'unsafe const fn' support

[thepowersgang]

It was noted in RFC 911 that unsafe const fn was an unresolved question.

Along with #913, adding support for unsafe const functions will allow constructing empty collections in statics (most likely wrapped with a mutex or similar). At the moment, this isn't possible as Unique's constructor is unsafe, and hence cannot be const (the compiler rejects such a definition)

[eddyb]

Unique? But that's just an almost pointless wrapper that adds some markers to a non-null pointer - surely constructing such a value is as unsafe as creating an arbitrary *mut T?

[thepowersgang]

@eddyb It also wraps NonZero, which is unsafe to construct.

[eddyb]

Fair enough. Feel free to open a RFC modifying #911 to allow unsafe const fn, subject to the usual unsafety checks.

[thepowersgang]

Closing issue, as the RFC PR has been filed.