diff --git a/src/Lean/Compiler/IR/FreeVars.lean b/src/Lean/Compiler/IR/FreeVars.lean
index af118f391c..05fab205f1 100644
--- a/src/Lean/Compiler/IR/FreeVars.lean
+++ b/src/Lean/Compiler/IR/FreeVars.lean
@@ -108,87 +108,84 @@ namespace FreeIndices
 /-! We say a variable (join point) index (aka name) is free in a function body
    if there isn't a `FnBody.vdecl` (`Fnbody.jdecl`) binding it. -/
 
-abbrev Collector := IndexSet → IndexSet → IndexSet
+structure State where
+  freeIndices : IndexSet := {}
 
-@[inline] private def skip : Collector :=
-  fun _ fv => fv
+abbrev M := StateM State
 
-@[inline] private def collectIndex (x : Index) : Collector :=
-  fun bv fv => if bv.contains x then fv else fv.insert x
+private def visitIndex (x : Index) : M Unit := do
+  modify fun s => { s with freeIndices := s.freeIndices.insert x }
 
-@[inline] private def collectVar (x : VarId) : Collector :=
-  collectIndex x.idx
+private def visitVar (x : VarId) : M Unit :=
+  visitIndex x.idx
 
-@[inline] private def collectJP (x : JoinPointId) : Collector :=
-  collectIndex x.idx
+private def visitJP (j : JoinPointId) : M Unit :=
+  visitIndex j.idx
 
-@[inline] private def withIndex (x : Index) : Collector → Collector :=
-  fun k bv fv => k (bv.insert x) fv
+private def visitArg (arg : Arg) : M Unit :=
+  match arg with
+  | .var x => visitVar x
+  | .erased => pure ()
 
-@[inline] private def withVar (x : VarId) : Collector → Collector :=
-  withIndex x.idx
+private def visitParam (p : Param) : M Unit :=
+  visitVar p.x
 
-@[inline] private def withJP (x : JoinPointId) : Collector → Collector :=
-  withIndex x.idx
-
-def insertParams (s : IndexSet) (ys : Array Param) : IndexSet :=
-  ys.foldl (init := s) fun s p => s.insert p.x.idx
-
-@[inline] private def withParams (ys : Array Param) : Collector → Collector :=
-  fun k bv fv => k (insertParams bv ys) fv
-
-@[inline] private def seq : Collector → Collector → Collector :=
-  fun k₁ k₂ bv fv => k₂ bv (k₁ bv fv)
-
-instance : AndThen Collector where
-  andThen a b := private seq a (b ())
-
-private def collectArg : Arg → Collector
-  | .var x  => collectVar x
-  | .erased => skip
-
-private def collectArray {α : Type} (as : Array α) (f : α → Collector) : Collector :=
-  fun bv fv => as.foldl (fun fv a => f a bv fv) fv
-
-private def collectArgs (as : Array Arg) : Collector :=
-  collectArray as collectArg
-
-private def collectExpr : Expr → Collector
+private def visitExpr (e : Expr) : M Unit := do
+  match e with
   | .proj _ x | .uproj _ x | .sproj _ _ x | .box _ x | .unbox x | .reset _ x | .isShared x =>
-    collectVar x
+    visitVar x
   | .ctor _ ys | .fap _ ys | .pap _ ys =>
-    collectArgs ys
+    ys.forM visitArg
   | .ap x ys | .reuse x _ _ ys =>
-    collectVar x >> collectArgs ys
-  | .lit _ => skip
+    visitVar x
+    ys.forM visitArg
+  | .lit _ => pure ()
 
-private def collectAlts (f : FnBody → Collector) (alts : Array Alt) : Collector :=
-  collectArray alts fun alt => f alt.body
-
-partial def collectFnBody : FnBody → Collector
+partial def visitFnBody (fnBody : FnBody) : M Unit := do
+  match fnBody with
   | .vdecl x _ v b =>
-    collectExpr v >> withVar x (collectFnBody b)
+    visitVar x
+    visitExpr v
+    visitFnBody b
   | .jdecl j ys v b =>
-    withParams ys (collectFnBody v) >> withJP j (collectFnBody b)
+    visitJP j
+    visitFnBody v
+    ys.forM visitParam
+    visitFnBody b
   | .set x _ y b =>
-    collectVar x >> collectArg y >> collectFnBody b
+    visitVar x
+    visitArg y
+    visitFnBody b
   | .uset x _ y b | .sset x _ _ y _ b =>
-    collectVar x >> collectVar y >> collectFnBody b
+    visitVar x
+    visitVar y
+    visitFnBody b
   | .setTag x _ b | .inc x _ _ _ b | .dec x _ _ _ b | .del x b =>
-    collectVar x >> collectFnBody b
+    visitVar x
+    visitFnBody b
   | .case _ x _ alts =>
-    collectVar x >>
-      collectAlts collectFnBody alts
+    visitVar x
+    alts.forM (visitFnBody ·.body)
   | .jmp j ys =>
-    collectJP j >> collectArgs ys
+    visitJP j
+    ys.forM visitArg
   | .ret x =>
-    collectArg x
-  | .unreachable => skip
+    visitArg x
+  | .unreachable => pure ()
+
+private def visitDecl (decl : Decl) : M Unit := do
+  match decl with
+  | .fdecl (xs := xs) (body := b) .. =>
+    xs.forM visitParam
+    visitFnBody b
+  | .extern (xs := xs) .. =>
+    xs.forM visitParam
 
 end FreeIndices
 
-def FnBody.collectFreeIndices (b : FnBody) (vs : IndexSet) : IndexSet :=
-  FreeIndices.collectFnBody b {} vs
+def FnBody.collectFreeIndices (b : FnBody) (init : IndexSet) : IndexSet := Id.run do
+  let ⟨_, { freeIndices }⟩ := FreeIndices.visitFnBody b |>.run { freeIndices := init }
+  return freeIndices
 
 def FnBody.freeIndices (b : FnBody) : IndexSet :=
   b.collectFreeIndices {}