Use #[derive] instead of custom syntax in all newtype_index

compiler/rustc_middle/src/middle/region.rs

@@ -147,8 +147,8 @@ rustc_index::newtype_index! {
     ///
     /// * The subscope with `first_statement_index == 1` is scope of `c`,
     ///   and thus does not include EXPR_2, but covers the `...`.
+    #[derive(HashStable)]
     pub struct FirstStatementIndex {
-        derive [HashStable]
     }
 }