomData<&T> };
+        StorageLive(_11);
+        _11 = _10;
+        _12 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _11, _marker: const ZeroSized: PhantomData<&T> };
+        StorageDead(_11);
+        StorageDead(_10);
         StorageDead(_5);
         StorageDead(_4);
-        StorageDead(_10);
         StorageDead(_6);
         StorageDead(_3);
-        _12 = Enumerate::<std::slice::Iter<'_, T>> { iter: _11, count: const 0_usize };
-        StorageDead(_11);
-        StorageLive(_13);
-        _13 = _12;
+        _13 = Enumerate::<std::slice::Iter<'_, T>> { iter: _12, count: const 0_usize };
+        StorageDead(_12);
+        StorageLive(_14);
+        _14 = _13;
         goto -> bb4;
     }
 
     bb4: {
+        StorageLive(_16);
         StorageLive(_15);
-        StorageLive(_14);
-        _14 = &mut _13;
-        _15 = <Enumerate<std::slice::Iter<'_, T>> as Iterator>::next(move _14) -> [return: bb5, unwind unreachable];
+        _15 = &mut _14;
+        _16 = <Enumerate<std::slice::Iter<'_, T>> as Iterator>::next(move _15) -> [return: bb5, unwind unreachable];
     }
 
     bb5: {
-        StorageDead(_14);
-        _16 = discriminant(_15);
-        switchInt(move _16) -> [0: bb6, 1: bb8, otherwise: bb10];
+        StorageDead(_15);
+        _17 = discriminant(_16);
+        switchInt(move _17) -> [0: bb6, 1: bb8, otherwise: bb10];
     }
 
     bb6: {
-        StorageDead(_15);
-        StorageDead(_13);
+        StorageDead(_16);
+        StorageDead(_14);
         drop(_2) -> [return: bb7, unwind unreachable];
     }
 
@@ -130,19 +134,19 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     }
 
     bb8: {
-        _17 = (((_15 as Some).0: (usize, &T)).0: usize);
-        _18 = (((_15 as Some).0: (usize, &T)).1: &T);
-        StorageLive(_19);
-        _19 = &_2;
+        _18 = (((_16 as Some).0: (usize, &T)).0: usize);
+        _19 = (((_16 as Some).0: (usize, &T)).1: &T);
         StorageLive(_20);
-        _20 = (_17, _18);
-        _21 = <impl Fn(usize, &T) as Fn<(usize, &T)>>::call(move _19, move _20) -> [return: bb9, unwind unreachable];
+        _20 = &_2;
+        StorageLive(_21);
+        _21 = (_18, _19);
+        _22 = <impl Fn(usize, &T) as Fn<(usize, &T)>>::call(move _20, move _21) -> [return: bb9, unwind unreachable];
     }
 
     bb9: {
+        StorageDead(_21);
         StorageDead(_20);
-        StorageDead(_19);
-        StorageDead(_15);
+        StorageDead(_16);
         goto -> bb4;
     }
 

tests/mir-opt/pre-codegen/slice_iter.enumerated_loop.PreCodegen.after.panic-unwind.mir

@@ -4,22 +4,22 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     debug slice => _1;
     debug f => _2;
     let mut _0: ();
-    let mut _11: std::slice::Iter<'_, T>;
-    let mut _12: std::iter::Enumerate<std::slice::Iter<'_, T>>;
+    let mut _12: std::slice::Iter<'_, T>;
     let mut _13: std::iter::Enumerate<std::slice::Iter<'_, T>>;
-    let mut _14: &mut std::iter::Enumerate<std::slice::Iter<'_, T>>;
-    let mut _15: std::option::Option<(usize, &T)>;
-    let mut _16: isize;
-    let mut _19: &impl Fn(usize, &T);
-    let mut _20: (usize, &T);
-    let _21: ();
+    let mut _14: std::iter::Enumerate<std::slice::Iter<'_, T>>;
+    let mut _15: &mut std::iter::Enumerate<std::slice::Iter<'_, T>>;
+    let mut _16: std::option::Option<(usize, &T)>;
+    let mut _17: isize;
+    let mut _20: &impl Fn(usize, &T);
+    let mut _21: (usize, &T);
+    let _22: ();
     scope 1 {
-        debug iter => _13;
-        let _17: usize;
-        let _18: &T;
+        debug iter => _14;
+        let _18: usize;
+        let _19: &T;
         scope 2 {
-            debug i => _17;
-            debug x => _18;
+            debug i => _18;
+            debug x => _19;
         }
     }
     scope 3 (inlined core::slice::<impl [T]>::iter) {
@@ -28,10 +28,11 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
             let mut _7: bool;
             let mut _8: *mut T;
             let mut _9: *mut T;
-            let mut _10: *const T;
+            let mut _11: *const T;
             scope 5 {
                 let _6: std::ptr::NonNull<T>;
                 scope 6 {
+                    let _10: *const T;
                     scope 7 {
                     }
                     scope 11 (inlined without_provenance::<T>) {
@@ -60,16 +61,16 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     }
 
