Fix #1568 - Filter LSP completion results with the prefixes

autoload/ale/completion.vim

@@ -296,6 +296,10 @@ function! ale#completion#ParseLSPCompletions(response) abort
         \})
     endfor
 
+    if has_key(l:info, 'prefix')
+        return ale#completion#Filter(l:buffer, l:results, l:info.prefix)
+    endif
+
     return l:results
 endfunction
 

test/completion/test_lsp_completion_parsing.vader

@@ -390,3 +390,42 @@ Execute(Should handle Python completion results correctly):
   \       ]
   \    }
   \ })
+
+Execute(Should handle Python completion results correctly):
+  let b:ale_completion_info = {
+  \ 'completion_filter': 'ale#completion#python#CompletionItemFilter',
+  \ 'prefix': 'mig',
+  \}
+
+  AssertEqual
+  \ [
+  \   {'word': 'migrations', 'menu': 'xxx', 'info': 'migrations', 'kind': 'f', 'icase': 1},
+  \   {'word': 'MigEngine', 'menu': 'xxx', 'info': 'mig engine', 'kind': 'f', 'icase': 1},
+  \ ],
+  \ ale#completion#ParseLSPCompletions({
+  \   'jsonrpc': '2.0',
+  \   'id': 6,
+  \   'result': {
+  \     'isIncomplete': v:false,
+  \     'items': [
+  \       {
+  \         'label': 'migrations',
+  \         'kind': 3,
+  \         'detail': 'xxx',
+  \         'documentation': 'migrations',
+  \       },
+  \       {
+  \         'label': 'MigEngine',
+  \         'kind': 3,
+  \         'detail': 'xxx',
+  \         'documentation': 'mig engine',
+  \       },
+  \       {
+  \         'label': 'ignore me',
+  \         'kind': 3,
+  \         'detail': 'nope',
+  \         'documentation': 'nope',
+  \       },
+  \     ]
+  \   }
+  \ })