Auto merge of #105880 - Nilstrieb:make-newtypes-less-not-rust, r=oli-obk

Improve syntax of `newtype_index`

This makes it more like proper Rust and also makes the implementation a lot simpler.

Mostly just turns weird flags in the body into proper attributes.

It should probably also be converted to an attribute macro instead of function-like, but that can be done in a future PR.

compiler/rustc_middle/src/middle/region.rs

@@ -147,9 +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 `...`.
-    pub struct FirstStatementIndex {
-        derive [HashStable]
-    }
+    #[derive(HashStable)]
+    pub struct FirstStatementIndex {}
 }
 
 // compilation error if size of `ScopeData` is not the same as a `u32`