#977 Complain about the lack of configuration files for scalastyle in such a way that users will be directed towards getting it to work

ale_linters/scala/scalastyle.vim

@@ -8,10 +8,21 @@ let g:ale_scalastyle_config_loc =
 \   get(g:, 'ale_scalastyle_config_loc', '')
 
 function! ale_linters#scala#scalastyle#Handle(buffer, lines) abort
+    " Look for help output from scalastyle first, which indicates that no
+    " configuration file was found.
+    for l:line in a:lines[:10]
+        if l:line =~# '-c, --config'
+            return [{
+            \   'lnum': 1,
+            \   'text': '(See :help ale-scala-scalastyle)'
+            \       . ' No scalastyle configuration file was found.',
+            \}]
+        endif
+    endfor
+
     " Matches patterns like the following:
     "
     " warning file=/home/blurble/Doop.scala message=Missing or badly formed ScalaDoc: Extra @param foobles line=190
-
     let l:patterns = [
     \   '^\(.\+\) .\+ message=\(.\+\) line=\(\d\+\)$',
     \   '^\(.\+\) .\+ message=\(.\+\) line=\(\d\+\) column=\(\d\+\)$',

doc/ale-scala.txt

@@ -5,6 +5,17 @@ ALE Scala Integration                                       *ale-scala-options*
 ===============================================================================
 scalastyle                                               *ale-scala-scalastyle*
 
+`scalastyle` requires a configuration file for a project to run. When no
+configuration file can be found, ALE will report a problem saying that a
+configuration file is required at line 1.
+
+To disable `scalastyle` globally, use |g:ale_linters| like so: >
+  let g:ale_linters = {'scala': ['scalac']} " Enable only scalac instead
+<
+
+See |g:ale_linters| for more information on disabling linters.
+
+
 g:ale_scalastyle_config_loc                       *g:ale_scalastyle_config_loc*
 
   Type: |String|

test/handler/test_scalastyle_handler.vader

@@ -1,6 +1,10 @@
-Execute(The scalastyle handler should parse lines correctly):
+Before:
   runtime! ale_linters/scala/scalastyle.vim
 
+After:
+  call ale#linter#Reset()
+
+Execute(The scalastyle handler should parse lines correctly):
   AssertEqual
   \ [
   \   {
@@ -34,5 +38,16 @@ Execute(The scalastyle handler should parse lines correctly):
   \    'Finished in 934 ms',
   \ ])
 
-After:
+Execute(The scalastyle linter should complain when there is no configuration file):
-  call ale#linter#Reset()
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 1,
+  \     'text': '(See :help ale-scala-scalastyle) No scalastyle configuration file was found.',
+  \   },
+  \ ],
+  \ ale_linters#scala#scalastyle#Handle(347, [
+  \   'scalastyle 1.0.0',
+  \   'Usage: scalastyle [options] <source directory>',
+  \   ' -c, --config FILE               configuration file (required)',
+  \ ])