Improve parameter hints a bit & add emacs support

- just include the name, not e.g. `mut`
 - don't return empty hints (or `_`)

crates/ra_ide/src/display/function_signature.rs

@@ -169,9 +169,22 @@ impl From<&'_ ast::FnDef> for FunctionSignature {
                     res.push(self_param.syntax().text().to_string())
                 }
 
-                res.extend(param_list.params().map(|param| {
-                    param.pat().map(|pat| pat.syntax().text().to_string()).unwrap_or_default()
-                }));
+                res.extend(
+                    param_list
+                        .params()
+                        .map(|param| {
+                            Some(
+                                param
+                                    .pat()?
+                                    .syntax()
+                                    .descendants()
+                                    .find_map(ast::Name::cast)?
+                                    .text()
+                                    .to_string(),
+                            )
+                        })
+                        .map(|param| param.unwrap_or_default()),
+                );
             }
             res
         }