Inline more functions

[devinus]

Inline more regularly used functions in the standard Elixir modules to their Erlang STDLIB counterparts:

• Float.to_charlist/1
• Float.to_string/1
• List.duplicate/2
• List.flatten/1
• List.flatten/2
• Map.from_keys/2
• Map.merge/3

Great care was taken to ensure function guards already applied to the Elixir functions were subsets of the function guards in Erlang's STDLIB. For example, Map.merge/3 specified the combiner function must be is_function(fun, 3) and Erlang's :maps.merge_with also already specifies that argument must be is_function(Combiner, 3).

The one exception to the above is the inlining of Float.to_charlist/1 and Float.to_string/1. The Elixir functions specified that the argument must be a float with is_float(float) and thus will raise a FunctionClauseError exception if e.g. an integer is passed, while :erlang.float_to_list/2 and :erlang.float_to_binary/2 will instead throw an ArgumentError exception. However, changing to the Erlang behavior brings those functions more in line with the behavior of the rest of the Elixir modules, e.g. Atom.to_charlist/1, Atom.to_string/1, Integer.to_charlist/1 and Integer.to_string/1 will all throw an ArgumentError exception on an unexpected argument type.

[josevalim]

Hi @devinus, nice to see you around.

We only inline functions if they are BIFs/NIFs on the Erlang side, otherwise it is easy to end-up inlining everything. For example, if we want to optimize to List.flatten, I would rather just implement it in Elixir.

So we should only keep the maps and float ones for now, thank you.

[sabiwara]

So we should only keep the maps and float ones for now

@josevalim I think only [:maps.from_keys/1](https://github.com/erlang/otp/blob/OTP-27.0/lib/stdlib/src/maps.erl#L167) is a BIF, :maps.merge_with/3 isn't?

[devinus]

@josevalim @sabiwara Updated PR with those guidelines.

[sabiwara]

LGTM! 💜

[josevalim]

💚 💙 💜 💛 ❤️