     bb0: {
-        StorageLive(_11);
+        StorageLive(_12);
         StorageLive(_3);
         StorageLive(_6);
-        StorageLive(_10);
         StorageLive(_4);
         StorageLive(_5);
         _3 = Len((*_1));
         _4 = &raw const (*_1);
         _5 = _4 as *const T (PtrToPtr);
         _6 = NonNull::<T> { pointer: _5 };
+        StorageLive(_10);
         StorageLive(_7);
         _7 = const <T as std::mem::SizedTypeProperties>::IS_ZST;
         switchInt(move _7) -> [0: bb1, otherwise: bb2];
@@ -93,35 +94,38 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
 
     bb3: {
         StorageDead(_7);
-        _11 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _10, _marker: const ZeroSized: PhantomData<&T> };
+        StorageLive(_11);
+        _11 = _10;
+        _12 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _11, _marker: const ZeroSized: PhantomData<&T> };
+        StorageDead(_11);
+        StorageDead(_10);
         StorageDead(_5);
         StorageDead(_4);
-        StorageDead(_10);
         StorageDead(_6);
         StorageDead(_3);
-        _12 = Enumerate::<std::slice::Iter<'_, T>> { iter: _11, count: const 0_usize };
-        StorageDead(_11);
-        StorageLive(_13);
-        _13 = _12;
+        _13 = Enumerate::<std::slice::Iter<'_, T>> { iter: _12, count: const 0_usize };
+        StorageDead(_12);
+        StorageLive(_14);
+        _14 = _13;
         goto -> bb4;
     }
 
     bb4: {
+        StorageLive(_16);
         StorageLive(_15);
-        StorageLive(_14);
-        _14 = &mut _13;
-        _15 = <Enumerate<std::slice::Iter<'_, T>> as Iterator>::next(move _14) -> [return: bb5, unwind: bb11];
+        _15 = &mut _14;
+        _16 = <Enumerate<std::slice::Iter<'_, T>> as Iterator>::next(move _15) -> [return: bb5, unwind: bb11];
     }
 
     bb5: {
-        StorageDead(_14);
-        _16 = discriminant(_15);
-        switchInt(move _16) -> [0: bb6, 1: bb8, otherwise: bb10];
+        StorageDead(_15);
+        _17 = discriminant(_16);
+        switchInt(move _17) -> [0: bb6, 1: bb8, otherwise: bb10];
     }
 
     bb6: {
-        StorageDead(_15);
-        StorageDead(_13);
+        StorageDead(_16);
+        StorageDead(_14);
         drop(_2) -> [return: bb7, unwind continue];
     }
 
@@ -130,19 +134,19 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     }
 
     bb8: {
-        _17 = (((_15 as Some).0: (usize, &T)).0: usize);
-        _18 = (((_15 as Some).0: (usize, &T)).1: &T);
-        StorageLive(_19);
-        _19 = &_2;
+        _18 = (((_16 as Some).0: (usize, &T)).0: usize);
+        _19 = (((_16 as Some).0: (usize, &T)).1: &T);
         StorageLive(_20);
-        _20 = (_17, _18);
-        _21 = <impl Fn(usize, &T) as Fn<(usize, &T)>>::call(move _19, move _20) -> [return: bb9, unwind: bb11];
+        _20 = &_2;
+        StorageLive(_21);
+        _21 = (_18, _19);
+        _22 = <impl Fn(usize, &T) as Fn<(usize, &T)>>::call(move _20, move _21) -> [return: bb9, unwind: bb11];
     }
 
     bb9: {
+        StorageDead(_21);
         StorageDead(_20);
-        StorageDead(_19);
-        StorageDead(_15);
+        StorageDead(_16);
         goto -> bb4;
     }
 

tests/mir-opt/pre-codegen/slice_iter.forward_loop.PreCodegen.after.panic-abort.mir

