ways insufficient for concatenating identifiers, and may be
removed entirely for something more wholesome.
+* `custom_attribute` - Allows the usage of attributes unknown to the compiler
+ so that new attributes can be added in a bacwards compatible
+ manner (RFC 572).
+
* `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
are inherently unstable and no promise about them is made.
implemented very poorly and will likely change significantly
with a proper implementation.
+* `rustc_attrs` - Gates internal `#[rustc_*]` attributes which may be
+ for internal use only or have meaning added to them in the future.
+
* `rustc_diagnostic_macros`- A mysterious feature, used in the implementation
of rustc, not meant for mortals.