Rollup merge of #88481 - bjorn3:remove_feature_gates, r=cjgillot

Remove some feature gates

The first commit removes various feature gates that are unused. The second commit replaces some `Fn` implementations with `Iterator` implementations, which is much cleaner IMO. The third commit replaces an unboxed_closures feature gate with min_specialization. For some reason the unboxed_closures feature gate suppresses the min_specialization feature gate from triggering on an `TrustedStep` impl. The last comment just turns a regular comment into a doc comment as drive by cleanup. I can move it to a separate PR if preferred.

compiler/rustc_middle/src/lib.rs

@@ -41,13 +41,11 @@
 #![feature(once_cell)]
 #![feature(min_specialization)]
 #![feature(trusted_len)]
-#![feature(test)]
 #![feature(in_band_lifetimes)]
 #![feature(crate_visibility_modifier)]
 #![feature(associated_type_bounds)]
 #![feature(rustc_attrs)]
 #![feature(half_open_range_patterns)]
-#![feature(exclusive_range_pattern)]
 #![feature(control_flow_enum)]
 #![feature(associated_type_defaults)]
 #![feature(iter_zip)]

compiler/rustc_middle/src/ty/codec.rs

@@ -1,10 +1,10 @@
-// This module contains some shared code for encoding and decoding various
-// things from the `ty` module, and in particular implements support for
-// "shorthands" which allow to have pointers back into the already encoded
-// stream instead of re-encoding the same thing twice.
-//
-// The functionality in here is shared between persisting to crate metadata and
-// persisting to incr. comp. caches.
+//! This module contains some shared code for encoding and decoding various
+//! things from the `ty` module, and in particular implements support for
+//! "shorthands" which allow to have pointers back into the already encoded
+//! stream instead of re-encoding the same thing twice.
+//!
+//! The functionality in here is shared between persisting to crate metadata and
+//! persisting to incr. comp. caches.
 
 use crate::arena::ArenaAllocatable;
 use crate::infer::canonical::{CanonicalVarInfo, CanonicalVarInfos};