Check tests for warnings (#418)

* Check tests for warnings

Code is based on this blog post:
https://dashbit.co/blog/tests-with-warnings-as-errors

* Don't check unused deps on unsupported versions

Also only use `--warnings-as-errors` on 1.11 where it is fully working

* Missed removing the unconditional check-unused

.travis.yml

@@ -9,6 +9,13 @@ script:
       echo "Not checking formatting"
     fi
   - mix test
+  - |
+    if [[ "$CHECK_UNUSED_DEPS" -eq 1 ]]
+    then
+      mix deps.unlock --check-unused
+    else
+      echo "Not checking unused deps"
+    fi
   - |
     if [[ "$RUN_DIALYZER" -eq 1 ]]
     then
@@ -32,8 +39,11 @@ matrix:
       elixir: 1.9.4
     - otp_release: 23.0.4
       elixir: 1.10.4
+      env:
+        - CHECK_UNUSED_DEPS=1
     - otp_release: 23.0.4
       elixir: 1.11.1
       env:
         - CHECK_FORMATTED=1
         - RUN_DIALYZER=1
+        - CHECK_UNUSED_DEPS=1

apps/elixir_ls_debugger/test/test_helper.exs

@@ -1,2 +1,6 @@
+if Version.match?(System.version(), ">= 1.11.0") do
+  Code.put_compiler_option(:warnings_as_errors, true)
+end
+
 Application.put_env(:elixir_ls_debugger, :test_mode, true)
 ExUnit.start(exclude: [pending: true])

apps/elixir_ls_utils/test/test_helper.exs

@@ -1 +1,5 @@
+if Version.match?(System.version(), ">= 1.11.0") do
+  Code.put_compiler_option(:warnings_as_errors, true)
+end
+
 ExUnit.start(exclude: [pending: true])

apps/language_server/test/test_helper.exs

@@ -1,3 +1,7 @@
+if Version.match?(System.version(), ">= 1.11.0") do
+  Code.put_compiler_option(:warnings_as_errors, true)
+end
+
 Application.put_env(:language_server, :test_mode, true)
 Application.ensure_started(:stream_data)
 ExUnit.start(exclude: [pending: true])