fix Registry::args for plugins loaded with --extra-plugins

Author: oli-obk

src/librustc_plugin/registry.rs

@@ -92,8 +92,11 @@ impl<'a> Registry<'a> {
     /// ```no_run
     /// #![plugin(my_plugin_name(... args ...))]
     /// ```
-    pub fn args<'b>(&'b self) -> &'b Vec<P<ast::MetaItem>> {
-        self.args_hidden.as_ref().expect("args not set")
+    ///
+    /// Returns empty slice in case the plugin was loaded
+    /// with `--extra-plugins`
+    pub fn args<'b>(&'b self) -> &'b [P<ast::MetaItem>] {
+        self.args_hidden.as_ref().map(|v| &v[..]).unwrap_or(&[])
     }
 
     /// Register a syntax extension of any kind.

src/test/run-pass-fulldeps/auxiliary/plugin_args.rs

@@ -45,7 +45,7 @@ impl TTMacroExpander for Expander {
 
 #[plugin_registrar]
 pub fn plugin_registrar(reg: &mut Registry) {
-    let args = reg.args().clone();
+    let args = reg.args().to_owned();
     reg.register_syntax_extension(token::intern("plugin_args"),
         // FIXME (#22405): Replace `Box::new` with `box` here when/if possible.
         NormalTT(Box::new(Expander { args: args, }), None, false));