new minor language feature: .noSideEffect blocks like .gcsafe blocks

lib/core/seqs.nim

@@ -82,7 +82,7 @@ type
     cap: int
     region: Allocator
 
-proc newSeqPayload(cap, elemSize: int): pointer {.compilerRtl.} =
+proc newSeqPayload(cap, elemSize: int): pointer {.compilerRtl, noSideEffect.} =
   # we have to use type erasure here as Nim does not support generic
   # compilerProcs. Oh well, this will all be inlined anyway.
   if cap <= 0:
@@ -94,7 +94,8 @@ proc newSeqPayload(cap, elemSize: int): pointer {.compilerRtl.} =
   else:
     result = nil
 
-proc prepareSeqAdd(len: int; p: pointer; addlen, elemSize: int): pointer {.compilerRtl.} =
+proc prepareSeqAdd(len: int; p: pointer; addlen, elemSize: int): pointer {.
+    compilerRtl, noSideEffect.} =
   if len+addlen <= len:
     result = p
   elif p == nil:
@@ -128,7 +129,7 @@ proc grow*[T](x: var seq[T]; newLen: Natural; value: T) =
   xu.p = cast[typeof(xu.p)](prepareSeqAdd(oldLen, xu.p, newLen - oldLen, sizeof(T)))
   xu.len = newLen
   for i in oldLen .. newLen-1:
-    x.data[i] = value
+    xu.p.data[i] = value
 
 proc setLen[T](s: var seq[T], newlen: Natural) =
   if newlen < s.len: