Do not run lints that cannot emit

Before this change, adding a lint was a difficult matter
because it always had some overhead involved. This was
because all lints would run, no matter their default level,
or if the user had #![allow]ed them. This PR changes that

compiler/rustc_lint/src/unused.rs

@@ -1025,8 +1025,8 @@ pub(crate) struct UnusedParens {
     parens_in_cast_in_lt: Vec<ast::NodeId>,
 }
 
-impl UnusedParens {
-    pub(crate) fn new() -> Self {
+impl Default for UnusedParens {
+    fpub(crate) fn default() -> Self {
         Self { with_self_ty_parens: false, parens_in_cast_in_lt: Vec::new() }
     }
 }