Feature gates
This crate declares the set of past and present unstable features in the compiler.
Feature gate checking itself is done in libsyntax/feature_gate/check.rs
at the moment.
Features are enabled in programs via the crate-level attributes of
#![feature(...)]
with a comma-separated list of features.
For the purpose of future feature-tracking, once a feature gate is added,
even if it is stabilized or removed, do not remove it. Instead, move the
symbol to the accepted
or removed
modules respectively.