Runtime crash on erlang when constructing bit arrays

[GearsDatapacks]

Currently, if you run the following code:

pub fn main() {
  let size = 2
  <<2.3:float-size(size)>>
}

The compiler will allow it, but when run, it produces an erlang Badarg error. This is probably not desired behaviour. I'm not sure if we consider this a breaking change, but since it never worked in the first place, I would lean no.

[lpil]

I agree, thank you. We need to work out what the right way to handle this would be as we may not be able to statically prevent this.

[richard-viney]

Another similar runtime crash related to invalid sizes:

pub fn main() {
  <<<<>>:bits-size(1)>>
}

This also crashes on Erlang with erlang:error(Badarg). On JavaScript it gives a more detailed runtime exception: Error: Invalid bit array slice: start = 0, end = 1, bit size = 0.

The general case can't be statically prevented because the size can be dynamic, but simple cases like the one in the initial description when constructing a float with an invalid constant size could be flagged.

The error on Erlang could be improved if Gleam inserted explicit size checks where relevant.