Rollup merge of rust-lang#119949 - clubby789:spec-newtype-comment, r=nnethercote

Add note on SpecOptionPartialEq to `newtype_index`

https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/.60SpecOptionPartialEq.60.20error

r? `@nnethercote`

Author: GuillaumeGomez

core/src/option.rs

@@ -2147,6 +2147,7 @@ impl<T: PartialEq> PartialEq for Option<T> {
 ///
 /// Once that's fixed, `Option` should go back to deriving `PartialEq`, as
 /// it used to do before <https://github.com/rust-lang/rust/pull/103556>.
+/// The comment regarding this trait on the `newtype_index` macro should be removed if this is done.
 #[unstable(feature = "spec_option_partial_eq", issue = "none", reason = "exposed only for rustc")]
 #[doc(hidden)]
 pub trait SpecOptionPartialEq: Sized {