Really appreciate all the help here. Looking at your post:

1. deps/argon2_elixir does contain a folder argon2. Within that folder there is an include folder that has a single file, argon2.h, and a src folder which has 14 files most of which are .c files, with some .h files.

Trying number 2 now, and will update with the results

EDIT: Thinking your git comment was right, as this is in the output when I tried recursively cloning argon2_elixir:

git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
fatal: clone of ‘git@github.com:riverrun/phc-winner-argon2.git’ into submodule path ‘/Users/steve/Sites/argon2_elixir/argon2’ failed
Failed to clone ‘argon2’. Retry scheduled

Ok so after adding an SSH key I was able to clone the repo properly. From within that directory I ran make and got

steves-mbp:argon2_elixir steve$ make
mkdir -p priv
cc -pthread -O3 -Wall -g -Iargon2/include -Iargon2/src -Ic_src -I"/usr/local/Cellar/erlang/22.2.8/lib/erlang/erts-10.6.4/include/" -dynamiclib -undefined dynamic_lookup argon2/src/argon2.c argon2/src/core.c argon2/src/blake2/blake2b.c argon2/src/thread.c argon2/src/encoding.c argon2/src/ref.c c_src/argon2_nif.c -o priv/argon2_nif.so
steves-mbp:argon2_elixir Steve$

Since this didn’t complain about any errors, I took it that mix was working. So I went back to my project, and completely removed argon2_elixir and comeonin. I then re-added argon2_elixir and ran mix deps.get and mix deps.compile and got the following output:

steves-mbp:rumbl steve$ mix deps.compile
===> Compiling ranch
===> Compiling telemetry
==> comeonin
Compiling 4 files (.ex)
Generated comeonin app
==> argon2_elixir
mkdir -p priv
cc -pthread -O3 -Wall -g -Iargon2/include -Iargon2/src -Ic_src -I"/usr/local/Cellar/erlang/22.2.8/lib/erlang/erts-10.6.4/include/" -dynamiclib -undefined dynamic_lookup argon2/src/argon2.c argon2/src/core.c argon2/src/blake2/blake2b.c argon2/src/thread.c argon2/src/encoding.c argon2/src/ref.c c_src/argon2_nif.c -o priv/argon2_nif.so
Compiling 3 files (.ex)
Generated argon2_elixir app
===> Compiling cowlib
===> Compiling cowboy

Everything seemed to work fine, but I’m still seeing the error, could not compile comeonin

No, not really, it is the recommended password hashing function for comeonin, and depends on it.

It would be nice to know how You did that… with asdf? packages? other?

Do You have XCode installed?

That sounds like a Mac related issue

Most people prefer to use a package manager…

I also prefer to use asdf-vm.

You probably can find some topics to help You if You choose this installation path.

I just tried to clone argon2_elixir recursively (to pick up the submodules) and that seems to break (permissions error). I’ll dig in a little more, but I don’t think it’s a Mac issue at this stage. Error I got was:

 !  ~/c/e/temp  git clone https://github.com/riverrun/argon2_elixir.git --recursive   Sat 28 Mar 2020 11:47:22 AWST
Cloning into 'argon2_elixir'...
remote: Enumerating objects: 17, done.
remote: Counting objects: 100% (17/17), done.
remote: Compressing objects: 100% (12/12), done.
remote: Total 657 (delta 7), reused 14 (delta 5), pack-reused 640
Receiving objects: 100% (657/657), 113.82 KiB | 282.00 KiB/s, done.
Resolving deltas: 100% (371/371), done.
Submodule 'argon2' (git@github.com:riverrun/phc-winner-argon2.git) registered for path 'argon2'
Cloning into '/home/john/code/elix/temp/argon2_elixir/argon2'...
Warning: Permanently added the RSA host key for IP address '52.64.108.95' to the list of known hosts.
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: clone of 'git@github.com:riverrun/phc-winner-argon2.git' into submodule path '/home/john/code/elix/temp/argon2_elixir/argon2' failed
Failed to clone 'argon2'. Retry scheduled
Cloning into '/home/john/code/elix/temp/argon2_elixir/argon2'...
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: clone of 'git@github.com:riverrun/phc-winner-argon2.git' into submodule path '/home/john/code/elix/temp/argon2_elixir/argon2' failed
Failed to clone 'argon2' a second time, aborting

So I manually cloned https://github.com/riverrun/phc-winner-argon2.git into an argon2 subdirectory of argon2_elixir, ran make and it built ok. For me it looks like there’s something weird with bringing in the git submodule automatically.

EDIT: Try git clone https://github.com/riverrun/argon2_elixir.git --recursive - if that complains then you might have to do some git kung-fu to get dependencies with git submodules to work properly. There’s a discussion here that might help: https://github.com/facebookresearch/deepmask/issues/23