@@ -4,19 +4,19 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     debug slice => _1;
     debug f => _2;
     let mut _0: ();
-    let mut _11: std::slice::Iter<'_, T>;
     let mut _12: std::slice::Iter<'_, T>;
-    let mut _13: &mut std::slice::Iter<'_, T>;
-    let mut _14: std::option::Option<&T>;
-    let mut _15: isize;
-    let mut _17: &impl Fn(&T);
-    let mut _18: (&T,);
-    let _19: ();
+    let mut _13: std::slice::Iter<'_, T>;
+    let mut _14: &mut std::slice::Iter<'_, T>;
+    let mut _15: std::option::Option<&T>;
+    let mut _16: isize;
+    let mut _18: &impl Fn(&T);
+    let mut _19: (&T,);
+    let _20: ();
     scope 1 {
-        debug iter => _12;
-        let _16: &T;
+        debug iter => _13;
+        let _17: &T;
         scope 2 {
-            debug x => _16;
+            debug x => _17;
         }
     }
     scope 3 (inlined core::slice::<impl [T]>::iter) {
@@ -25,10 +25,11 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
             let mut _7: bool;
             let mut _8: *mut T;
             let mut _9: *mut T;
-            let mut _10: *const T;
+            let mut _11: *const T;
             scope 5 {
                 let _6: std::ptr::NonNull<T>;
                 scope 6 {
+                    let _10: *const T;
                     scope 7 {
                     }
                     scope 11 (inlined without_provenance::<T>) {
@@ -55,13 +56,13 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     bb0: {
         StorageLive(_3);
         StorageLive(_6);
-        StorageLive(_10);
         StorageLive(_4);
         StorageLive(_5);
         _3 = Len((*_1));
         _4 = &raw const (*_1);
         _5 = _4 as *const T (PtrToPtr);
         _6 = NonNull::<T> { pointer: _5 };
+        StorageLive(_10);
         StorageLive(_7);
         _7 = const <T as std::mem::SizedTypeProperties>::IS_ZST;
         switchInt(move _7) -> [0: bb1, otherwise: bb2];
@@ -85,33 +86,36 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
 
     bb3: {
         StorageDead(_7);
-        _11 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _10, _marker: const ZeroSized: PhantomData<&T> };
+        StorageLive(_11);
+        _11 = _10;
+        _12 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _11, _marker: const ZeroSized: PhantomData<&T> };
+        StorageDead(_11);
+        StorageDead(_10);
         StorageDead(_5);
         StorageDead(_4);
-        StorageDead(_10);
         StorageDead(_6);
         StorageDead(_3);
-        StorageLive(_12);
-        _12 = _11;
+        StorageLive(_13);
+        _13 = _12;
         goto -> bb4;
     }
 
     bb4: {
+        StorageLive(_15);
         StorageLive(_14);
-        StorageLive(_13);
-        _13 = &mut _12;
-        _14 = <std::slice::Iter<'_, T> as Iterator>::next(move _13) -> [return: bb5, unwind unreachable];
+        _14 = &mut _13;
+        _15 = <std::slice::Iter<'_, T> as Iterator>::next(move _14) -> [return: bb5, unwind unreachable];
     }
 
     bb5: {
-        StorageDead(_13);
-        _15 = discriminant(_14);
-        switchInt(move _15) -> [0: bb6, 1: bb8, otherwise: bb10];
+        StorageDead(_14);
+        _16 = discriminant(_15);
+        switchInt(move _16) -> [0: bb6, 1: bb8, otherwise: bb10];
     }
 
     bb6: {
-        StorageDead(_14);
-        StorageDead(_12);
+        StorageDead(_15);
+        StorageDead(_13);
         drop(_2) -> [return: bb7, unwind unreachable];
     }
 
@@ -120,18 +124,18 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     }
 
     bb8: {
-        _16 = ((_14 as Some).0: &T);
-        StorageLive(_17);
-        _17 = &_2;
+        _17 = ((_15 as Some).0: &T);
         StorageLive(_18);
-        _18 = (_16,);
-        _19 = <impl Fn(&T) as Fn<(&T,)>>::call(move _17, move _18) -> [return: bb9, unwind unreachable];
+        _18 = &_2;
+        StorageLive(_19);
+        _19 = (_17,);
+        _20 = <impl Fn(&T) as Fn<(&T,)>>::call(move _18, move _19) -> [return: bb9, unwind unreachable];
     }
 
     bb9: {
+        StorageDead(_19);
         StorageDead(_18);
-        StorageDead(_17);
-        StorageDead(_14);
+        StorageDead(_15);
         goto -> bb4;
     }
 

tests/mir-opt/pre-codegen/slice_iter.forward_loop.PreCodegen.after.panic-unwind.mir

