[kibwen]

I think some clarification is warranted here, because Miri is intended to be used for more than just constant evaluation. In particular, Miri wants to be able to dynamically verify `unsafe` code in order to detect undefined behavior, which means that it will eventually want to support the entire Rust language. However, just because Miri supports any given Rust feature does not necessarily mean that that feature will be made available for constant evaluation; consider features that are non-deterministic (like RNG) or architecture-dependent/nonportable (like floating-point operations), and how const-evaluating such things could potentially cause memory-unsafety to occur when runtime output and const output differ.