Honestly, my best guess is that wasn't needed for icu4x (the project it was written for), and no-one's implemented it yet. I can't see any reason why it wouldn't work. You could probably just call alloc::collections::BTreeMap::from([...]). @manishearth.bsky.social probably know more.

yeah, this is the reason. happy to accept a PR with those as well though in general we don't even use databake with types that mandatorily allocate, the Vec impl is just there because it's one of the types it feels silly to not impl this stuff on

I think you're right, I can get most of the way towards impling Bake for BTreeSet (though the unit test is failing about something that I don't think should fail)