better beginStore API

lib/std/formatfloat.nim

@@ -18,12 +18,12 @@ proc addCstringN(result: var string, buf: cstring; buflen: int) =
   # no nimvm support needed, so it doesn't need to be fast here either
   let oldLen = result.len
   let newLen = oldLen + buflen
-  result.setLen newLen
   {.cast(noSideEffect).}:
-    when declared(completeStore):
-      c_memcpy(beginStore(result, buflen, oldLen), buf, buflen.csize_t)
+    when declared(beginStore):
+      c_memcpy(beginStore(result, newLen, oldLen), buf, buflen.csize_t)
       endStore(result)
     else:
+      result.setLen newLen
       discard c_memcpy(result[oldLen].addr, buf, buflen.csize_t)
 
 import std/private/[dragonbox, schubfach]

lib/std/strbasics.nim

@@ -84,7 +84,7 @@ func setSlice*(s: var string, slice: Slice[int]) =
       when not declared(moveMem):
         impl()
       else:
-        let p = beginStore(s, last - first + 1)
+        let p = beginStore(s, s.len)
         moveMem(p, addr p[first], last - first + 1)
         endStore(s)
   s.setLen(last - first + 1)

lib/std/syncio.nim

@@ -485,7 +485,7 @@ proc readLine*(f: File, line: var string): bool {.tags: [ReadIOEffect],
     while true:
       # fixes #9634; this pattern may need to be abstracted as a template if reused;
       # likely other io procs need this for correctness.
-      fgetsSuccess = c_fgets(cast[cstring](beginStore(line, sp, pos)), sp.cint, f) != nil
+      fgetsSuccess = c_fgets(cast[cstring](beginStore(line, pos + sp, pos)), sp.cint, f) != nil
       endStore(line)
       if fgetsSuccess: break
       when not defined(nimscript):