Rollup merge of #87769 - m-ou-se:alloc-features-cleanup, r=yaahc,dtolnay

Alloc features cleanup

This sorts and categorizes the `#![features]` in `alloc` and removes unused ones.

This is part of #87766

The following feature attributes were unnecessary and are removed:

```diff
// Library features:
-#![feature(cow_is_borrowed)]
-#![feature(maybe_uninit_uninit_array)]
-#![feature(slice_partition_dedup)]

// Language features:
-#![feature(arbitrary_self_types)]
-#![feature(auto_traits)]
-#![feature(box_patterns)]
-#![feature(decl_macro)]
-#![feature(nll)]
```

library/alloc/src/lib.rs

@@ -80,87 +80,90 @@
 )]
 #![no_std]
 #![needs_allocator]
+//
+// Lints:
+#![deny(unsafe_op_in_unsafe_fn)]
 #![warn(deprecated_in_future)]
-#![warn(missing_docs)]
 #![warn(missing_debug_implementations)]
+#![warn(missing_docs)]
 #![allow(explicit_outlives_requirements)]
-#![deny(unsafe_op_in_unsafe_fn)]
-#![feature(rustc_allow_const_fn_unstable)]
-#![cfg_attr(not(test), feature(generator_trait))]
-#![cfg_attr(test, feature(test))]
-#![cfg_attr(test, feature(new_uninit))]
+//
+// Library features:
+#![feature(alloc_layout_extra)]
 #![feature(allocator_api)]
 #![feature(array_chunks)]
 #![feature(array_methods)]
 #![feature(array_windows)]
-#![feature(allow_internal_unstable)]
-#![feature(arbitrary_self_types)]
 #![feature(async_stream)]
-#![feature(box_patterns)]
-#![feature(box_syntax)]
-#![feature(cfg_sanitize)]
-#![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
 #![cfg_attr(not(no_global_oom_handling), feature(const_btree_new))]
-#![feature(const_fn_trait_bound)]
-#![feature(cow_is_borrowed)]
 #![feature(const_cow_is_borrowed)]
-#![feature(const_trait_impl)]
-#![feature(destructuring_assignment)]
-#![feature(dispatch_from_dyn)]
 #![feature(core_intrinsics)]
-#![feature(dropck_eyepatch)]
+#![feature(dispatch_from_dyn)]
 #![feature(exact_size_is_empty)]
-#![feature(exclusive_range_pattern)]
 #![feature(extend_one)]
 #![feature(fmt_internals)]
 #![feature(fn_traits)]
-#![feature(fundamental)]
 #![feature(inplace_iteration)]
-// Technically, this is a bug in rustdoc: rustdoc sees the documentation on `#[lang = slice_alloc]`
-// blocks is for `&[T]`, which also has documentation using this feature in `core`, and gets mad
-// that the feature-gate isn't enabled. Ideally, it wouldn't check for the feature gate for docs
-// from other crates, but since this can only appear for lang items, it doesn't seem worth fixing.
-#![feature(intra_doc_pointers)]
 #![feature(iter_advance_by)]
 #![feature(iter_zip)]
-#![feature(lang_items)]
 #![feature(layout_for_ptr)]
-#![feature(negative_impls)]
-#![feature(never_type)]
-#![feature(nll)]
+#![feature(maybe_uninit_extra)]
+#![feature(maybe_uninit_slice)]
+#![cfg_attr(test, feature(new_uninit))]
 #![feature(nonnull_slice_from_raw_parts)]
-#![feature(auto_traits)]
 #![feature(option_result_unwrap_unchecked)]
 #![feature(pattern)]
 #![feature(ptr_internals)]
-#![feature(rustc_attrs)]
 #![feature(receiver_trait)]
-#![feature(min_specialization)]
 #![feature(set_ptr_value)]
+#![feature(slice_group_by)]
 #![feature(slice_ptr_get)]
 #![feature(slice_ptr_len)]
 #![feature(slice_range)]
-#![feature(staged_api)]
 #![feature(str_internals)]
 #![feature(trusted_len)]
-#![feature(unboxed_closures)]
+#![feature(trusted_random_access)]
+#![feature(try_trait_v2)]
 #![feature(unicode_internals)]
 #![feature(unsize)]
-#![feature(unsized_fn_params)]
+//
+// Language features:
 #![feature(allocator_internals)]
-#![feature(slice_partition_dedup)]
-#![feature(maybe_uninit_extra, maybe_uninit_slice, maybe_uninit_uninit_array)]
-#![feature(alloc_layout_extra)]
-#![feature(trusted_random_access)]
-#![feature(try_trait_v2)]
+#![feature(allow_internal_unstable)]
 #![feature(associated_type_bounds)]
-#![feature(slice_group_by)]
-#![feature(decl_macro)]
+#![feature(box_syntax)]
+#![feature(cfg_sanitize)]
+#![feature(cfg_target_has_atomic)]
+#![feature(const_fn_trait_bound)]
+#![feature(const_trait_impl)]
+#![feature(destructuring_assignment)]
+#![feature(dropck_eyepatch)]
+#![feature(exclusive_range_pattern)]
+#![feature(fundamental)]
+#![cfg_attr(not(test), feature(generator_trait))]
+#![feature(lang_items)]
+#![feature(min_specialization)]
+#![feature(negative_impls)]
+#![feature(never_type)]
+#![feature(nll)] // Not necessary, but here to test the `nll` feature.
+#![feature(rustc_allow_const_fn_unstable)]
+#![feature(rustc_attrs)]
+#![feature(staged_api)]
+#![cfg_attr(test, feature(test))]
+#![feature(unboxed_closures)]
+#![feature(unsized_fn_params)]
+//
+// Rustdoc features:
 #![feature(doc_cfg)]
 #![cfg_attr(not(bootstrap), feature(doc_cfg_hide))]
-// Allow testing this library
+// Technically, this is a bug in rustdoc: rustdoc sees the documentation on `#[lang = slice_alloc]`
+// blocks is for `&[T]`, which also has documentation using this feature in `core`, and gets mad
+// that the feature-gate isn't enabled. Ideally, it wouldn't check for the feature gate for docs
+// from other crates, but since this can only appear for lang items, it doesn't seem worth fixing.
+#![feature(intra_doc_pointers)]
 
+// Allow testing this library
 #[cfg(test)]
 #[macro_use]
 extern crate std;