now bootstraps with -d:nimsso

compiler/layeredtable.nim

@@ -46,12 +46,11 @@ proc setToPreviousLayer*(pt: var LayeredIdTable) {.inline.} =
   when useRef:
     pt = pt.nextLayer
   else:
-    when defined(gcDestructors):
-      pt = pt.nextLayer[]
-    else:
-      # workaround refc
-      let tmp = pt.nextLayer[]
-      pt = tmp
+    # Must read nextLayer into a temp before destroying pt:
+    # `pt = pt.nextLayer[]` would call eqcopy(&pt, &(*pt.nextLayer)) which
+    # decrements pt.nextLayer's rc (freeing it) before reading pt.nextLayer.nextLayer.
+    let tmp = pt.nextLayer[]
+    pt = tmp
 
 iterator pairs*(pt: LayeredIdTable): (ItemId, PType) =
   var tm = pt

lib/system/strs_v3.nim

@@ -133,7 +133,9 @@ proc ensureUniqueLong(s: var SmallString; oldLen, newLen: int) =
   if isHeap and s.more.rc == 1 and newLen <= cap:
     s.more.fullLen = newLen
   else:
-    let newCap = max(newLen, resize(cap))
+    # Only grow capacity when actually needed; pure COW copies (newLen <= cap)
+    # preserve the existing capacity to avoid exponential growth via repeated COW.
+    let newCap = if newLen > cap: max(newLen, resize(cap)) else: cap
     let p = cast[ptr LongString](alloc(sizeof(int) * 3 + newCap + 1))
     p.rc = 1
     p.fullLen = newLen