compiler/msgs: Default to stderr for diagnostics

Previously we were defaulting to stdout for diagnostics, which could interfere
with scripts or `nim c -r' programs outputting their results to stdout,
possibly mixing their output with compiler messages.

This change makes now Nim to be inline with other compilers emitting
diagnostics to stderr. Also now --stdout option has proper meaning making all
diagnostics to be sent to stdout instead.

NOTE: Tester now calls compiler with --stdout.

compiler/msgs.nim

@@ -734,20 +734,22 @@ proc outWriteln*(s: string) =
     flushFile(stdout)
 
 proc msgWriteln*(s: string) =
-  ## Writes to stdout. If --stdout option is given, writes to stderr instead.
+  ## Writes to stderr. If --stdout option is given, writes to stdout instead.
 
   #if gCmd == cmdIdeTools and optCDebug notin gGlobalOptions: return
 
   if not isNil(writelnHook):
     writelnHook(s)
   elif optStdout in gGlobalOptions:
-    if eStdErr in errorOutputs:
-      writeLine(stderr, s)
-      flushFile(stderr)
-  else:
     if eStdOut in errorOutputs:
       writeLine(stdout, s)
       flushFile(stdout)
+  else:
+    if eStdErr in errorOutputs:
+      writeLine(stderr, s)
+      # On Windows stderr is fully-buffered when piped, regardless of C std.
+      when defined(windows):
+        flushFile(stderr)
 
 macro callIgnoringStyle(theProc: typed, first: typed,
                         args: varargs[expr]): stmt =
@@ -767,8 +769,9 @@ macro callIgnoringStyle(theProc: typed, first: typed,
        typ != typTerminalCmd:
       result.add(arg)
 
-macro callStyledEcho(args: varargs[expr]): stmt =
-  result = newCall(bindSym"styledEcho")
+macro callStyledWriteLineStderr(args: varargs[expr]): stmt =
+  result = newCall(bindSym"styledWriteLine")
+  result.add(bindSym"stderr")
   for arg in children(args[0][1]):
     result.add(arg)
 
@@ -782,16 +785,18 @@ template styledMsgWriteln*(args: varargs[expr]) =
   if not isNil(writelnHook):
     callIgnoringStyle(callWritelnHook, nil, args)
   elif optStdout in gGlobalOptions:
-    if eStdErr in errorOutputs:
-      callIgnoringStyle(writeLine, stderr, args)
-      flushFile(stderr)
-  else:
     if eStdOut in errorOutputs:
+      callIgnoringStyle(writeLine, stdout, args)
+      flushFile(stdout)
+  else:
+    if eStdErr in errorOutputs:
       if optUseColors in gGlobalOptions:
-        callStyledEcho(args)
+        callStyledWriteLineStderr(args)
       else:
-        callIgnoringStyle(writeLine, stdout, args)
-      flushFile stdout
+        callIgnoringStyle(writeLine, stderr, args)
+      # On Windows stderr is fully-buffered when piped, regardless of C std.
+      when defined(windows):
+        flushFile(stderr)
 
 proc coordToStr(coord: int): string =
   if coord == -1: result = "???"
@@ -885,8 +890,7 @@ proc rawMessage*(msg: TMsgKind, arg: string) =
 
 proc resetAttributes* =
   if {optUseColors, optStdout} * gGlobalOptions == {optUseColors}:
-    terminal.resetAttributes()
-    stdout.flushFile()
+    terminal.resetAttributes(stderr)
 
 proc writeSurroundingSrc(info: TLineInfo) =
   const indent = "  "
@@ -1032,5 +1036,5 @@ proc listHints*() =
     ])
 
 # enable colors by default on terminals
-if terminal.isatty(stdout):
+if terminal.isatty(stderr):
   incl(gGlobalOptions, optUseColors)