Fix argument alias handling for -O and -C opt-level

Author: varkor

src/librustc/session/config.rs

@@ -2181,10 +2181,15 @@ pub fn build_session_options_and_crate_config(
         TargetTriple::from_triple(host_triple())
     };
     let opt_level = {
-        if matches.opt_present("O") {
-            if cg.opt_level.is_some() {
-                early_error(error_format, "-O and -C opt-level both provided");
+        let max_o = matches.opt_positions("O").into_iter().max();
+        let max_c = matches.opt_strs_pos("C").into_iter().flat_map(|(i, s)| {
+            if let Some("opt-level") = s.splitn(2, '=').next() {
+                Some(i)
+            } else {
+                None
             }
+        }).max();
+        if max_o > max_c {
             OptLevel::Default
         } else {
             match cg.opt_level.as_ref().map(String::as_ref) {