From 36487c75956bfe2476d1f813dff6903b104066b9 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Fri, 3 Jul 2026 14:58:41 +1000
Subject: [PATCH] Shrink `ast::Expr`

From 72 bytes to 64 bytes, by boxing `ForLoop`, which is the biggest
variant.

There are now 11 `ExprKind` variants that are 31 bytes, so future
improvements here will be very difficult.
---
 compiler/rustc_ast/src/ast.rs                 | 21 +++++++++--------
 compiler/rustc_ast/src/util/classify.rs       |  5 +++-
 compiler/rustc_ast/src/visit.rs               |  2 +-
 compiler/rustc_ast_lowering/src/expr.rs       |  2 +-
 .../rustc_ast_pretty/src/pprust/state/expr.rs |  2 +-
 compiler/rustc_lint/src/unused.rs             |  6 ++---
 compiler/rustc_parse/src/parser/expr.rs       | 22 ++++++++++++++----
 compiler/rustc_resolve/src/late.rs            |  2 +-
 .../clippy/clippy_utils/src/ast_utils/mod.rs  | 23 ++++++-------------
 src/tools/rustfmt/src/expr.rs                 | 10 ++------
 tests/ui/stats/input-stats.stderr             | 16 ++++++-------
 11 files changed, 57 insertions(+), 54 deletions(-)

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 3293da27d2912..deaafa8ab2e36 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -1655,6 +1655,15 @@ impl From<Box<Expr>> for Expr {
     }
 }
 
+#[derive(Clone, Encodable, Decodable, Debug, Walkable)]
+pub struct ForLoop {
+    pub pat: Box<Pat>,
+    pub iter: Box<Expr>,
+    pub body: Box<Block>,
+    pub label: Option<Label>,
+    pub kind: ForLoopKind,
+}
+
 #[derive(Clone, Encodable, Decodable, Debug, Walkable)]
 pub struct Closure {
     pub binder: ClosureBinder,
@@ -1780,13 +1789,7 @@ pub enum ExprKind {
     /// `'label: for await? pat in iter { block }`
     ///
     /// This is desugared to a combination of `loop` and `match` expressions.
-    ForLoop {
-        pat: Box<Pat>,
-        iter: Box<Expr>,
-        body: Box<Block>,
-        label: Option<Label>,
-        kind: ForLoopKind,
-    },
+    ForLoop(Box<ForLoop>),
     /// Conditionless loop (can be exited with `break`, `continue`, or `return`).
     ///
     /// `'label: loop { block }`
@@ -4385,8 +4388,8 @@ mod size_asserts {
     static_assert_size!(AttrKind, 16);
     static_assert_size!(Attribute, 32);
     static_assert_size!(Block, 24);
-    static_assert_size!(Expr, 72);
-    static_assert_size!(ExprKind, 40);
+    static_assert_size!(Expr, 64);
+    static_assert_size!(ExprKind, 32);
     static_assert_size!(Fn, 192);
     static_assert_size!(FnDecl, 24);
     static_assert_size!(FnHeader, 76);
diff --git a/compiler/rustc_ast/src/util/classify.rs b/compiler/rustc_ast/src/util/classify.rs
index 0c2218e557f23..e799f73ff544f 100644
--- a/compiler/rustc_ast/src/util/classify.rs
+++ b/compiler/rustc_ast/src/util/classify.rs
@@ -101,7 +101,10 @@ pub fn expr_requires_semi_to_be_stmt(e: &ast::Expr) -> bool {
 pub fn leading_labeled_expr(mut expr: &ast::Expr) -> bool {
     loop {
         match &expr.kind {
-            Block(_, label) | ForLoop { label, .. } | Loop(_, label, _) | While(_, _, label) => {
+            Block(_, label)
+            | ForLoop(ast::ForLoop { label, .. })
+            | Loop(_, label, _)
+            | While(_, _, label) => {
                 return label.is_some();
             }
 