@@ -4,19 +4,19 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     debug slice => _1;
     debug f => _2;
     let mut _0: ();
-    let mut _11: std::slice::Iter<'_, T>;
     let mut _12: std::slice::Iter<'_, T>;
-    let mut _13: &mut std::slice::Iter<'_, T>;
-    let mut _14: std::option::Option<&T>;
-    let mut _15: isize;
-    let mut _17: &impl Fn(&T);
-    let mut _18: (&T,);
-    let _19: ();
+    let mut _13: std::slice::Iter<'_, T>;
+    let mut _14: &mut std::slice::Iter<'_, T>;
+    let mut _15: std::option::Option<&T>;
+    let mut _16: isize;
+    let mut _18: &impl Fn(&T);
+    let mut _19: (&T,);
+    let _20: ();
     scope 1 {
-        debug iter => _12;
-        let _16: &T;
+        debug iter => _13;
+        let _17: &T;
         scope 2 {
-            debug x => _16;
+            debug x => _17;
         }
     }
     scope 3 (inlined core::slice::<impl [T]>::iter) {
@@ -25,10 +25,11 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
             let mut _7: bool;
             let mut _8: *mut T;
             let mut _9: *mut T;
-            let mut _10: *const T;
+            let mut _11: *const T;
             scope 5 {
                 let _6: std::ptr::NonNull<T>;
                 scope 6 {
+                    let _10: *const T;
                     scope 7 {
                     }
                     scope 11 (inlined without_provenance::<T>) {
@@ -55,13 +56,13 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     bb0: {
         StorageLive(_3);
         StorageLive(_6);
-        StorageLive(_10);
         StorageLive(_4);
         StorageLive(_5);
         _3 = Len((*_1));
         _4 = &raw const (*_1);
         _5 = _4 as *const T (PtrToPtr);
         _6 = NonNull::<T> { pointer: _5 };
+        StorageLive(_10);
         StorageLive(_7);
         _7 = const <T as std::mem::SizedTypeProperties>::IS_ZST;
         switchInt(move _7) -> [0: bb1, otherwise: bb2];
@@ -85,33 +86,36 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
 
     bb3: {
         StorageDead(_7);
-        _11 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _10, _marker: const ZeroSized: PhantomData<&T> };
+        StorageLive(_11);
+        _11 = _10;
+        _12 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _11, _marker: const ZeroSized: PhantomData<&T> };
+        StorageDead(_11);
+        StorageDead(_10);
         StorageDead(_5);
         StorageDead(_4);
-        StorageDead(_10);
         StorageDead(_6);
         StorageDead(_3);
-        StorageLive(_12);
-        _12 = _11;
+        StorageLive(_13);
+        _13 = _12;
         goto -> bb4;
     }
 
     bb4: {
+        StorageLive(_15);
         StorageLive(_14);
-        StorageLive(_13);
-        _13 = &mut _12;
-        _14 = <std::slice::Iter<'_, T> as Iterator>::next(move _13) -> [return: bb5, unwind: bb11];
+        _14 = &mut _13;
+        _15 = <std::slice::Iter<'_, T> as Iterator>::next(move _14) -> [return: bb5, unwind: bb11];
     }
 
     bb5: {
-        StorageDead(_13);
-        _15 = discriminant(_14);
-        switchInt(move _15) -> [0: bb6, 1: bb8, otherwise: bb10];
+        StorageDead(_14);
+        _16 = discriminant(_15);
+        switchInt(move _16) -> [0: bb6, 1: bb8, otherwise: bb10];
     }
 
     bb6: {
-        StorageDead(_14);
-        StorageDead(_12);
+        StorageDead(_15);
+        StorageDead(_13);
         drop(_2) -> [return: bb7, unwind continue];
     }
 
@@ -120,18 +124,18 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     }
 
     bb8: {
-        _16 = ((_14 as Some).0: &T);
-        StorageLive(_17);
-        _17 = &_2;
+        _17 = ((_15 as Some).0: &T);
         StorageLive(_18);
-        _18 = (_16,);
-        _19 = <impl Fn(&T) as Fn<(&T,)>>::call(move _17, move _18) -> [return: bb9, unwind: bb11];
+        _18 = &_2;
+        StorageLive(_19);
+        _19 = (_17,);
+        _20 = <impl Fn(&T) as Fn<(&T,)>>::call(move _18, move _19) -> [return: bb9, unwind: bb11];
     }
 
     bb9: {
+        StorageDead(_19);
         StorageDead(_18);
-        StorageDead(_17);
-        StorageDead(_14);
+        StorageDead(_15);
         goto -> bb4;
     }
 

