Convert newtype_index to a proc macro

The `macro_rules!` implementation was becomng excessively complicated,
and difficult to modify. The new proc macro implementation should make
it much easier to add new features (e.g. skipping certain `#[derive]`s)

compiler/rustc_macros/src/lib.rs

@@ -1,4 +1,5 @@
 #![feature(proc_macro_diagnostic)]
+#![feature(allow_internal_unstable)]
 #![allow(rustc::default_hash_types)]
 #![recursion_limit = "128"]
 
@@ -8,6 +9,7 @@ use proc_macro::TokenStream;
 
 mod hash_stable;
 mod lift;
+mod newtype;
 mod query;
 mod serialize;
 mod session_diagnostic;
@@ -24,6 +26,27 @@ pub fn symbols(input: TokenStream) -> TokenStream {
     symbols::symbols(input.into()).into()
 }
 
+/// Creates a struct type `S` that can be used as an index with
+/// `IndexVec` and so on.
+///
+/// There are two ways of interacting with these indices:
+///
+/// - The `From` impls are the preferred way. So you can do
+///   `S::from(v)` with a `usize` or `u32`. And you can convert back
+///   to an integer with `u32::from(s)`.
+///
+/// - Alternatively, you can use the methods `S::new(v)` and `s.index()`
+///   to create/return a value.
+///
+/// Internally, the index uses a u32, so the index must not exceed
+/// `u32::MAX`. You can also customize things like the `Debug` impl,
+/// what traits are derived, and so forth via the macro.
+#[proc_macro]
+#[allow_internal_unstable(step_trait, rustc_attrs, trusted_step)]
+pub fn newtype_index(input: TokenStream) -> TokenStream {
+    newtype::newtype(input).into()
+}
+
 decl_derive!([HashStable, attributes(stable_hasher)] => hash_stable::hash_stable_derive);
 decl_derive!(
     [HashStable_Generic, attributes(stable_hasher)] =>