Rollup merge of #95308 - bjorn3:more_stable_proc_macro, r=Mark-Simulacrum

Reduce the amount of unstable features used in libproc_macro

This makes it easier to adapt the source for stable when copying it into rust-analyzer to load rustc compiled proc macros.

library/proc_macro/src/lib.rs

@@ -17,18 +17,18 @@
     test(no_crate_inject, attr(deny(warnings))),
     test(attr(allow(dead_code, deprecated, unused_variables, unused_mut)))
 )]
+// This library is copied into rust-analyzer to allow loading rustc compiled proc macros.
+// Please avoid unstable features where possible to minimize the amount of changes necessary
+// to make it compile with rust-analyzer on stable.
 #![feature(rustc_allow_const_fn_unstable)]
 #![feature(nll)]
 #![feature(staged_api)]
 #![feature(allow_internal_unstable)]
 #![feature(decl_macro)]
-#![feature(extern_types)]
 #![feature(negative_impls)]
-#![feature(auto_traits)]
 #![feature(restricted_std)]
 #![feature(rustc_attrs)]
 #![feature(min_specialization)]
-#![feature(panic_update_hook)]
 #![recursion_limit = "256"]
 
 #[unstable(feature = "proc_macro_internals", issue = "27812")]