tests/mir-opt/pre-codegen/slice_iter.range_loop.PreCodegen.after.panic-abort.mir

@@ -6,27 +6,28 @@ fn range_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     let mut _0: ();
     let mut _3: usize;
     let mut _4: usize;
-    let mut _8: std::option::Option<usize>;
-    let mut _10: usize;
-    let mut _11: bool;
-    let mut _13: &impl Fn(usize, &T);
-    let mut _14: (usize, &T);
-    let _15: ();
+    let mut _9: std::option::Option<usize>;
+    let mut _11: usize;
+    let mut _12: bool;
+    let mut _14: &impl Fn(usize, &T);
+    let mut _15: (usize, &T);
+    let _16: ();
     scope 1 {
         debug ((iter: std::ops::Range<usize>).0: usize) => _4;
         debug ((iter: std::ops::Range<usize>).1: usize) => _3;
-        let _9: usize;
+        let _10: usize;
         scope 2 {
-            debug i => _9;
-            let _12: &T;
+            debug i => _10;
+            let _13: &T;
             scope 3 {
-                debug x => _12;
+                debug x => _13;
             }
         }
         scope 5 (inlined iter::range::<impl Iterator for std::ops::Range<usize>>::next) {
             scope 6 (inlined <std::ops::Range<usize> as iter::range::RangeIteratorImpl>::spec_next) {
                 let mut _6: bool;
                 let _7: usize;
+                let mut _8: usize;
                 scope 7 {
                 }
                 scope 8 (inlined std::cmp::impls::<impl PartialOrd for usize>::lt) {
@@ -40,12 +41,13 @@ fn range_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
 
     bb0: {
         _3 = Len((*_1));
+        StorageLive(_4);
         _4 = const 0_usize;
         goto -> bb1;
     }
 
     bb1: {
-        StorageLive(_8);
+        StorageLive(_9);
         StorageLive(_7);
         StorageLive(_6);
         StorageLive(_5);
@@ -58,7 +60,8 @@ fn range_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     bb2: {
         StorageDead(_6);
         StorageDead(_7);
-        StorageDead(_8);
+        StorageDead(_9);
+        StorageDead(_4);
         drop(_2) -> [return: bb3, unwind unreachable];
     }
 
@@ -68,32 +71,35 @@ fn range_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
 
     bb4: {
         _7 = _4;
-        _4 = <usize as Step>::forward_unchecked(_7, const 1_usize) -> [return: bb5, unwind unreachable];
+        StorageLive(_8);
+        _8 = <usize as Step>::forward_unchecked(_7, const 1_usize) -> [return: bb5, unwind unreachable];
     }
 
     bb5: {
-        _8 = Option::<usize>::Some(_7);
+        _4 = move _8;
+        StorageDead(_8);
+        _9 = Option::<usize>::Some(_7);
         StorageDead(_6);
         StorageDead(_7);
-        _9 = ((_8 as Some).0: usize);
-        _10 = Len((*_1));
-        _11 = Lt(_9, _10);
-        assert(move _11, "index out of bounds: the length is {} but the index is {}", move _10, _9) -> [success: bb6, unwind unreachable];
+        _10 = ((_9 as Some).0: usize);
+        _11 = Len((*_1));
+        _12 = Lt(_10, _11);
+        assert(move _12, "index out of bounds: the length is {} but the index is {}", move _11, _10) -> [success: bb6, unwind unreachable];
     }
 
     bb6: {
-        _12 = &(*_1)[_9];
-        StorageLive(_13);
-        _13 = &_2;
+        _13 = &(*_1)[_10];
         StorageLive(_14);
-        _14 = (_9, _12);
-        _15 = <impl Fn(usize, &T) as Fn<(usize, &T)>>::call(move _13, move _14) -> [return: bb7, unwind unreachable];
+        _14 = &_2;
+        StorageLive(_15);
+        _15 = (_10, _13);
+        _16 = <impl Fn(usize, &T) as Fn<(usize, &T)>>::call(move _14, move _15) -> [return: bb7, unwind unreachable];
     }
 
     bb7: {
+        StorageDead(_15);
         StorageDead(_14);
-        StorageDead(_13);
-        StorageDead(_8);
+        StorageDead(_9);
         goto -> bb1;
     }
 }

tests/mir-opt/pre-codegen/slice_iter.range_loop.PreCodegen.after.panic-unwind.mir

@@ -6,27 +6,28 @@ fn range_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     let mut _0: ();
     let mut _3: usize;
     let mut _4: usize;
-    let mut _8: std::option::Option<usize>;
-    let mut _10: usize;
-    let mut _11: bool;
-    let mut _13: &impl Fn(usize, &T);
-    let mut _14: (usize, &T);
-    let _15: ();
+    let mut _9: std::option::Option<usize>;
+    let mut _11: usize;
+    let mut _12: bool;
+    let mut _14: &impl Fn(usize, &T);
+    let mut _15: (usize, &T);
+    let _16: ();
     scope 1 {
         debug ((iter: std::ops::Range<usize>).0: usize) => _4;
         debug ((iter: std::ops::Range<usize>).1: usize) => _3;
-        let _9: usize;
+        let _10: usize;
         scope 2 {
-            debug i => _9;
-            let _12: &T;
+            debug i => _10;
+            let _13: &T;
             scope 3 {
-                debug x => _12;
+                debug x => _13;
             }
         }
         scope 5 (inlined iter::range::<impl Iterator for std::ops::Range<usize>>::next) {
             scope 6 (inlined <std::ops::Range<usize> as iter::range::RangeIteratorImpl>::spec_next) {
                 let mut _6: bool;
                 let _7: usize;
+                let mut _8: usize;
                 scope 7 {
                 }
                 scope 8 (inlined std::cmp::impls::<impl PartialOrd for usize>::lt) {
@@ -40,12 +41,13 @@ fn range_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
 