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index e25c1a0b31937..16145385b97b5 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -1000,7 +1000,7 @@ macro_rules! common_visitor_and_walkers {
                     visit_visitable!($($mut)? vis, head_expression, if_block, optional_else),
                 ExprKind::While(subexpression, block, opt_label) =>
                     visit_visitable!($($mut)? vis, subexpression, block, opt_label),
-                ExprKind::ForLoop { pat, iter, body, label, kind } =>
+                ExprKind::ForLoop(ForLoop { pat, iter, body, label, kind }) =>
                     visit_visitable!($($mut)? vis, pat, iter, body, label, kind),
                 ExprKind::Loop(block, opt_label, span) =>
                     visit_visitable!($($mut)? vis, block, opt_label, span),
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 4ed23e032d234..875dbb010b5b3 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -191,7 +191,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 //
                 // This also needs special handling because the HirId of the returned `hir::Expr` will not
                 // correspond to the `e.id`, so `lower_expr_for` handles attribute lowering itself.
-                ExprKind::ForLoop { pat, iter, body, label, kind } => {
+                ExprKind::ForLoop(ForLoop { pat, iter, body, label, kind }) => {
                     return self.lower_expr_for(e, pat, iter, body, *label, *kind);
                 }
                 ExprKind::Closure(closure) => return self.lower_expr_closure_expr(e, closure),
diff --git a/compiler/rustc_ast_pretty/src/pprust/state/expr.rs b/compiler/rustc_ast_pretty/src/pprust/state/expr.rs
index c3a197eaf3ae8..238e349ab5acf 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state/expr.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state/expr.rs
@@ -526,7 +526,7 @@ impl<'a> State<'a> {
                 self.space();
                 self.print_block_with_attrs(blk, attrs, cb, ib);
             }
-            ast::ExprKind::ForLoop { pat, iter, body, label, kind } => {
+            ast::ExprKind::ForLoop(ast::ForLoop { pat, iter, body, label, kind }) => {
                 if let Some(label) = label {
                     self.print_ident(label.ident);
                     self.word_space(":");
diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
index 4c22310571f1a..a24bf6c20c4eb 100644
--- a/compiler/rustc_lint/src/unused.rs
+++ b/compiler/rustc_lint/src/unused.rs
@@ -1,5 +1,5 @@
 use rustc_ast::util::{classify, parser};
-use rustc_ast::{self as ast, ExprKind, FnRetTy, HasAttrs as _, StmtKind};
+use rustc_ast::{self as ast, ExprKind, FnRetTy, ForLoop, HasAttrs as _, StmtKind};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::MultiSpan;
 use rustc_hir::{self as hir};
@@ -381,7 +381,7 @@ trait UnusedDelimLint {
                 (cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right), true)
             }
 
-            ForLoop { ref iter, ref body, .. } => {
+            ForLoop(ast::ForLoop { ref iter, ref body, .. }) => {
                 (iter, UnusedDelimsCtx::ForIterExpr, true, None, Some(body.span.lo()), true)
             }
 
