Checks whether the running or installed
rustc supports feature flags.
In other words, if the channel is either “nightly” or “dev”.
Note that support for specific
rustc features can be enabled or disabled
allow-features compiler flag, which this function does not
check. That is, this function does not check whether a specific feature
is supported, but instead whether features are supported at all. To check
for support for a specific feature, use
If the version could not be determined, returns
None. Otherwise returns
true if the running version supports feature flags and