     bb0: {
         _3 = Len((*_1));
+        StorageLive(_4);
         _4 = const 0_usize;
         goto -> bb1;
     }
 
     bb1: {
-        StorageLive(_8);
+        StorageLive(_9);
         StorageLive(_7);
         StorageLive(_6);
         StorageLive(_5);
@@ -58,7 +60,8 @@ fn range_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
     bb2: {
         StorageDead(_6);
         StorageDead(_7);
-        StorageDead(_8);
+        StorageDead(_9);
+        StorageDead(_4);
         drop(_2) -> [return: bb3, unwind continue];
     }
 
@@ -68,32 +71,35 @@ fn range_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
 
     bb4: {
         _7 = _4;
-        _4 = <usize as Step>::forward_unchecked(_7, const 1_usize) -> [return: bb5, unwind: bb8];
+        StorageLive(_8);
+        _8 = <usize as Step>::forward_unchecked(_7, const 1_usize) -> [return: bb5, unwind: bb8];
     }
 
     bb5: {
-        _8 = Option::<usize>::Some(_7);
+        _4 = move _8;
+        StorageDead(_8);
+        _9 = Option::<usize>::Some(_7);
         StorageDead(_6);
         StorageDead(_7);
-        _9 = ((_8 as Some).0: usize);
-        _10 = Len((*_1));
-        _11 = Lt(_9, _10);
-        assert(move _11, "index out of bounds: the length is {} but the index is {}", move _10, _9) -> [success: bb6, unwind: bb8];
+        _10 = ((_9 as Some).0: usize);
+        _11 = Len((*_1));
+        _12 = Lt(_10, _11);
+        assert(move _12, "index out of bounds: the length is {} but the index is {}", move _11, _10) -> [success: bb6, unwind: bb8];
     }
 
     bb6: {
-        _12 = &(*_1)[_9];
-        StorageLive(_13);
-        _13 = &_2;
+        _13 = &(*_1)[_10];
         StorageLive(_14);
-        _14 = (_9, _12);
-        _15 = <impl Fn(usize, &T) as Fn<(usize, &T)>>::call(move _13, move _14) -> [return: bb7, unwind: bb8];
+        _14 = &_2;
+        StorageLive(_15);
+        _15 = (_10, _13);
+        _16 = <impl Fn(usize, &T) as Fn<(usize, &T)>>::call(move _14, move _15) -> [return: bb7, unwind: bb8];
     }
 
     bb7: {
+        StorageDead(_15);
         StorageDead(_14);
-        StorageDead(_13);
-        StorageDead(_8);
+        StorageDead(_9);
         goto -> bb1;
     }
 

tests/mir-opt/pre-codegen/slice_iter.reverse_loop.PreCodegen.after.panic-abort.mir

@@ -4,22 +4,22 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     debug slice => _1;
     debug f => _2;
     let mut _0: ();
-    let mut _11: std::slice::Iter<'_, T>;
-    let mut _12: std::iter::Rev<std::slice::Iter<'_, T>>;
+    let mut _12: std::slice::Iter<'_, T>;
     let mut _13: std::iter::Rev<std::slice::Iter<'_, T>>;
-    let mut _15: std::option::Option<&T>;
-    let mut _16: isize;
-    let mut _18: &impl Fn(&T);
-    let mut _19: (&T,);
-    let _20: ();
+    let mut _14: std::iter::Rev<std::slice::Iter<'_, T>>;
+    let mut _16: std::option::Option<&T>;
+    let mut _17: isize;
+    let mut _19: &impl Fn(&T);
+    let mut _20: (&T,);
+    let _21: ();
     scope 1 {
-        debug iter => _13;
-        let _17: &T;
+        debug iter => _14;
+        let _18: &T;
         scope 2 {
-            debug x => _17;
+            debug x => _18;
         }
         scope 17 (inlined <Rev<std::slice::Iter<'_, T>> as Iterator>::next) {
-            let mut _14: &mut std::slice::Iter<'_, T>;
+            let mut _15: &mut std::slice::Iter<'_, T>;
         }
     }
     scope 3 (inlined core::slice::<impl [T]>::iter) {
@@ -28,10 +28,11 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
             let mut _7: bool;
             let mut _8: *mut T;
             let mut _9: *mut T;
-            let mut _10: *const T;
+            let mut _11: *const T;
             scope 5 {
                 let _6: std::ptr::NonNull<T>;
                 scope 6 {
+                    let _10: *const T;
                     scope 7 {
                     }
                     scope 11 (inlined without_provenance::<T>) {
@@ -60,16 +61,16 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     }
 
