std uses a lot of unstable language features. While core will most likely always have to use language features because it defines things that are part of the core of the language, we can strive for std to eventually be free of unstable language features.

Some of these we'll need to stabilize, but some others we might be able to simply remove with some small changes to the code.

(See also #94971 for the library features used by std.)

These are the language features we currently use in std: (This does not include core or alloc.)

• Macro related:
  • allow_internal_unsafe
  • allow_internal_unstable
  • concat_idents
  • decl_macro
  • rustc_attrs: rustc_macro_transparency
• Const eval:
  • const_mut_refs
  • const_trait_impl
• Panic related:
  • rustc_attrs: rustc_builtin_macro(std_panic)
  • lang_items: begin_panic
  • rustc_attrs: rustc_do_not_const_check
  • needs_panic_runtime
• Global allocator: (RFC 2492 might be of help here)
  • alloc_error_handler
  • allocator_internals
• Diagnostics:
  • deprecated_suggestion
  • must_not_suspend
  • rustc_attrs: rustc_conversion_suggestion
  • rustc_attrs: rustc_diagnostic_item
  • rustc_attrs: rustc_insignificant_dtor
  • rustc_attrs: rustc_lint_query_instability
• Rustdoc:
  • doc_cfg
  • doc_cfg_hide
  • rustdoc_internals
  • doc_masked
  • doc_notable_trait
  • intra_doc_pointers
• Other language features:
  • lang_items: start
  • lang_items: termination
  • lang_items: f32_runtime, f64_runtime
    • Fixed if we move all f32 and f64 methods to core. Libm -> compiler_builtins?
  • associated_type_bounds: Remove need for associated_type_bounds in std. #95587
  • box_syntax
  • try_blocks
  • c_unwind: Stabilise c_unwind #116088
  • dropck_eyepatch
  • exhaustive_patterns
  • linkage
  • min_specialization
  • negative_impls
    • We can remove this if we switch to just using PhantomData everywhere.
  • never_type
  • nll
  • platform_intrinsics: remove platform-intrinsics ABI; make SIMD intrinsics be regular intrinsics #121516
  • prelude_import
  • staged_api
  • thread_local
  • cfg_target_thread_local
  • rustc_attrs: rustc_std_internal_symbol
  • rustc_attrs: rustc_specialization_trait
  • rustc_attrs: rustc_layout_scalar_valid_range_start, rustc_layout_scalar_valid_range_end