From 6e754565d8488665ab213bf1f49764ee7570d5a2 Mon Sep 17 00:00:00 2001
From: Sebastian Ullrich <sebasti@nullri.ch>
Date: Sat, 29 Aug 2026 13:04:33 +0000
Subject: [PATCH] perf: move `currRecDepth` from `Core.Context` into
 `Core.State`

Variant A of removing the per-recursion-step `Core.Context` rebuild: 96% of reconstructions exist only to bump the depth counter, and a `StateRefT` field can be updated in place.
---
 src/Lean/CoreM.lean        | 20 +++++++++++++++-----
 src/Lean/Elab/Command.lean |  4 ++--
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/src/Lean/CoreM.lean b/src/Lean/CoreM.lean
index c625885b98b7..a52d87b4be49 100644
--- a/src/Lean/CoreM.lean
+++ b/src/Lean/CoreM.lean
@@ -187,6 +187,15 @@ structure Cache where
 structure State where
   /-- Current environment. -/
   env             : Environment
+  /--
+  Current recursion depth, maintained by `MonadRecDepth`.
+
+  This lives in the state rather than the context because `withReader` cannot reuse the context
+  record, so keeping it there made every recursion step rebuild `Context` and pay one reference
+  count increment per pointer field. The state is held in a `StateRefT` ref and can be updated in
+  place instead.
+  -/
+  currRecDepth    : Nat := 0
   /-- Next macro scope. We use macro scopes to avoid accidental name capture. -/
   nextMacroScope  : MacroScope     := firstFrontendMacroScope + 1
   /-- Name generator for producing unique `FVarId`s, `MVarId`s, and `LMVarId`s -/
@@ -235,7 +244,6 @@ structure Context.Cold where
 /-- Context for the CoreM monad. -/
 structure Context extends Context.Cold where
   options        : Options := {}
-  currRecDepth   : Nat := 0
   maxRecDepth    : Nat := 1000
   ref            : Syntax := Syntax.missing
   currNamespace  : Name := Name.anonymous
@@ -316,8 +324,10 @@ instance : MonadDeclNameGenerator CoreM where
   setDeclNGen ngen := modify fun s => { s with auxDeclNGen := ngen }
 
 instance : MonadRecDepth CoreM where
-  withRecDepth d x := withReader (fun ctx => { ctx with currRecDepth := d }) x
-  getRecDepth := return (← read).currRecDepth
+  withRecDepth d x := do
+    let old ← modifyGet fun s => (s.currRecDepth, { s with currRecDepth := d })
+    try x finally modify fun s => { s with currRecDepth := old }
+  getRecDepth := return (← get).currRecDepth
   getMaxRecDepth := return (← read).maxRecDepth
 
 instance : MonadResolveName CoreM where
@@ -338,8 +348,8 @@ instance : Elab.MonadInfoTree CoreM where
   modifyInfoState f := modify fun s => { s with infoState := f s.infoState }
 
 @[inline] def modifyCache (f : Cache → Cache) : CoreM Unit :=
-  modify fun ⟨env, next, ngen, auxDeclNGen, trace, cache, messages, infoState, snaps⟩ =>
-   ⟨env, next, ngen, auxDeclNGen, trace, f cache, messages, infoState, snaps⟩
+  modify fun ⟨env, recDepth, next, ngen, auxDeclNGen, trace, cache, messages, infoState, snaps⟩ =>
+   ⟨env, recDepth, next, ngen, auxDeclNGen, trace, f cache, messages, infoState, snaps⟩
 
 @[inline] def modifyInstLevelTypeCache (f : InstantiateLevelCache → InstantiateLevelCache) : CoreM Unit :=
   modifyCache fun ⟨c₁, c₂⟩ => ⟨f c₁, c₂⟩
diff --git a/src/Lean/Elab/Command.lean b/src/Lean/Elab/Command.lean
index d1bb776105ed..081c4f189842 100644
--- a/src/Lean/Elab/Command.lean
+++ b/src/Lean/Elab/Command.lean
@@ -267,7 +267,6 @@ private def runCore (x : CoreM α) : CommandElabM α := do
   let coreCtx : Core.Context := {
     fileName           := ctx.fileName
     fileMap            := ctx.fileMap
-    currRecDepth       := ctx.currRecDepth
     maxRecDepth        := s.maxRecDepth
     ref                := ctx.ref
     currNamespace      := scope.currNamespace
@@ -280,6 +279,7 @@ private def runCore (x : CoreM α) : CommandElabM α := do
     suppressElabErrors := ctx.suppressElabErrors }
   let x : EIO _ _ := x.run coreCtx {
     env
+    currRecDepth := ctx.currRecDepth
     ngen := s.ngen
     auxDeclNGen := s.auxDeclNGen
     nextMacroScope := s.nextMacroScope
@@ -1096,7 +1096,7 @@ private def liftCommandElabMCore (cmd : CommandElabM α) (throwOnError : Bool) :
     cmd.run {
       fileName := ctx.fileName
       fileMap := ctx.fileMap
-      currRecDepth := ctx.currRecDepth
+      currRecDepth := s.currRecDepth
       currMacroScope := ctx.currMacroScope
       ref := ctx.ref
       snap? := none