     bb0: {
-        StorageLive(_11);
+        StorageLive(_12);
         StorageLive(_3);
         StorageLive(_6);
-        StorageLive(_10);
         StorageLive(_4);
         StorageLive(_5);
         _3 = Len((*_1));
         _4 = &raw const (*_1);
         _5 = _4 as *const T (PtrToPtr);
         _6 = NonNull::<T> { pointer: _5 };
+        StorageLive(_10);
         StorageLive(_7);
         _7 = const <T as std::mem::SizedTypeProperties>::IS_ZST;
         switchInt(move _7) -> [0: bb1, otherwise: bb2];
@@ -93,35 +94,38 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
 
     bb3: {
         StorageDead(_7);
-        _11 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _10, _marker: const ZeroSized: PhantomData<&T> };
+        StorageLive(_11);
+        _11 = _10;
+        _12 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _11, _marker: const ZeroSized: PhantomData<&T> };
+        StorageDead(_11);
+        StorageDead(_10);
         StorageDead(_5);
         StorageDead(_4);
-        StorageDead(_10);
         StorageDead(_6);
         StorageDead(_3);
-        _12 = Rev::<std::slice::Iter<'_, T>> { iter: _11 };
-        StorageDead(_11);
-        StorageLive(_13);
-        _13 = _12;
+        _13 = Rev::<std::slice::Iter<'_, T>> { iter: _12 };
+        StorageDead(_12);
+        StorageLive(_14);
+        _14 = _13;
         goto -> bb4;
     }
 
     bb4: {
+        StorageLive(_16);
         StorageLive(_15);
-        StorageLive(_14);
-        _14 = &mut (_13.0: std::slice::Iter<'_, T>);
-        _15 = <std::slice::Iter<'_, T> as DoubleEndedIterator>::next_back(move _14) -> [return: bb5, unwind unreachable];
+        _15 = &mut (_14.0: std::slice::Iter<'_, T>);
+        _16 = <std::slice::Iter<'_, T> as DoubleEndedIterator>::next_back(move _15) -> [return: bb5, unwind unreachable];
     }
 
     bb5: {
-        StorageDead(_14);
-        _16 = discriminant(_15);
-        switchInt(move _16) -> [0: bb6, 1: bb8, otherwise: bb10];
+        StorageDead(_15);
+        _17 = discriminant(_16);
+        switchInt(move _17) -> [0: bb6, 1: bb8, otherwise: bb10];
     }
 
     bb6: {
-        StorageDead(_15);
-        StorageDead(_13);
+        StorageDead(_16);
+        StorageDead(_14);
         drop(_2) -> [return: bb7, unwind unreachable];
     }
 
@@ -130,18 +134,18 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     }
 
     bb8: {
-        _17 = ((_15 as Some).0: &T);
-        StorageLive(_18);
-        _18 = &_2;
+        _18 = ((_16 as Some).0: &T);
         StorageLive(_19);
-        _19 = (_17,);
-        _20 = <impl Fn(&T) as Fn<(&T,)>>::call(move _18, move _19) -> [return: bb9, unwind unreachable];
+        _19 = &_2;
+        StorageLive(_20);
+        _20 = (_18,);
+        _21 = <impl Fn(&T) as Fn<(&T,)>>::call(move _19, move _20) -> [return: bb9, unwind unreachable];
     }
 
     bb9: {
+        StorageDead(_20);
         StorageDead(_19);
-        StorageDead(_18);
-        StorageDead(_15);
+        StorageDead(_16);
         goto -> bb4;
     }
 

tests/mir-opt/pre-codegen/slice_iter.reverse_loop.PreCodegen.after.panic-unwind.mir

