From 6543040d40b063ce2c872f582c153c6cff9ef0aa Mon Sep 17 00:00:00 2001
From: Peter Munch-Ellingsen <peterme@peterme.net>
Date: Fri, 21 Nov 2025 21:26:43 +0100
Subject: [PATCH] Fixes #25304 proper test for hlo recursion limit (#25305)

The `warnUser` message kind is probably not the right one, but I left it
as a placeholder. It should probably at least warn if not just straight
up throw an error, was very hard to figure out what went wrong without
any indication. The hard coded 300 should possibly also be changed to
`evalTemplateLimit` or the VM call recursion limit or something.
---
 compiler/hlo.nim     | 19 +++++++++----------
 compiler/semdata.nim |  1 -
 2 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/compiler/hlo.nim b/compiler/hlo.nim
index 9fdec38c0e..94ee96f541 100644
--- a/compiler/hlo.nim
+++ b/compiler/hlo.nim
@@ -10,7 +10,7 @@
 # This include implements the high level optimization pass.
 # included from sem.nim
 
-proc hlo(c: PContext, n: PNode): PNode
+proc hlo(c: PContext, n: PNode, loopDetector: int): PNode
 
 proc evalPattern(c: PContext, n, orig: PNode): PNode =
   internalAssert c.config, n.kind == nkCall and n[0].kind == nkSym
@@ -61,10 +61,11 @@ proc applyPatterns(c: PContext, n: PNode): PNode =
         # activate this pattern again:
         c.patterns[i] = pattern
 
-proc hlo(c: PContext, n: PNode): PNode =
-  inc(c.hloLoopDetector)
+proc hlo(c: PContext, n: PNode, loopDetector: int): PNode =
   # simply stop and do not perform any further transformations:
-  if c.hloLoopDetector > 300: return n
+  if loopDetector > 300:
+    message(c.config, n.info, warnUser, "term rewrite macro instantiation too nested")
+    return n
   case n.kind
   of nkMacroDef, nkTemplateDef, procDefs:
     # already processed (special cases in semstmts.nim)
@@ -80,7 +81,7 @@ proc hlo(c: PContext, n: PNode): PNode =
       # no optimization applied, try subtrees:
       for i in 0..<result.safeLen:
         let a = result[i]
-        let h = hlo(c, a)
+        let h = hlo(c, a, loopDetector)
         if h != a: result[i] = h
     else:
       # perform type checking, so that the replacement still fits:
@@ -90,17 +91,15 @@ proc hlo(c: PContext, n: PNode): PNode =
         result = fitNode(c, n.typ, result, n.info)
       # optimization has been applied so check again:
       result = commonOptimizations(c.graph, c.idgen, c.module, result)
-      result = hlo(c, result)
+      result = hlo(c, result, loopDetector + 1)
       result = commonOptimizations(c.graph, c.idgen, c.module, result)
 
 proc hloBody(c: PContext, n: PNode): PNode =
   # fast exit:
   if c.patterns.len == 0 or optTrMacros notin c.config.options: return n
-  c.hloLoopDetector = 0
-  result = hlo(c, n)
+  result = hlo(c, n, 0)
 
 proc hloStmt(c: PContext, n: PNode): PNode =
   # fast exit:
   if c.patterns.len == 0 or optTrMacros notin c.config.options: return n
-  c.hloLoopDetector = 0
-  result = hlo(c, n)
+  result = hlo(c, n, 0)
diff --git a/compiler/semdata.nim b/compiler/semdata.nim
index 5f26d2d6e7..4dc7e3e26a 100644
--- a/compiler/semdata.nim
+++ b/compiler/semdata.nim
@@ -153,7 +153,6 @@ type
     generics*: seq[TInstantiationPair] # pending list of instantiated generics to compile
     topStmts*: int # counts the number of encountered top level statements
     lastGenericIdx*: int      # used for the generics stack
-    hloLoopDetector*: int     # used to prevent endless loops in the HLO
     inParallelStmt*: int
     instTypeBoundOp*: proc (c: PContext; dc: PSym; t: PType; info: TLineInfo;
                             op: TTypeAttachedOp; col: int): PSym {.nimcall.}