@@ -728,7 +728,7 @@ impl EarlyLintPass for UnusedParens {
         }
 
         match e.kind {
-            ExprKind::Let(ref pat, _, _, _) | ExprKind::ForLoop { ref pat, .. } => {
+            ExprKind::Let(ref pat, _, _, _) | ExprKind::ForLoop(ForLoop { ref pat, .. }) => {
                 self.check_unused_parens_pat(cx, pat, false, false, (true, true));
             }
             // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index e151899a57228..da053a2175552 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -15,8 +15,8 @@ use rustc_ast::visit::{Visitor, walk_expr};
 use rustc_ast::{
     self as ast, AnonConst, Arm, AssignOp, AssignOpKind, AttrStyle, AttrVec, BinOp, BinOpKind,
     BlockCheckMode, CaptureBy, ClosureBinder, DUMMY_NODE_ID, Expr, ExprField, ExprKind, FnDecl,
-    FnRetTy, Guard, Label, MacCall, MetaItemLit, Movability, Param, RangeLimits, StmtKind, Ty,
-    TyKind, UnOp, UnsafeBinderCastKind, YieldKind,
+    FnRetTy, ForLoop, Guard, Label, MacCall, MetaItemLit, Movability, Param, RangeLimits, StmtKind,
+    Ty, TyKind, UnOp, UnsafeBinderCastKind, YieldKind,
 };
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::stack::ensure_sufficient_stack;
@@ -1950,7 +1950,7 @@ impl<'a> Parser<'a> {
                 if label.is_some()
                     && match &expr.kind {
                         ExprKind::While(_, _, None)
-                        | ExprKind::ForLoop { label: None, .. }
+                        | ExprKind::ForLoop(ForLoop { label: None, .. })
                         | ExprKind::Loop(_, None, _) => true,
                         ExprKind::Block(block, None) => {
                             matches!(block.rules, BlockCheckMode::Default)
@@ -3048,7 +3048,13 @@ impl<'a> Parser<'a> {
             let block = self.mk_block(thin_vec![], BlockCheckMode::Default, self.prev_token.span);
             return Ok(self.mk_expr(
                 lo.to(self.prev_token.span),
-                ExprKind::ForLoop { pat, iter: err_expr, body: block, label: opt_label, kind },
+                ExprKind::ForLoop(Box::new(ForLoop {
+                    pat,
+                    iter: err_expr,
+                    body: block,
+                    label: opt_label,
+                    kind,
+                })),
             ));
         }
 
@@ -3058,7 +3064,13 @@ impl<'a> Parser<'a> {
             opt_label.is_none().then_some(lo),
         )?;
 
-        let kind = ExprKind::ForLoop { pat, iter: expr, body: loop_block, label: opt_label, kind };
+        let kind = ExprKind::ForLoop(Box::new(ForLoop {
+            pat,
+            iter: expr,
+            body: loop_block,
+            label: opt_label,
+            kind,
+        }));
 
         self.recover_loop_else("for", lo)?;
 
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 4ae56ac7c6f3d..9d9278b125549 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -5262,7 +5262,7 @@ impl<'a, 'ast, 'ra, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                 });
             }
 
