Fix #988 - Support --fix-dry-run for ESLint by processing the JSON output

autoload/ale/fixers/eslint.vim

@@ -14,6 +14,14 @@ function! ale#fixers#eslint#Fix(buffer) abort
     \}
 endfunction
 
+function! ale#fixers#eslint#ProcessFixDryRunOutput(buffer, output) abort
+    for l:item in ale#util#FuzzyJSONDecode(a:output, [])
+        return split(get(l:item, 'output', ''), "\n")
+    endfor
+
+    return []
+endfunction
+
 function! ale#fixers#eslint#ApplyFixForVersion(buffer, version_output) abort
     let l:executable = ale#handlers#eslint#GetExecutable(a:buffer)
     let l:version = ale#semver#GetVersion(l:executable, a:version_output)
@@ -32,6 +40,15 @@ function! ale#fixers#eslint#ApplyFixForVersion(buffer, version_output) abort
         \}
     endif
 
+    " 4.9.0 is the first version with --fix-dry-run
+    if ale#semver#GTE(l:version, [4, 9, 0])
+        return {
+        \   'command': ale#node#Executable(a:buffer, l:executable)
+        \       . ' --stdin-filename %s --stdin --fix-dry-run --format=json',
+        \   'process_with': 'ale#fixers#eslint#ProcessFixDryRunOutput',
+        \}
+    endif
+
     return {
     \   'command': ale#node#Executable(a:buffer, l:executable)
     \       . ' -c ' . ale#Escape(l:config)

autoload/ale/util.vim

@@ -250,7 +250,14 @@ function! ale#util#FuzzyJSONDecode(data, default) abort
     let l:str = type(a:data) == type('') ? a:data : join(a:data, '')
 
     try
-        return json_decode(l:str)
+        let l:result = json_decode(l:str)
+
+        " Vim 8 only uses the value v:none for decoding blank strings.
+        if !has('nvim') && l:result is v:none
+            return a:default
+        endif
+
+        return l:result
     catch /E474/
         return a:default
     endtry