Damn, I've been doing a bunch of exploring of this exact topic over the past week. This would have saved me a bunch of time. Unlucky for me it wasn't submitted last Sunday!

One thing that's killed me a bit is the core::/std:: split, when the core:: versions have the exact same semantics, or perhaps are equivalent for the things that they do implement (for example, the core version might be missing the serde parts, but is otherwise the same). There's a few crates I tried out that wouldn't compile with no_std, but with a simple text substitution, would work. I'm sure there's a million edge cases I'm not aware of, but if cargo or rustc could provide an option to just sub in the core version, that'd be really nice.