Use MirPatch in EnumSizeOpt.

Instead of `expand_statements`. This makes the code shorter and
consistent with other MIR transform passes.

The tests require updating because there is a slight change in
MIR output:
- the old code replaced the original statement with twelve new
  statements.
- the new code inserts converts the original statement to a `nop` and
  then insert twelve new statements in front of it.

I.e. we now end up with an extra `nop`, which doesn't matter at all.

compiler/rustc_mir_transform/src/large_enums.rs

@@ -6,6 +6,8 @@ use rustc_middle::ty::util::IntTypeExt;
 use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt};
 use rustc_session::Session;
 
+use crate::patch::MirPatch;
+
 /// A pass that seeks to optimize unnecessary moves of large enum types, if there is a large
 /// enough discrepancy between them.
 ///
@@ -41,31 +43,34 @@ impl<'tcx> crate::MirPass<'tcx> for EnumSizeOpt {
         let mut alloc_cache = FxHashMap::default();
         let typing_env = body.typing_env(tcx);
 
-        let blocks = body.basic_blocks.as_mut();
-        let local_decls = &mut body.local_decls;
+        let mut patch = MirPatch::new(body);
 
-        for bb in blocks {
-            bb.expand_statements(|st| {
+        for (block, data) in body.basic_blocks.as_mut().iter_enumerated_mut() {
+            for (statement_index, st) in data.statements.iter_mut().enumerate() {
                 let StatementKind::Assign(box (
                     lhs,
                     Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)),
                 )) = &st.kind
                 else {
-                    return None;
+                    continue;
                 };
 
-                let ty = lhs.ty(local_decls, tcx).ty;
+                let location = Location { block, statement_index };
 
-                let (adt_def, num_variants, alloc_id) =
-                    self.candidate(tcx, typing_env, ty, &mut alloc_cache)?;
+                let ty = lhs.ty(&body.local_decls, tcx).ty;
 
-                let source_info = st.source_info;
-                let span = source_info.span;
+                let Some((adt_def, num_variants, alloc_id)) =
+                    self.candidate(tcx, typing_env, ty, &mut alloc_cache)
+                else {
+                    continue;
+                };
+
+                let span = st.source_info.span;
 
                 let tmp_ty = Ty::new_array(tcx, tcx.types.usize, num_variants as u64);
-                let size_array_local = local_decls.push(LocalDecl::new(tmp_ty, span));
-                let store_live =
-                    Statement { source_info, kind: StatementKind::StorageLive(size_array_local) };
+                let size_array_local = patch.new_temp(tmp_ty, span);
+
+                let store_live = StatementKind::StorageLive(size_array_local);
 
                 let place = Place::from(size_array_local);
                 let constant_vals = ConstOperand {
@@ -77,108 +82,63 @@ impl<'tcx> crate::MirPass<'tcx> for EnumSizeOpt {
                     ),
                 };
                 let rval = Rvalue::Use(Operand::Constant(Box::new(constant_vals)));
-                let const_assign =
-                    Statement { source_info, kind: StatementKind::Assign(Box::new((place, rval))) };
+                let const_assign = StatementKind::Assign(Box::new((place, rval)));
 
-                let discr_place = Place::from(
-                    local_decls.push(LocalDecl::new(adt_def.repr().discr_type().to_ty(tcx), span)),
-                );
-                let store_discr = Statement {
-                    source_info,
-                    kind: StatementKind::Assign(Box::new((
-                        discr_place,
-                        Rvalue::Discriminant(*rhs),
-                    ))),
-                };
+                let discr_place =
+                    Place::from(patch.new_temp(adt_def.repr().discr_type().to_ty(tcx), span));
+                let store_discr =
+                    StatementKind::Assign(Box::new((discr_place, Rvalue::Discriminant(*rhs))));
 
