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_borrowck/src/member_constraints.rs

@@ -144,7 +144,7 @@ impl<R> MemberConstraintSet<'tcx, R>
 where
     R: Copy + Hash + Eq,
 {
-    crate fn all_indices(&self) -> impl Iterator<Item = NllMemberConstraintIndex> {
+    crate fn all_indices(&self) -> impl Iterator<Item = NllMemberConstraintIndex> + '_ {
         self.constraints.indices()
     }
 

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -497,7 +497,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     }
 
     /// Returns an iterator over all the region indices.
-    pub fn regions(&self) -> impl Iterator<Item = RegionVid> {
+    pub fn regions(&self) -> impl Iterator<Item = RegionVid> + '_ {
         self.definitions.indices()
     }