`mix compile`'s `--warnings-as-errors` doesn't work for errors reported by `--all-warnings`

[Gladear]

Environment

• Elixir & Erlang/OTP versions (elixir --version):

Erlang/OTP 24 [erts-12.2.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Elixir 1.13.3 (compiled with Erlang/OTP 24)

• Operating system:
  macOS Monterney 12.2.1
  or Ubuntu 20.04.3 LTS on Windows 10 x86_64 (WSL 2) / 4.19.128-microsoft-standard

Current behavior

When running mix compile, it looks like warnings reported by --all-warnings aren't considered by --warnings-as-errors.
For example, create a new mix project and add an unused variable anywhere:

$ mix clean
$ mix compile --all-warnings --warnings-as-errors # note that `--all-warning` won't do anything, since we cleaned the project before
Compiling 1 file (.ex)
warning: variable "args" is unused (if the variable is not meant to be used, prefix it with an underscore)
  lib/testmix.ex:15: Testmix.hello/1

Compilation failed due to warnings while using the --warnings-as-errors option
$ # failed as expected
$ mix compile
Compiling 1 file (.ex)
warning: variable "args" is unused (if the variable is not meant to be used, prefix it with an underscore)
  lib/testmix.ex:15: Testmix.hello/1

Generated testmix app
$ mix compile --all-warnings --warnings-as-errors
warning: variable "args" is unused (if the variable is not meant to be used, prefix it with an underscore)
  lib/testmix.ex:15
(compilation went fine)

Expected behavior

Warning reported by the --all-warnings options should be treated the same as others by the --warnings-as-errors option, and therefore that the compilation crashed

[josevalim]

Good call. A PR is welcome! :)