Correctly detect color support

Author: GuillaumeGomez

src/librustdoc/doctest.rs

@@ -428,11 +428,13 @@ crate fn make_test(
             // Any errors in parsing should also appear when the doctest is compiled for real, so just
             // send all the errors that librustc_ast emits directly into a `Sink` instead of stderr.
             let sm = Lrc::new(SourceMap::new(FilePathMapping::empty()));
+            supports_color =
+                EmitterWriter::stderr(ColorConfig::Auto, None, false, false, Some(80), false)
+                    .supports_color();
+
             let emitter =
                 EmitterWriter::new(box io::sink(), None, false, false, false, None, false);
 
-            supports_color = emitter.supports_color();
-
             // FIXME(misdreavus): pass `-Z treat-err-as-bug` to the doctest parser
             let handler = Handler::with_emitter(false, None, box emitter);
             let sess = ParseSess::with_span_handler(handler, sm);