-                let discr_cast_place =
-                    Place::from(local_decls.push(LocalDecl::new(tcx.types.usize, span)));
-                let cast_discr = Statement {
-                    source_info,
-                    kind: StatementKind::Assign(Box::new((
-                        discr_cast_place,
-                        Rvalue::Cast(
-                            CastKind::IntToInt,
-                            Operand::Copy(discr_place),
-                            tcx.types.usize,
-                        ),
-                    ))),
-                };
+                let discr_cast_place = Place::from(patch.new_temp(tcx.types.usize, span));
+                let cast_discr = StatementKind::Assign(Box::new((
+                    discr_cast_place,
+                    Rvalue::Cast(CastKind::IntToInt, Operand::Copy(discr_place), tcx.types.usize),
+                )));
 
-                let size_place =
-                    Place::from(local_decls.push(LocalDecl::new(tcx.types.usize, span)));
-                let store_size = Statement {
-                    source_info,
-                    kind: StatementKind::Assign(Box::new((
-                        size_place,
-                        Rvalue::Use(Operand::Copy(Place {
-                            local: size_array_local,
-                            projection: tcx
-                                .mk_place_elems(&[PlaceElem::Index(discr_cast_place.local)]),
-                        })),
-                    ))),
-                };
+                let size_place = Place::from(patch.new_temp(tcx.types.usize, span));
+                let store_size = StatementKind::Assign(Box::new((
+                    size_place,
+                    Rvalue::Use(Operand::Copy(Place {
+                        local: size_array_local,
+                        projection: tcx.mk_place_elems(&[PlaceElem::Index(discr_cast_place.local)]),
+                    })),
+                )));
 
-                let dst =
-                    Place::from(local_decls.push(LocalDecl::new(Ty::new_mut_ptr(tcx, ty), span)));
-                let dst_ptr = Statement {
-                    source_info,
-                    kind: StatementKind::Assign(Box::new((
-                        dst,
-                        Rvalue::RawPtr(RawPtrKind::Mut, *lhs),
-                    ))),
-                };
+                let dst = Place::from(patch.new_temp(Ty::new_mut_ptr(tcx, ty), span));
+                let dst_ptr =
+                    StatementKind::Assign(Box::new((dst, Rvalue::RawPtr(RawPtrKind::Mut, *lhs))));
 
                 let dst_cast_ty = Ty::new_mut_ptr(tcx, tcx.types.u8);
-                let dst_cast_place =
-                    Place::from(local_decls.push(LocalDecl::new(dst_cast_ty, span)));
-                let dst_cast = Statement {
-                    source_info,
-                    kind: StatementKind::Assign(Box::new((
-                        dst_cast_place,
-                        Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(dst), dst_cast_ty),
-                    ))),
-                };
+                let dst_cast_place = Place::from(patch.new_temp(dst_cast_ty, span));
+                let dst_cast = StatementKind::Assign(Box::new((
+                    dst_cast_place,
+                    Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(dst), dst_cast_ty),
+                )));
 
-                let src =
-                    Place::from(local_decls.push(LocalDecl::new(Ty::new_imm_ptr(tcx, ty), span)));
-                let src_ptr = Statement {
-                    source_info,
-                    kind: StatementKind::Assign(Box::new((
-                        src,
-                        Rvalue::RawPtr(RawPtrKind::Const, *rhs),
-                    ))),
-                };
+                let src = Place::from(patch.new_temp(Ty::new_imm_ptr(tcx, ty), span));
+                let src_ptr =
+                    StatementKind::Assign(Box::new((src, Rvalue::RawPtr(RawPtrKind::Const, *rhs))));
 
                 let src_cast_ty = Ty::new_imm_ptr(tcx, tcx.types.u8);
-                let src_cast_place =
-                    Place::from(local_decls.push(LocalDecl::new(src_cast_ty, span)));
-                let src_cast = Statement {
-                    source_info,
-                    kind: StatementKind::Assign(Box::new((
-                        src_cast_place,
-                        Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(src), src_cast_ty),
-                    ))),
-                };
+                let src_cast_place = Place::from(patch.new_temp(src_cast_ty, span));
+                let src_cast = StatementKind::Assign(Box::new((
+                    src_cast_place,
+                    Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(src), src_cast_ty),
+                )));
 
-                let deinit_old =
-                    Statement { source_info, kind: StatementKind::Deinit(Box::new(dst)) };
+                let deinit_old = StatementKind::Deinit(Box::new(dst));
 
