Typing violation in elixir 1.18.0-rc.0

[ashkhn]

Compiling a rustler project using the latest elixir 1.18-rc.0
generates the following compile warning at the use Rustler injection site

    warning: the following clause will never match:

        {^default_load_data_value, {module, function}}

    because it attempts to match on the result of:

        {0, nil}

    which has type:

        {integer(), nil}

    where "default_load_data_value" (context Rustler) was given the type:

        # type: integer()
        # from: lib/rustler_mix_test.ex:1
        default_load_data_value = 0

    typing violation found at:
    │
  1 │ defmodule RustlerMixTest do
    │ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    │
    └─ lib/rustler_mix_test.ex:1: RustlerMixTest._construct_load_data/0

The warning originates at this pattern match
presumably because the compiler infers the types of load_data and load_data_fun based on the default values set in struct definition? 🤔

I'm not sure if it's a false positive in the 1.18 compiler so I opened an issue here first for confirmation 🙇

[filmor]

I looked a bit into this. I think the issue is that we generate a macro within __before_compile__ which then uses case expressions in its body where the values to match are completely static. I'll rewrite this section to "run" the case directly in __before_compile__ instead.

[filmor]

I'll see to it that a new version of rustler is released around the 1.18 final release. Maybe we can get some other smaller changes in before.