Mix fails to build Erlang application with locally defined and referenced behaviours

[eseres]

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit] [dtrace]

Elixir 1.19.5 (compiled with Erlang/OTP 28)

Operating system

linux

Current behavior

If an Erlang application both defines and references behaviour modules, Mix will throw a warning like this:

0.522 apps/my_app/src/a_module.erl:2:2: behaviour my_behaviour undefined                                                                                        
0.522 %    2| -behaviour(my_behaviour).
0.522 %     |  ^
0.522 
0.620 compile: warnings being treated as errors

This is because Mix compiles the files in alphabetical order and a_module.erl comes before my_behaviour.erl.

We have been using rebar3 for a long time without this issue. We are trying to switch to Mix and can't seem to get around this problem.

Are we missing something or is this really a limitation? If it is a limitation, could it be addressed in a future Mix version?

Expected behavior

The expectation is that regardless of the relative alphabetical order between the behaviour module and the module referencing it, the application should be able to be successfully compiled.

[josevalim]

Can you please provide a minimal reproduction? We slightly sort files to support behaviors and parse transforms, but we may be missing something. Thanks!

[josevalim]

Here is the reference to the code which should have handled behaviors: https://github.com/elixir-lang/elixir/blob/main/lib/mix/lib/mix/tasks/compile.erlang.ex#L176

[eseres]

Attached is a minimal application with which you can reproduce the issue. Please note that I had to run mix clean --all && mix compile several times before the issue happened. And if you run it again, it will likely succeed for many attempts before it fails again. It is unpredictable.

behaviour-test.tar.gz

[eseres]

The best way we have found to trigger the failure is to repeatedly call:

rm -rf _build && mix compile