MSVC send part of the error/warning messages to STDOUT, so concat wit…

…h STDERR...

ccache.c

@@ -1214,7 +1214,49 @@ to_cache(struct args *args)
 		}
 		failed();
 	}
-	if (st.st_size != 0) {
+
+	// MSVC compiler always print the input file name to stdout,
+	// plus parts of the warnings/error messages.
+	// So we have to fusion that into stderr...
+	if (compiler_is_msvc(args)) {
+		char *tmp_stderr2 = format("%s.2", tmp_stderr);
+		if (x_rename(tmp_stderr, tmp_stderr2)) {
+			cc_log("Failed to rename %s to %s: %s", tmp_stderr, tmp_stderr2,
+				   strerror(errno));
+			failed();
+		}
+
+		int fd_result =
+		  open(tmp_stderr, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY, 0666);
+		if (fd_result == -1) {
+			cc_log("Failed opening %s: %s", tmp_stderr, strerror(errno));
+			failed();
+		}
+
+		int fd_stdout = open(tmp_stdout, O_RDONLY | O_BINARY);
+		if (fd_stdout == -1) {
+			cc_log("Failed opening %s: %s", tmp_stdout, strerror(errno));
+			failed();
+		}
+		copy_fd(fd_stdout, fd_result);
+		close(fd_stdout);
+
+		int fd_stderr = open(tmp_stderr2, O_RDONLY | O_BINARY);
+		if (fd_stderr == -1) {
+			cc_log("Failed opening %s: %s", tmp_stderr2, strerror(errno));
+			failed();
+		}
+		copy_fd(fd_stderr, fd_result);
+		close(fd_stderr);
+
+		close(fd_result);
+		tmp_unlink(tmp_stderr2);
+		free(tmp_stderr2);
+
+		st.st_size = 0;
+	}
+
+	if (st.st_size > 0) {
 		cc_log("Compiler produced stdout");
 		stats_update(STATS_STDOUT);
 		tmp_unlink(tmp_stdout);