@@ -4,22 +4,22 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     debug slice => _1;
     debug f => _2;
     let mut _0: ();
-    let mut _11: std::slice::Iter<'_, T>;
-    let mut _12: std::iter::Rev<std::slice::Iter<'_, T>>;
+    let mut _12: std::slice::Iter<'_, T>;
     let mut _13: std::iter::Rev<std::slice::Iter<'_, T>>;
-    let mut _15: std::option::Option<&T>;
-    let mut _16: isize;
-    let mut _18: &impl Fn(&T);
-    let mut _19: (&T,);
-    let _20: ();
+    let mut _14: std::iter::Rev<std::slice::Iter<'_, T>>;
+    let mut _16: std::option::Option<&T>;
+    let mut _17: isize;
+    let mut _19: &impl Fn(&T);
+    let mut _20: (&T,);
+    let _21: ();
     scope 1 {
-        debug iter => _13;
-        let _17: &T;
+        debug iter => _14;
+        let _18: &T;
         scope 2 {
-            debug x => _17;
+            debug x => _18;
         }
         scope 17 (inlined <Rev<std::slice::Iter<'_, T>> as Iterator>::next) {
-            let mut _14: &mut std::slice::Iter<'_, T>;
+            let mut _15: &mut std::slice::Iter<'_, T>;
         }
     }
     scope 3 (inlined core::slice::<impl [T]>::iter) {
@@ -28,10 +28,11 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
             let mut _7: bool;
             let mut _8: *mut T;
             let mut _9: *mut T;
-            let mut _10: *const T;
+            let mut _11: *const T;
             scope 5 {
                 let _6: std::ptr::NonNull<T>;
                 scope 6 {
+                    let _10: *const T;
                     scope 7 {
                     }
                     scope 11 (inlined without_provenance::<T>) {
@@ -60,16 +61,16 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     }
 
     bb0: {
-        StorageLive(_11);
+        StorageLive(_12);
         StorageLive(_3);
         StorageLive(_6);
-        StorageLive(_10);
         StorageLive(_4);
         StorageLive(_5);
         _3 = Len((*_1));
         _4 = &raw const (*_1);
         _5 = _4 as *const T (PtrToPtr);
         _6 = NonNull::<T> { pointer: _5 };
+        StorageLive(_10);
         StorageLive(_7);
         _7 = const <T as std::mem::SizedTypeProperties>::IS_ZST;
         switchInt(move _7) -> [0: bb1, otherwise: bb2];
@@ -93,35 +94,38 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
 
     bb3: {
         StorageDead(_7);
-        _11 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _10, _marker: const ZeroSized: PhantomData<&T> };
+        StorageLive(_11);
+        _11 = _10;
+        _12 = std::slice::Iter::<'_, T> { ptr: _6, end_or_len: move _11, _marker: const ZeroSized: PhantomData<&T> };
+        StorageDead(_11);
+        StorageDead(_10);
         StorageDead(_5);
         StorageDead(_4);
-        StorageDead(_10);
         StorageDead(_6);
         StorageDead(_3);
-        _12 = Rev::<std::slice::Iter<'_, T>> { iter: _11 };
-        StorageDead(_11);
-        StorageLive(_13);
-        _13 = _12;
+        _13 = Rev::<std::slice::Iter<'_, T>> { iter: _12 };
+        StorageDead(_12);
+        StorageLive(_14);
+        _14 = _13;
         goto -> bb4;
     }
 
     bb4: {
+        StorageLive(_16);
         StorageLive(_15);
-        StorageLive(_14);
-        _14 = &mut (_13.0: std::slice::Iter<'_, T>);
-        _15 = <std::slice::Iter<'_, T> as DoubleEndedIterator>::next_back(move _14) -> [return: bb5, unwind: bb11];
+        _15 = &mut (_14.0: std::slice::Iter<'_, T>);
+        _16 = <std::slice::Iter<'_, T> as DoubleEndedIterator>::next_back(move _15) -> [return: bb5, unwind: bb11];
     }
 
     bb5: {
-        StorageDead(_14);
-        _16 = discriminant(_15);
-        switchInt(move _16) -> [0: bb6, 1: bb8, otherwise: bb10];
+        StorageDead(_15);
+        _17 = discriminant(_16);
+        switchInt(move _17) -> [0: bb6, 1: bb8, otherwise: bb10];
     }
 
     bb6: {
-        StorageDead(_15);
-        StorageDead(_13);
+        StorageDead(_16);
+        StorageDead(_14);
         drop(_2) -> [return: bb7, unwind continue];
     }
 
@@ -130,18 +134,18 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
     }
 
     bb8: {
-        _17 = ((_15 as Some).0: &T);
-        StorageLive(_18);
-        _18 = &_2;
+        _18 = ((_16 as Some).0: &T);
         StorageLive(_19);
-        _19 = (_17,);
-        _20 = <impl Fn(&T) as Fn<(&T,)>>::call(move _18, move _19) -> [return: bb9, unwind: bb11];
+        _19 = &_2;
+        StorageLive(_20);
+        _20 = (_18,);
+        _21 = <impl Fn(&T) as Fn<(&T,)>>::call(move _19, move _20) -> [return: bb9, unwind: bb11];
     }
 
     bb9: {
+        StorageDead(_20);
         StorageDead(_19);
-        StorageDead(_18);
-        StorageDead(_15);
+        StorageDead(_16);
         goto -> bb4;
     }