auto merge of #6719 : kud1ing/rust/rustc_w, r=bstrie

Fixes #6697

src/librustc/driver/driver.rs

@@ -811,7 +811,7 @@ pub fn optgroups() -> ~[getopts::groups::OptGroup] {
                           for detail)", "FEATURE"),
   optopt("", "android-cross-path",
          "The path to the Android NDK", "PATH"),
-  optmulti("W", "warn",
+  optflagopt("W", "warn",
                         "Set lint warnings", "OPT"),
   optmulti("A", "allow",
                         "Set lint allowed", "OPT"),

src/librustc/rustc.rc

@@ -228,9 +228,14 @@ pub fn run_compiler(args: &~[~str], demitter: diagnostic::Emitter) {
         return;
     }
 
+    // Display the available lint options if "-W help" or only "-W" is given.
     let lint_flags = vec::append(getopts::opt_strs(matches, "W"),
                                  getopts::opt_strs(matches, "warn"));
-    if lint_flags.contains(&~"help") {
+
+    let show_lint_options = lint_flags.contains(&~"help") ||
+        (opt_present(matches, "W") && lint_flags.is_empty());
+
+    if show_lint_options {
         describe_warnings();
         return;
     }