Cleanup diagnostics.diag function. NFC

Now the ANSI colors are always used we can make this function much
simpler.

Followup to #25502.

Author: sbc100

tools/diagnostics.py

@@ -57,33 +57,29 @@ def reset_color():
   return '\033[0m'
 
 
-def diag(level, msg, *args):
-  # Format output message as:
-  # <tool>: <level>: msg
-  # With the `<level>:` part being colored accordingly.
-  sys.stderr.write(tool_name + ': ')
+def with_bold_color(color, string):
+  if not color_enabled:
+    return string
+  return output_color(color) + bold() + string + reset_color()
 
-  if color_enabled:
-    output = output_color(level_colors[level]) + bold()
-    if output:
-      sys.stderr.write(output)
 
-  sys.stderr.write(level_prefixes[level])
+def with_bold(string):
+  if not color_enabled:
+    return string
+  return bold() + string + reset_color()
 
-  if color_enabled:
-    output = reset_color() + bold()
-    if output:
-      sys.stderr.write(output)
 
+def diag(level, msg, *args):
+  # Format output message as:
+  # <tool>: <level>: msg
+  # With the `<level>:` part being colored accordingly, and the message itself in bold.
+  prefix = level_prefixes[level]
+  color = level_colors[level]
   if args:
     msg = msg % args
-  sys.stderr.write(str(msg))
-  sys.stderr.write('\n')
 
-  if color_enabled:
-    output = reset_color()
-    if output:
-      sys.stderr.write(output)
+  full_msg = f'{tool_name}: {with_bold_color(color, prefix)}{with_bold(msg)}\n'
+  sys.stderr.write(full_msg)
 
 
 def error(msg, *args):