Add basic linting for .app file

[ankhers]

Resolves #980

This currently just checks for the existence of the description and applications
keys and that the applications list has kernel and stdlib in it.

If there are other things that could be linted, I would be happy to add them.

[ferd]

Can you think of any way to test this so we can make sure linting works as expected? I can help add the tests if you need assistance as well.

[ankhers]

I am not familiar enough with common test. Is there a smiliar function to ExUnit's capture_io/3? If not, would you be open to having something similar be written for this project as a test helper?

Other than that I am unable to think of something at the moment.

[ferd]

There's a bunch of IO-capturing functions -- ct:capture_start/0, ct:capture_get/0-1 and ct:capture_stop/0, but the current test suites capture things through mocking with meck, something like:

%% override the logging module
meck:new(rebar_log, [no_link, passthrough]),
_ = rebar_test_utils:run_and_check(CTConfig, RebarConfig, ["compile"], return),
History = meck:history(rebar_log),
Warnings = [{Str, Args} || {_, {rebar_log, log, [warn, Str, Args]}, _} <- History],
Errors = [{Str, Args} || {_, {rebar_log, log, [error, Str, Args]}, _} <- History],
%% do some checking here

The rebar_test_utils stuff is documented in https://github.com/erlang/rebar3/blob/master/CONTRIBUTING.md#tests

But if you want to do something like create one or two sample OTP apps, I can transform them to tests as well.

[ankhers]

I'm happy to write the tests, but should the test(s) go into the rebar_compile_SUITE or elsewhere?

[ferd]

Yeah that should be fine. You may want to use the earliest command that can give the linting result for test execution speed (i.e. just 'get-deps' might be sufficient as a command if you are also testing deps in the linting), but the compile suite is fine.

[ankhers]

Sorry for the long delay on this.

I plan on writing a couple more tests for the different warnings that are being output at the moment. I just wanted to first ask if this is the proper way to check that the warnings are being output, or if I should be doing something slightly different.

[ferd]

"for applications value" maybe would be clearer

[ferd]

That's a perfectly good approach to test the warnings.

[ankhers]

After going through the test helpers, I think this is the best we can do while still using them. Because it writes out all the keys. I'm happy to write tests that just manually does the application setup if that is preferred.

[ferd]

Thanks for the contribution!