sysstr: fix (new) cast bug for old strings

Author: ZoomRmc

changelog.md

@@ -34,7 +34,6 @@ errors.
   to more efficiently calculate the symmetric difference of bitsets.
 - `strutils.multiReplace` overload for character set replacements in a single pass.
 	Useful for string sanitation. Follows existing multiReplace semantics.
-- `system.setLenUninit` for the `string` type. Allows setting length without initializing new memory on growth.
 
 - `std/files` adds:
   - Exports `CopyFlag` enum and `FilePermission` type for fine-grained control of file operations
@@ -51,6 +50,7 @@ errors.
     - `copyDirWithPermissions` to recursively preserve attributes
 
 - `system.setLenUninit` now supports refc, JS and VM backends.
+- `system.setLenUninit` for the `string` type. Allows setting length without initializing new memory on growth.
 
 [//]: # "Changes:"
 

lib/system/sysstr.nim

@@ -232,7 +232,7 @@ proc setLengthStr(s: NimString, newLen: int): NimString {.compilerRtl.} =
 proc setLengthStrUninit(s: var string, newlen: Natural) {.nodestroy.} =
   ## Sets the `s` length to `newlen` without zeroing memory on growth.
   ## Terminating zero for cstring compatibility is set.
-  var str = cast[NimString](s.unsafeAddr)
+  var str = cast[NimString](s)
   let n = max(newLen, 0)
   if str == nil:
     if n == 0: return

tests/stdlib/tstring.nim

@@ -122,7 +122,7 @@ proc main() =
 
   block: # setLen #setLenUninit
     proc checkStrInternals(s: string; expectedLen: int) =
-      doAssert s.len == expectedLen
+      doAssert s.len == expectedLen, "expected:" & $expectedLen & " s.len:" & $s.len
       when nimvm: discard
       else:
         when defined(UncheckedArray): # skip JS