Mark system.newStringUninit sideeffect-free (#24813)

- Allows using with `--experimental:strictFuncs`
- `{.cast(noSideEffect).}:` inside the proc was required to mutate
`s.len`, same as used in `newSeqImpl`.
- Removed now unnecessary `noSideEffect` casts in `system.nim`
- 
Closes #24811

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

Author: ZoomRmc

changelog.md

@@ -34,6 +34,7 @@ errors.
 - `std/math` The `^` symbol now supports floating-point as exponent in addition to the Natural type.
 
 - `system.substr` implementation now uses `copymem` (wrapped C `memcpy`) for copying data, if available at compilation.
+- `system.newStringUninit` is now considered free of side-effects allowing it to be used with `--experimental:strictFuncs`.
 
 ## Language changes
 

lib/pure/strutils.nim

@@ -2356,8 +2356,7 @@ func multiReplace*(s: openArray[char]; replacements: varargs[(set[char], char)])
     # Sanitize a filename with Windows-incompatible characters
     const file = "a/file:with?invalid*chars.txt"
     doAssert file.multiReplace(WinSanitationRules) == "a-file-with_invalid_chars.txt"
-  {.cast(noSideEffect).}:
-    result = newStringUninit(s.len)
+  result = newStringUninit(s.len)
   for i in 0..<s.len:
     var nextChar = s[i]
     for subs, by in replacements.items:

lib/system.nim

@@ -1685,7 +1685,7 @@ when not defined(js):
     else:
       {.error: "The type T cannot contain managed memory or have destructors".}
 
-  proc newStringUninit*(len: Natural): string =
+  proc newStringUninit*(len: Natural): string {.noSideEffect.} =
     ## Returns a new string of length `len` but with uninitialized
     ## content. One needs to fill the string character after character
     ## with the index operator `s[i]`.
@@ -1696,15 +1696,16 @@ when not defined(js):
       result = newString(len)
     else:
       result = newStringOfCap(len)
-      when defined(nimSeqsV2):
-        let s = cast[ptr NimStringV2](addr result)
-        if len > 0:
+      {.cast(noSideEffect).}:
+        when defined(nimSeqsV2):
+          let s = cast[ptr NimStringV2](addr result)
+          if len > 0:
+            s.len = len
+            s.p.data[len] = '\0'
+        else:
+          let s = cast[NimString](result)
           s.len = len
-          s.p.data[len] = '\0'
-      else:
-        let s = cast[NimString](result)
-        s.len = len
-        s.data[len] = '\0'
+          s.data[len] = '\0'
 else:
   proc newStringUninit*(len: Natural): string {.
     magic: "NewString", importc: "mnewString", noSideEffect.}
@@ -2794,8 +2795,7 @@ proc substr*(a: openArray[char]): string =
     assert a.toOpenArray(2, 5).substr() == "cdef"
     assert a.toOpenArray(2, high(a)).substr() == "cdefgh"  # From index 2 to `high(a)`
     doAssertRaises(IndexDefect): discard a.toOpenArray(5, 99).substr()
-  {.cast(noSideEffect).}:
-    result = newStringUninit(a.len)
+  result = newStringUninit(a.len)
   when NotJSnotVMnotNims:
     if a.len > 0:
       copyMem(result[0].addr, a[0].unsafeAddr, a.len)
@@ -2830,8 +2830,7 @@ proc substr*(s: string; first, last: int): string = # A bug with `magic: Slice`
     first = max(first, 0)
     last = min(last, high(s))
     L = max(last - first + 1, 0)
-  {.cast(noSideEffect).}:
-    result = newStringUninit(L)
+  result = newStringUninit(L)
   when NotJSnotVMnotNims:
     if L > 0:
       copyMem(result[0].addr, s[first].unsafeAddr, L)