Here’s a list of the unstable features used by the 100 most popular crates on crates.io (by reverse deps). Perhaps this query and others like it can help drive stabilization decisions.
30 crates report using some unstable feature. A number are just using the test
feature, presumably only in the test suite, mostly for benchmarking it seems. Some also seem to be cfgd to optionally support nightly features.
work/ex/unstable/res/stable/reg/bincode-0.6.0/log.txt
2:boom! unstable-feature: custom_attribute
3:boom! unstable-feature: custom_derive
4:boom! unstable-feature: plugin
work/ex/unstable/res/stable/reg/nom-2.0.1/log.txt
2:boom! unstable-feature: trace_macros
work/ex/unstable/res/stable/reg/pkg-config-0.3.8/log.txt
2:boom! unstable-feature: static_mutex
work/ex/unstable/res/stable/reg/bit-vec-0.4.3/log.txt
2:boom! unstable-feature: bit_vec_append_split_off
3:boom! unstable-feature: collections
4:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/tokio-core-0.1.2/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/rand-0.3.15/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/serde_derive-0.8.21/log.txt
2:boom! unstable-feature: proc_macro
3:boom! unstable-feature: proc_macro_lib
work/ex/unstable/res/stable/reg/image-0.10.4/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/csv-0.14.7/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/rustc-serialize-0.3.22/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/cgmath-0.12.0/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/quickcheck_macros-0.4.0/log.txt
2:boom! unstable-feature: custom_attribute
3:boom! unstable-feature: plugin
4:boom! unstable-feature: plugin_registrar
5:boom! unstable-feature: rustc_private
work/ex/unstable/res/stable/reg/clippy-0.0.104/log.txt
2:boom! unstable-feature: associated_consts
3:boom! unstable-feature: associated_type_defaults
4:boom! unstable-feature: attr_literals
5:boom! unstable-feature: borrow_state
6:boom! unstable-feature: box_syntax
7:boom! unstable-feature: collections
8:boom! unstable-feature: const_fn
9:boom! unstable-feature: custom_attribute
10:boom! unstable-feature: inclusive_range_syntax
11:boom! unstable-feature: never_type
12:boom! unstable-feature: non_ascii_idents
13:boom! unstable-feature: plugin
14:boom! unstable-feature: plugin_registrar
15:boom! unstable-feature: rustc_private
16:boom! unstable-feature: static_in_const
17:boom! unstable-feature: step_by
18:boom! unstable-feature: test
19:boom! unstable-feature: untagged_unions
work/ex/unstable/res/stable/reg/regex_macros-0.1.38/log.txt
2:boom! unstable-feature: plugin_registrar
3:boom! unstable-feature: quote
4:boom! unstable-feature: rustc_private
work/ex/unstable/res/stable/reg/byteorder-0.5.3/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/time-0.1.35/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/json-tools-0.3.0/log.txt
2:boom! unstable-feature: collections
3:boom! unstable-feature: core
4:boom! unstable-feature: io
5:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/lazy_static-0.2.2/log.txt
2:boom! unstable-feature: const_fn
work/ex/unstable/res/stable/reg/num-0.1.36/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/itertools-0.5.8/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/serde_macros-0.8.9/log.txt
2:boom! unstable-feature: plugin_registrar
3:boom! unstable-feature: rustc_private
work/ex/unstable/res/stable/reg/glium-0.15.0/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/error-chain-0.7.2/log.txt
2:boom! unstable-feature: trace_macros
work/ex/unstable/res/stable/reg/regex-0.1.80/log.txt
2:boom! unstable-feature: plugin
3:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/docopt-0.6.86/log.txt
2:boom! unstable-feature: plugin
work/ex/unstable/res/stable/reg/nalgebra-0.10.1/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/compiletest_rs-0.2.5/log.txt
2:boom! unstable-feature: rustc_private
3:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/uuid-0.3.1/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/hyper-0.9.14/log.txt
2:boom! unstable-feature: test
work/ex/unstable/res/stable/reg/syntex_syntax-0.52.0/log.txt
2:boom! unstable-feature: ...
3:boom! unstable-feature: prelude_import
4:boom! unstable-feature: {}
(Clearly my parser needs some work)