-                let copy_bytes = Statement {
-                    source_info,
-                    kind: StatementKind::Intrinsic(Box::new(
-                        NonDivergingIntrinsic::CopyNonOverlapping(CopyNonOverlapping {
-                            src: Operand::Copy(src_cast_place),
-                            dst: Operand::Copy(dst_cast_place),
-                            count: Operand::Copy(size_place),
-                        }),
-                    )),
-                };
+                let copy_bytes = StatementKind::Intrinsic(Box::new(
+                    NonDivergingIntrinsic::CopyNonOverlapping(CopyNonOverlapping {
+                        src: Operand::Copy(src_cast_place),
+                        dst: Operand::Copy(dst_cast_place),
+                        count: Operand::Copy(size_place),
+                    }),
+                ));
 
-                let store_dead =
-                    Statement { source_info, kind: StatementKind::StorageDead(size_array_local) };
+                let store_dead = StatementKind::StorageDead(size_array_local);
 
-                let iter = [
+                let stmts = [
                     store_live,
                     const_assign,
                     store_discr,
@@ -191,14 +151,16 @@ impl<'tcx> crate::MirPass<'tcx> for EnumSizeOpt {
                     deinit_old,
                     copy_bytes,
                     store_dead,
-                ]
-                .into_iter();
+                ];
+                for stmt in stmts {
+                    patch.add_statement(location, stmt);
+                }
 
                 st.make_nop();
-
-                Some(iter)
-            });
+            }
         }
+
+        patch.apply(body);
     }
 
     fn is_required(&self) -> bool {

tests/mir-opt/enum_opt.cand.EnumSizeOpt.32bit.diff

@@ -47,6 +47,7 @@
 +         Deinit(_8);
 +         copy_nonoverlapping(dst = copy _9, src = copy _11, count = copy _7);
 +         StorageDead(_4);
++         nop;
           StorageDead(_2);
 -         _0 = move _1;
 +         StorageLive(_12);
@@ -61,6 +62,7 @@
 +         Deinit(_16);
 +         copy_nonoverlapping(dst = copy _17, src = copy _19, count = copy _15);
 +         StorageDead(_12);
++         nop;
           StorageDead(_1);
           return;
       }

tests/mir-opt/enum_opt.cand.EnumSizeOpt.64bit.diff

@@ -47,6 +47,7 @@
 +         Deinit(_8);
 +         copy_nonoverlapping(dst = copy _9, src = copy _11, count = copy _7);
 +         StorageDead(_4);
++         nop;
           StorageDead(_2);
 -         _0 = move _1;
 +         StorageLive(_12);
@@ -61,6 +62,7 @@
 +         Deinit(_16);
 +         copy_nonoverlapping(dst = copy _17, src = copy _19, count = copy _15);
 +         StorageDead(_12);
++         nop;
           StorageDead(_1);
           return;
       }

tests/mir-opt/enum_opt.unin.EnumSizeOpt.32bit.diff

@@ -47,6 +47,7 @@
 +         Deinit(_8);
 +         copy_nonoverlapping(dst = copy _9, src = copy _11, count = copy _7);
 +         StorageDead(_4);
++         nop;
           StorageDead(_2);
 -         _0 = move _1;
 +         StorageLive(_12);
@@ -61,6 +62,7 @@
 +         Deinit(_16);
 +         copy_nonoverlapping(dst = copy _17, src = copy _19, count = copy _15);
 +         StorageDead(_12);
++         nop;
           StorageDead(_1);
           return;
       }

tests/mir-opt/enum_opt.unin.EnumSizeOpt.64bit.diff

@@ -47,6 +47,7 @@
 +         Deinit(_8);
 +         copy_nonoverlapping(dst = copy _9, src = copy _11, count = copy _7);
 +         StorageDead(_4);
++         nop;
           StorageDead(_2);
 -         _0 = move _1;
 +         StorageLive(_12);
@@ -61,6 +62,7 @@
 +         Deinit(_16);
 +         copy_nonoverlapping(dst = copy _17, src = copy _19, count = copy _15);
 +         StorageDead(_12);
++         nop;
           StorageDead(_1);
           return;
       }