From c2231df8644d276638120571066bb51490741944 Mon Sep 17 00:00:00 2001
From: Araq <rumpf_a@web.de>
Date: Sat, 21 Feb 2026 09:20:34 +0100
Subject: [PATCH] progress

---
 compiler/semmagic.nim         |  2 ++
 lib/pure/typetraits.nim       |  3 +++
 lib/system/seqs_v2.nim        | 27 ++++++++++++++++++++-------
 lib/system/seqs_v2_reimpl.nim |  2 +-
 4 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/compiler/semmagic.nim b/compiler/semmagic.nim
index 8a91d820f0..87e085d4fd 100644
--- a/compiler/semmagic.nim
+++ b/compiler/semmagic.nim
@@ -236,6 +236,8 @@ proc evalTypeTrait(c: PContext; traitCall: PNode, operand: PType, context: PSym)
     let complexObj = containsGarbageCollectedRef(t) or
                      hasDestructor(t)
     result = newIntNodeT(toInt128(ord(not complexObj)), traitCall, c.idgen, c.graph)
+  of "canFormCycles":
+    result = newIntNodeT(toInt128(ord(types.canFormAcycle(c.graph, operand))), traitCall, c.idgen, c.graph)
   of "hasDefaultValue":
     result = newIntNodeT(toInt128(ord(not operand.requiresInit)), traitCall, c.idgen, c.graph)
   of "isNamedTuple":
diff --git a/lib/pure/typetraits.nim b/lib/pure/typetraits.nim
index 508181316e..3043754f03 100644
--- a/lib/pure/typetraits.nim
+++ b/lib/pure/typetraits.nim
@@ -96,6 +96,9 @@ proc supportsCopyMem*(t: typedesc): bool {.magic: "TypeTrait".}
   ##
   ## Other languages name a type like these `blob`:idx:.
 
+proc canFormCycles*(t: typedesc): bool {.magic: "TypeTrait".}
+  ## Returns true if `t` can form cycles.
+
 proc hasDefaultValue*(t: typedesc): bool {.magic: "TypeTrait".} =
   ## Returns true if `t` has a valid default value.
   runnableExamples:
diff --git a/lib/system/seqs_v2.nim b/lib/system/seqs_v2.nim
index 5a3e437012..f81ac57d59 100644
--- a/lib/system/seqs_v2.nim
+++ b/lib/system/seqs_v2.nim
@@ -35,7 +35,18 @@ when defined(gcYrc):
       lockState = HasNoLock
       releaseRead gYrcGlobalLock
 
-  template yrcMutatorLock*(body: untyped) =
+  template yrcMutatorLock*(t: typedesc; body: untyped) =
+    {.noSideEffect.}:
+      when canFormCycles(t):
+        acquireMutatorLock()
+      try:
+        body
+      finally:
+        {.noSideEffect.}:
+          when canFormCycles(t):
+            releaseMutatorLock()
+
+  template yrcMutatorLockUntyped(body: untyped) =
     {.noSideEffect.}:
       acquireMutatorLock()
       try:
@@ -60,9 +71,11 @@ when defined(gcYrc):
       lockState = prevState
 
 else:
-  template yrcMutatorLock*(body: untyped) =
+  template yrcMutatorLock*(t: typedesc; body: untyped) =
     body
 
+  template yrcMutatorLockUntyped(body: untyped) =
+    body
 
 # Some optimizations here may be not to empty-seq-initialize some symbols, then StrictNotNil complains.
 {.push warning[StrictNotNil]: off.}  # See https://github.com/nim-lang/Nim/issues/21401
@@ -174,7 +187,7 @@ proc shrink*[T](x: var seq[T]; newLen: Natural) {.tags: [], raises: [], noSideEf
       setLen(x, newLen)
   else:
     #sysAssert newLen <= x.len, "invalid newLen parameter for 'shrink'"
-    yrcMutatorLock:
+    yrcMutatorLock(T):
       when not supportsCopyMem(T):
         for i in countdown(x.len - 1, newLen):
           reset x[i]
@@ -186,7 +199,7 @@ proc grow*[T](x: var seq[T]; newLen: Natural; value: T) {.nodestroy.} =
   let oldLen = x.len
   #sysAssert newLen >= x.len, "invalid newLen parameter for 'grow'"
   if newLen <= oldLen: return
-  yrcMutatorLock:
+  yrcMutatorLock(T):
     var xu = cast[ptr NimSeqV2[T]](addr x)
     if xu.p == nil or (xu.p.cap and not strlitFlag) < newLen:
       xu.p = cast[typeof(xu.p)](prepareSeqAddUninit(oldLen, xu.p, newLen - oldLen, sizeof(T), alignof(T)))
@@ -206,7 +219,7 @@ proc add*[T](x: var seq[T]; y: sink T) {.magic: "AppendSeqElem", noSideEffect, n
   ## Generic code becomes much easier to write if the Nim naming scheme is
   ## respected.
   {.cast(noSideEffect).}:
-    yrcMutatorLock:
+    yrcMutatorLock(T):
       let oldLen = x.len
       var xu = cast[ptr NimSeqV2[T]](addr x)
       if xu.p == nil or (xu.p.cap and not strlitFlag) < oldLen+1:
@@ -223,7 +236,7 @@ proc setLen[T](s: var seq[T], newlen: Natural) {.nodestroy.} =
     if newlen < s.len:
       shrink(s, newlen)
     else:
-      yrcMutatorLock:
+      yrcMutatorLock(T):
         let oldLen = s.len
         if newlen <= oldLen: return
         var xu = cast[ptr NimSeqV2[T]](addr s)
@@ -270,7 +283,7 @@ func setLenUninit[T](s: var seq[T], newlen: Natural) {.nodestroy.} =
     if newlen < s.len:
       shrink(s, newlen)
     else:
-      yrcMutatorLock:
+      yrcMutatorLock(T):
         let oldLen = s.len
         if newlen <= oldLen: return
         var xu = cast[ptr NimSeqV2[T]](addr s)
diff --git a/lib/system/seqs_v2_reimpl.nim b/lib/system/seqs_v2_reimpl.nim
index 8dc62f3aad..8e1d4383db 100644
--- a/lib/system/seqs_v2_reimpl.nim
+++ b/lib/system/seqs_v2_reimpl.nim
@@ -18,7 +18,7 @@ type
 
 template frees(s: NimSeqV2Reimpl) =
   if s.p != nil and (s.p.cap and strlitFlag) != strlitFlag:
-    yrcMutatorLock:
+    yrcMutatorLockUntyped:
       when compileOption("threads"):
         deallocShared(s.p)
       else: