Rollup merge of #119963 - clubby789:spec-allow-internal-unstable, r=compiler-errors

Fix `allow_internal_unstable` for `(min_)specialization`

Fixes #119950

Blocked on #119949 (comment doesn't make sense until that merges)

I'd like to follow this up and look for more instances of not properly checking spans for features but I wanted to fix the motivating issue.

compiler/rustc_index_macros/src/lib.rs

@@ -39,6 +39,9 @@ mod newtype;
     feature = "nightly",
     allow_internal_unstable(step_trait, rustc_attrs, trusted_step, spec_option_partial_eq)
 )]
+// FIXME: Remove the above comment about `min_specialization` once bootstrap is bumped,
+// and the corresponding one on SpecOptionPartialEq
+#[cfg_attr(all(feature = "nightly", not(bootstrap)), allow_internal_unstable(min_specialization))]
 pub fn newtype_index(input: TokenStream) -> TokenStream {
     newtype::newtype(input)
 }