-            ExprKind::ForLoop { ref pat, ref iter, ref body, label, kind: _ } => {
+            ExprKind::ForLoop(ForLoop { ref pat, ref iter, ref body, label, kind: _ }) => {
                 self.visit_expr(iter);
                 self.with_rib(ValueNS, RibKind::Normal, |this| {
                     this.resolve_pattern_top(pat, PatternSource::For);
diff --git a/src/tools/clippy/clippy_utils/src/ast_utils/mod.rs b/src/tools/clippy/clippy_utils/src/ast_utils/mod.rs
index c89130fb647a4..fe51f841bc8b6 100644
--- a/src/tools/clippy/clippy_utils/src/ast_utils/mod.rs
+++ b/src/tools/clippy/clippy_utils/src/ast_utils/mod.rs
@@ -183,22 +183,13 @@ fn eq_expr(l: &Expr, r: &Expr) -> bool {
         (While(lc, lt, ll), While(rc, rt, rl)) => {
             eq_label(ll.as_ref(), rl.as_ref()) && eq_expr(lc, rc) && eq_block(lt, rt)
         },
-        (
-            ForLoop {
-                pat: lp,
-                iter: li,
-                body: lt,
-                label: ll,
-                kind: lk,
-            },
-            ForLoop {
-                pat: rp,
-                iter: ri,
-                body: rt,
-                label: rl,
-                kind: rk,
-            },
-        ) => eq_label(ll.as_ref(), rl.as_ref()) && eq_pat(lp, rp) && eq_expr(li, ri) && eq_block(lt, rt) && lk == rk,
+        (ForLoop(lf), ForLoop(rf)) => {
+            eq_label(lf.label.as_ref(), rf.label.as_ref())
+                && eq_pat(&lf.pat, &rf.pat)
+                && eq_expr(&lf.iter, &rf.iter)
+                && eq_block(&lf.body, &rf.body)
+                && lf.kind == rf.kind
+        }
         (Loop(lt, ll, _), Loop(rt, rl, _)) => eq_label(ll.as_ref(), rl.as_ref()) && eq_block(lt, rt),
         (Block(lb, ll), Block(rb, rl)) => eq_label(ll.as_ref(), rl.as_ref()) && eq_block(lb, rb),
         (TryBlock(lb, lt), TryBlock(rb, rt)) => eq_block(lb, rb) && both(lt.as_deref(), rt.as_deref(), eq_ty),
diff --git a/src/tools/rustfmt/src/expr.rs b/src/tools/rustfmt/src/expr.rs
index cc7fdaefc8fee..5ecb68078561b 100644
--- a/src/tools/rustfmt/src/expr.rs
+++ b/src/tools/rustfmt/src/expr.rs
@@ -779,14 +779,8 @@ fn to_control_flow(expr: &ast::Expr, expr_type: ExprType) -> Option<ControlFlow<
                 expr.span,
             ))
         }
-        ast::ExprKind::ForLoop {
-            ref pat,
-            ref iter,
-            ref body,
-            label,
-            kind,
-        } => Some(ControlFlow::new_for(
-            pat, iter, body, label, expr.span, kind,
+        ast::ExprKind::ForLoop(ref f) => Some(ControlFlow::new_for(
+            &f.pat, &f.iter, &f.body, f.label, expr.span, f.kind,
         )),
         ast::ExprKind::Loop(ref block, label, _) => {
             Some(ControlFlow::new_loop(block, label, expr.span))
diff --git a/tests/ui/stats/input-stats.stderr b/tests/ui/stats/input-stats.stderr
index 53ae5bc156c0b..40a0a957296ab 100644
--- a/tests/ui/stats/input-stats.stderr
+++ b/tests/ui/stats/input-stats.stderr
@@ -16,13 +16,13 @@ ast-stats - Ptr                       56 (NN.N%)             1
 ast-stats - Ref                       56 (NN.N%)             1
 ast-stats - ImplicitSelf             112 (NN.N%)             2
 ast-stats - Path                     560 (NN.N%)            10
-ast-stats Expr                     648 (NN.N%)             9            72
-ast-stats - InlineAsm                 72 (NN.N%)             1
-ast-stats - Match                     72 (NN.N%)             1
-ast-stats - Path                      72 (NN.N%)             1
-ast-stats - Struct                    72 (NN.N%)             1
-ast-stats - Lit                      144 (NN.N%)             2
-ast-stats - Block                    216 (NN.N%)             3
+ast-stats Expr                     576 (NN.N%)             9            64
+ast-stats - InlineAsm                 64 (NN.N%)             1
+ast-stats - Match                     64 (NN.N%)             1
+ast-stats - Path                      64 (NN.N%)             1
+ast-stats - Struct                    64 (NN.N%)             1
+ast-stats - Lit                      128 (NN.N%)             2
+ast-stats - Block                    192 (NN.N%)             3
 ast-stats Pat                      448 (NN.N%)             7            64
 ast-stats - Struct                    64 (NN.N%)             1
 ast-stats - Wild                      64 (NN.N%)             1
@@ -57,7 +57,7 @@ ast-stats GenericArgs               40 (NN.N%)             1            40
 ast-stats - AngleBracketed            40 (NN.N%)             1
 ast-stats Crate                     40 (NN.N%)             1            40
 ast-stats ----------------------------------------------------------------
-ast-stats Total                  7_048                   127
+ast-stats Total                  6_976                   127
 ast-stats ================================================================
 hir-stats ================================================================
 hir-stats HIR STATS: input_stats