Rhai `no_std` appears to rely on `once_cell/alloc` but does not request it

[cbiffle]

Hi! Playing with Rhai on no_std over here (on an actual platform that doesn't provide std).

Currently, 1.18.0 won't build on a platform that doesn't provide std, because it appears to be relying on the once_cell crate having the alloc feature set (to get once_cell::race::OnceBox), but isn't setting that feature itself -- at least when depended on as

rhai = { version = "1.18.0", default-features = false, features = ["no_std"] }

Adding a redundant dep to once_cell and setting the alloc feature in my build causes it to start working:

# TODO: had to add this because rhai isn't requesting a feature it relies on.
once_cell = { version = "1.19.0", default-features = false, features = ["alloc"] }

[cbiffle]

From the commit history it looks like this might be a known bug; might be worth adding something to the README that the packaged versions require this workaround, lest it trip up more people. Feel free to close if you feel it's fixed.

[schungx]

Hhhmmm... that's strange, as the example under no_std has exactly the same default-features = false, features = ["no_std"] and it builds fine...

[schungx]

@cbiffle do you have more info on this?