Here's the list of features ordered by the number of times its used by a crate on crates.io, counting all crates, reproduced below.
498 test
268 plugin
164 rustc_private
156 plugin_registrar
131 core
97 collections
85 custom_derive
84 quote
84 box_syntax
65 libc
64 alloc
63 phase
59 std_misc
53 proc_macro
50 const_fn
49 unboxed_closures
48 io
44 custom_attribute
42 macro_rules
41 asm
40 slice_patterns
36 core_intrinsics
36 convert
33 heap_api
31 unsafe_destructor
31 globs
29 question_mark
26 unique
26 box_patterns
24 step_by
24 old_io
23 unicode
23 associated_type_defaults
22 associated_consts
21 slicing_syntax
19 zero_one
19 proc_macro_lib
18 path
18 no_std
15 trace_macros
15 stmt_expr_attributes
15 specialization
15 optin_builtin_traits
15 fs
14 path_ext
14 filling_drop
13 unsafe_no_drop_flag
13 type_ascription
13 fnbox
13 conservative_impl_trait
13 alloc_system
12 oom
12 collections_bound
11 env
10 rand
10 if_let
10 hash
10 allocator
9 try_from
9 raw
9 os
9 lang_items
9 concat_idents
8 old_path
8 iter_arith
8 inclusive_range_syntax
8 hashmap_hasher
8 core_slice_ext
8 btree_range
7 type_macros
7 str_char
7 prelude_import
7 non_ascii_idents
7 fn_traits
7 coerce_unsized
6 unsize
6 start
6 staged_api
6 simd
6 reflect_marker
6 placement_in_syntax
6 linkage
6 drain
6 associated_types
5 vec_push_all
5 scoped
5 repr_simd
5 old_orphan_check
5 nonzero
5 macro_reexport
5 get_type_id
5 dropck_parametricity
5 default_type_params
5 cfg_target_feature
5 catch_panic
5 box_raw
5 augmented_assignments
5 append
5 {}
4 untagged_unions
4 tuple_indexing
4 simd_ffi
4 shared
4 rt
4 platform_intrinsics
4 op_assign_traits
4 mpsc_select
4 main
4 link_llvm_intrinsics
4 ip_addr
4 int_uint
4 fs_walk
4 fixed_size_array
4 exit_status
4 core_prelude
4 ...
3 thread_local
3 tempdir
3 tcp
3 str_escape
3 std_panic
3 static_in_const
3 slice_bytes
3 scheme
3 rustc_macro
3 result_expect
3 recover
3 read_exact
3 rc_weak
3 pub_restricted
3 ptr_as_ref
3 process_exec
3 process
3 placement_new_protocol
3 pattern
3 path_relative_from
3 num_bits_bytes
3 net
3 naked_functions
3 lookup_host
3 link_args
3 ip
3 intrinsics
3 into_cow
3 integer_atomics
3 fs_time
3 enumset
3 duration
3 drop_types_in_const
3 core_float
3 compile
3 collections_range
3 arc_counts
3
2 unmarked_api
2 time2
2 step_trait
2 static_mutex
2 slice_concat_ext
2 sip_hash_13
2 set_stdio
2 set_recovery
2 rustdoc
2 rustc_attrs
2 page_size
2 on_unimplemented
2 no_core
2 never_type
2 iter_order
2 iter_cmp
2 forktable
2 float_extras
2 field_init_shorthand
2 clone_from_slice
2 braced_empty_structs
2 bit_vec_append_split_off
2 advanced_slice_patterns
1 wrapping
1 while_let
1 wait_timeout
1 vecmap
1 unwind_attributes
1 unboxed_closure_sugar
1 types]
1 thread_sleep
1 str_words
1 structural_match
1 string_split_off
1 split_off
1 slice_splits
1 scoped_tls
1 rustc_macro_lib
1 rustc_builtins
1 repeat_str
1 read_and_zero
1 rc_unique
1 rc_counts
1 range_inclusive
1 panic_propagate
1 ordering_chaining
1 negate_unsigned
1 map_values_mut
1 map_in_place
1 map_entry_recover_keys
1 managed_boxes
1 log_syntax
1 list
1 linked_list_extras
1 iter_idx
1 iter_arith_traits
1 io_take_into_inner
1 inclusive_range
1 import_shadowing
1 hasher_write
1 hash_default
1 fused
1 fs_ext
1 fs_canonicalize
1 float_from_str_radix
1 drop_in_place
1 dir_entry_ext
1 dir_builder
1 deque_extras
1 deprecated
1 debug_builders
1 cstr_to_str
1 cstr_memory
1 copy_from_slice
1 collections_drain
1 cfg_target_vendor
1 cell_extras
1 btree_append
1 borrow_state
1 binary_heap_extras
1 attr_literals
1 as_unsafe_cell