Mix xref and Elixir compiler warnings

The release of Elixir v1.3 added a mix task called xref for performing cross reference checks of your code. mix xref gives us some handy tools!

Find unreachable code:

❯ mix xref unreachable
lib/exonk8s/foo.ex:16: SomeModule.non_existant/0

Find callers of modules and functions:

❯ mix xref callers SomeModule
lib/exonk8s/foo.ex:16: SomeModule.non_existant/0

Generate a dependency graph:

> mix xref graph
lib/exonk8s.ex
├── lib/exonk8s/repo.ex
└── lib/router.ex
    └── lib/exonk8s/musician_controller.ex (compile)
        ├── lib/exonk8s/foo.ex
        ├── lib/exonk8s/musician.ex
        └── lib/exonk8s/repo.ex
...snipped for brevity

The release of Elixir v1.6 added the module attribute @deprecated to enable library authors to warn users of deprecated functions.

❯ mix xref deprecated
Compiling 2 files (.ex)
Foo Loaded
warning: ExOnK8s.Foo.lazy/0 is deprecated. You should be more eager.
  lib/exonk8s/musician_controller.ex:10

lib/exonk8s/musician_controller.ex:10: ExOnK8s.Foo.lazy/0

Lastly I should note that xref deprecated and xref unreachable are included in the compiler warnings, so you you've likely seen these at work even if you didn't know they were there.

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