Document newtype_index attributes.

Author: nnethercote

compiler/rustc_macros/src/lib.rs

@@ -57,8 +57,18 @@ pub fn symbols(input: TokenStream) -> TokenStream {
 ///   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.
+/// `u32::MAX`.
+///
+/// The impls provided by default are Clone, Copy, PartialEq, Eq, and Hash.
+///
+/// Accepted attributes for customization:
+/// - `#[derive(HashStable_Generic)]`/`#[derive(HashStable)]`: derives
+///   `HashStable`, as normal.
+/// - `#[encodable]`: derives `Encodable`/`Decodable`.
+/// - `#[orderable]`: derives `PartialOrd`/`Ord`, plus step-related methods.
+/// - `#[debug_format = "Foo({})"]`: derives `Debug` with particular output.
+/// - `#[max = 0xFFFF_FFFD]` specifies the max value, which allows niche
+///   optimizations. The default max value is 0xFFFF_FF00.
 #[proc_macro]
 #[allow_internal_unstable(step_trait, rustc_attrs, trusted_step, spec_option_partial_eq)]
 pub fn newtype_index(input: TokenStream) -> TokenStream {