A binary doesn't match even though it should

[mat-hek]

The following code prints the binaries when run on the BEAM, and prints no_match on AtomVM

binary = <<1,2,3,4>>
skip = 1
size = 2

case binary do
  <<_::binary-size(skip), bin::binary-size(size), bin2::binary>> ->
    IO.inspect({bin, bin2})
  _ ->
    IO.inspect(:no_match)
end

[FranekKubis]

binary = <<1,2,3,4>>
skip = 8
size = 16

case binary do
  <<_::size(skip), bin::size(size), bin2::binary>> ->
    IO.inspect({bin, bin2})
  _ ->
    IO.inspect(:no_match)
end

This is working just fine - I think the problem is around using binary option inside bitstrings.

[mat-hek]

So this seems only to be a problem outside of a module. The following code works fine:

defmodule M do
  def m do
    binary = <<1, 2, 3, 4>>
    skip = 1
    size = 2
  
    case binary do
      <<_::binary-size(skip), bin::binary-size(size), bin2::binary>> ->
        IO.inspect({bin, bin2})
  
      _ ->
        IO.inspect(:no_match)
    end
  end
end

M.m()

[mat-hek]

The reason is that erl_eval code is generic and thus it uses bitstrings to match on both bitstrings and binaries, while AtomVM doesn't support bitstrings. We already patched eval_bits:eval_exp_field, but there's also eval_bits:get_value. Patching it seems to fix the issue, but I'll try to check how hard it is to support bitstrings match specifiers and binary constructors if the size (computed at runtime) is divisible by 8, so we can avoid such errors in the future.

[mat-hek]

Opened atomvm/AtomVM#1978