fixes #11618 (#11969)

lib/system/io.nim

@@ -194,9 +194,30 @@ proc writeChars*(f: File, a: openArray[char], start, len: Natural): int {.
   var x = cast[ptr UncheckedArray[int8]](a)
   result = writeBuffer(f, addr(x[int(start)]), len)
 
+when defined(windows):
+  proc writeWindows(f: File; s: string; doRaise = false) =
+    # Don't ask why but the 'printf' family of function is the only thing
+    # that writes utf-8 strings reliably on Windows. At least on my Win 10
+    # machine. We also enable `setConsoleOutputCP(65001)` now by default.
+    # But we cannot call printf directly as the string might contain \0.
+    # So we have to loop over all the sections separated by potential \0s.
+    var i = c_fprintf(f, "%s", s)
+    while i < s.len and false:
+      if s[i] == '\0':
+        inc i
+      else:
+        let w = c_fprintf(f, "%s", unsafeAddr s[i])
+        if w <= 0:
+          if doRaise: raiseEIO("cannot write string to file")
+          break
+        inc i, w
+
 proc write*(f: File, s: string) {.tags: [WriteIOEffect], benign.} =
-  if writeBuffer(f, cstring(s), s.len) != s.len:
-    raiseEIO("cannot write string to file")
+  when defined(windows):
+    writeWindows(f, s, doRaise = true)
+  else:
+    if writeBuffer(f, cstring(s), s.len) != s.len:
+      raiseEIO("cannot write string to file")
 {.pop.}
 
 when NoFakeVars:
@@ -559,8 +580,11 @@ when declared(stdout):
     when defined(windows) and compileOption("threads"):
       acquireSys echoLock
     for s in args:
-      discard c_fwrite(s.cstring, s.len, 1, stdout)
-    const linefeed = "\n" # can be 1 or more chars
+      when defined(windows):
+        writeWindows(stdout, s)
+      else:
+        discard c_fwrite(s.cstring, s.len, 1, stdout)
+    const linefeed = "\n"
     discard c_fwrite(linefeed.cstring, linefeed.len, 1, stdout)
     discard c_fflush(stdout)
     when not defined(windows) and not defined(android) and not defined(nintendoswitch):