Auto merge of #86266 - LeSeulArtichaut:box-thir-adt, r=davidtwco

Box `thir::ExprKind::Adt` for performance

`Adt` is the biggest variant in the enum and probably isn't used very often compared to the other expr kinds, so boxing it should be beneficial for performance. We need a perf test to be sure.

compiler/rustc_middle/src/thir.rs

@@ -97,6 +97,20 @@ pub struct Block {
     pub safety_mode: BlockSafety,
 }
 
+#[derive(Debug, HashStable)]
+pub struct Adt<'tcx> {
+    pub adt_def: &'tcx AdtDef,
+    pub variant_index: VariantIdx,
+    pub substs: SubstsRef<'tcx>,
+
+    /// Optional user-given substs: for something like `let x =
+    /// Bar::<T> { ... }`.
+    pub user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
+
+    pub fields: Box<[FieldExpr]>,
+    pub base: Option<FruInfo<'tcx>>,
+}
+
 #[derive(Copy, Clone, Debug, HashStable)]
 pub enum BlockSafety {
     Safe,
@@ -145,7 +159,7 @@ pub enum StmtKind<'tcx> {
 
 // `Expr` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-rustc_data_structures::static_assert_size!(Expr<'_>, 144);
+rustc_data_structures::static_assert_size!(Expr<'_>, 104);
 
 /// The Thir trait implementor lowers their expressions (`&'tcx H::Expr`)
 /// into instances of this `Expr` enum. This lowering can be done
@@ -304,18 +318,7 @@ pub enum ExprKind<'tcx> {
     Tuple {
         fields: Box<[ExprId]>,
     },
-    Adt {
-        adt_def: &'tcx AdtDef,
-        variant_index: VariantIdx,
-        substs: SubstsRef<'tcx>,
-
-        /// Optional user-given substs: for something like `let x =
-        /// Bar::<T> { ... }`.
-        user_ty: Option<Canonical<'tcx, UserType<'tcx>>>,
-
-        fields: Box<[FieldExpr]>,
-        base: Option<FruInfo<'tcx>>,
-    },
+    Adt(Box<Adt<'tcx>>),
     PlaceTypeAscription {
         source: ExprId,
         /// Type that the user gave to this expression

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -264,7 +264,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 this.cfg.push_assign(block, source_info, destination, address_of);
                 block.unit()
             }
-            ExprKind::Adt { adt_def, variant_index, substs, user_ty, ref fields, ref base } => {
+            ExprKind::Adt(box Adt {
+                adt_def,
+                variant_index,
+                substs,
+                user_ty,
+                ref fields,
+                ref base,
+            }) => {
                 // See the notes for `ExprKind::Array` in `as_rvalue` and for
                 // `ExprKind::Borrow` above.
                 let is_union = adt_def.is_union();

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -195,14 +195,14 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
             ExprKind::InlineAsm { .. } | ExprKind::LlvmInlineAsm { .. } => {
                 self.requires_unsafe(expr.span, UseOfInlineAssembly);
             }
-            ExprKind::Adt {
+            ExprKind::Adt(box Adt {
                 adt_def,
                 variant_index: _,
                 substs: _,
                 user_ty: _,
                 fields: _,
                 base: _,
-            } => match self.tcx.layout_scalar_valid_range(adt_def.did) {
+            }) => match self.tcx.layout_scalar_valid_range(adt_def.did) {
                 (Bound::Unbounded, Bound::Unbounded) => {}
                 _ => self.requires_unsafe(expr.span, InitializingTypeWith),
             },

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -228,14 +228,14 @@ impl<'tcx> Cx<'tcx> {
                                 expr: self.mirror_expr(e),
                             })
                             .collect();
-                        ExprKind::Adt {
+                        ExprKind::Adt(Box::new(Adt {
                             adt_def,
                             substs,
                             variant_index: index,
                             fields: field_refs,
                             user_ty,
                             base: None,
-                        }
+                        }))
                     } else {
                         ExprKind::Call {
                             ty: self.typeck_results().node_type(fun.hir_id),
@@ -362,7 +362,7 @@ impl<'tcx> Cx<'tcx> {
                         let user_provided_types = self.typeck_results().user_provided_types();
                         let user_ty = user_provided_types.get(expr.hir_id).copied();
                         debug!("make_mirror_unadjusted: (struct/union) user_ty={:?}", user_ty);
-                        ExprKind::Adt {
+                        ExprKind::Adt(Box::new(Adt {
                             adt_def: adt,
                             variant_index: VariantIdx::new(0),
                             substs,
@@ -375,7 +375,7 @@ impl<'tcx> Cx<'tcx> {
                                     .copied()
                                     .collect(),
                             }),
-                        }
+                        }))
                     }
                     AdtKind::Enum => {
                         let res = self.typeck_results().qpath_res(qpath, expr.hir_id);
@@ -388,14 +388,14 @@ impl<'tcx> Cx<'tcx> {
                                     self.typeck_results().user_provided_types();
                                 let user_ty = user_provided_types.get(expr.hir_id).copied();
                                 debug!("make_mirror_unadjusted: (variant) user_ty={:?}", user_ty);
-                                ExprKind::Adt {
+                                ExprKind::Adt(Box::new(Adt {
                                     adt_def: adt,
                                     variant_index: index,
                                     substs,
                                     user_ty,
                                     fields: self.field_refs(fields),
                                     base: None,
-                                }
+                                }))
                             }
                             _ => {
                                 span_bug!(expr.span, "unexpected res: {:?}", res);
@@ -906,14 +906,14 @@ impl<'tcx> Cx<'tcx> {
                 match ty.kind() {
                     // A unit struct/variant which is used as a value.
                     // We return a completely different ExprKind here to account for this special case.
-                    ty::Adt(adt_def, substs) => ExprKind::Adt {
+                    ty::Adt(adt_def, substs) => ExprKind::Adt(Box::new(Adt {
                         adt_def,
                         variant_index: adt_def.variant_index_with_ctor_id(def_id),
                         substs,
                         user_ty: user_provided_type,
                         fields: box [],
                         base: None,
-                    },
+                    })),
                     _ => bug!("unexpected ty: {:?}", ty),
                 }
             }

compiler/rustc_mir_build/src/thir/visit.rs

@@ -1,4 +1,4 @@
-use rustc_middle::thir::*;
+use rustc_middle::thir::{self, *};
 use rustc_middle::ty::Const;
 
 pub trait Visitor<'a, 'tcx: 'a>: Sized {
@@ -94,7 +94,14 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
                 visitor.visit_expr(&visitor.thir()[field]);
             }
         }
-        Adt { ref fields, ref base, adt_def: _, variant_index: _, substs: _, user_ty: _ } => {
+        Adt(box thir::Adt {
+            ref fields,
+            ref base,
+            adt_def: _,
+            variant_index: _,
+            substs: _,
+            user_ty: _,
+        }) => {
             for field in &**fields {
                 visitor.visit_expr(&visitor.thir()[field.expr]);
             }