@RalfJung while you’re asking about needed language features, I would also add that recently I have been writing a bunch of code that depends on calling opaque FFI functions (either directly or indirectly) from inside lazy_statics. I have not been tracking the RFCs and implementation of const_fn, but I feel somewhat safe in assuming that the feature will not be including the ability to call unsafe extern "C" functions. It’s not clear to me that this use case (plus others which are also more niche than “const_fn isn’t stable yet”) would have lazy_static as an important enough crate to move into std (and hopefully core at that point, I think leaving that feature split between std/crates.io for std/no_std would be 
).