Rollup merge of #85850 - bjorn3:less_feature_gates, r=jyn514

Remove unused feature gates

The first commit removes a usage of a feature gate, but I don't expect it to be controversial as the feature gate was only used to workaround a limitation of rust in the past. (closures never being `Clone`)

The second commit uses `#[allow_internal_unstable]` to avoid leaking the `trusted_step` feature gate usage from inside the index newtype macro. It didn't work for the `min_specialization` feature gate though.

The third commit removes (almost) all feature gates from the compiler that weren't used anyway.

compiler/rustc_interface/src/lib.rs

@@ -1,6 +1,5 @@
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
-#![feature(box_syntax)]
 #![feature(internal_output_capture)]
 #![feature(nll)]
 #![feature(generator_trait)]