Add basic code action support

apps/language_server/lib/language_server/protocol.ex

@@ -200,6 +200,17 @@ defmodule ElixirLS.LanguageServer.Protocol do
     end
   end
 
+  defmacro code_action_req(id, uri, diagnostics) do
+    quote do
+      request(unquote(id), "textDocument/codeAction", %{
+        "context" => %{"diagnostics" => unquote(diagnostics)},
+        "textDocument" => %{
+          "uri" => unquote(uri)
+        }
+      })
+    end
+  end
+
   # Other utilities
 
   defmacro range(start_line, start_character, end_line, end_character) do

apps/language_server/lib/language_server/providers/code_action.ex

@@ -0,0 +1,61 @@
+defmodule ElixirLS.LanguageServer.Providers.CodeAction do
+  use ElixirLS.LanguageServer.Protocol
+
+  def code_actions(uri, diagnostics) do
+    actions =
+      diagnostics
+      |> Enum.map(fn diagnostic -> actions(uri, diagnostic) end)
+      |> List.flatten()
+
+    {:ok, actions}
+  end
+
+  defp actions(uri, %{"message" => message, "range" => range} = diagnostic) do
+    [
+      {~r/variable "(.*)" is unused/, &prefix_with_underscore/2},
+      {~r/variable "(.*)" is unused/, &remove_variable/2}
+    ]
+    |> Enum.filter(fn {r, _fun} -> String.match?(message, r) end)
+    |> Enum.map(fn {_r, fun} -> fun.(uri, diagnostic) end)
+  end
+
+  defp prefix_with_underscore(uri, %{"message" => message, "range" => range} = diagnostic) do
+    %{
+      "title" => "Add '_' to unused variable",
+      "kind" => "quickfix",
+      "edit" => %{
+        "changes" => %{
+          uri => [
+            %{
+              "newText" => "_",
+              "range" =>
+                range(
+                  range["start"]["line"],
+                  range["start"]["character"],
+                  range["start"]["line"],
+                  range["start"]["character"]
+                )
+            }
+          ]
+        }
+      }
+    }
+  end
+
+  defp remove_variable(uri, %{"range" => range} = diagnostic) do
+    %{
+      "title" => "Remove unused variable",
+      "kind" => "quickfix",
+      "edit" => %{
+        "changes" => %{
+          uri => [
+            %{
+              "newText" => "",
+              "range" => range
+            }
+          ]
+        }
+      }
+    }
+  end
+end

apps/language_server/lib/language_server/server.ex

@@ -31,7 +31,8 @@ defmodule ElixirLS.LanguageServer.Server do
     OnTypeFormatting,
     CodeLens,
     ExecuteCommand,
-    FoldingRange
+    FoldingRange,
+    CodeAction
   }
 
   alias ElixirLS.Utils.Launch
@@ -325,7 +326,9 @@ defmodule ElixirLS.LanguageServer.Server do
       # close notification send before
       JsonRpc.log_message(
         :warning,
-        "Received textDocument/didOpen for file that is already open. Received uri: #{inspect(uri)}"
+        "Received textDocument/didOpen for file that is already open. Received uri: #{
+          inspect(uri)
+        }"
       )
 
       state
@@ -769,6 +772,10 @@ defmodule ElixirLS.LanguageServer.Server do
     end
   end
 
+  defp handle_request(code_action_req(id, uri, diagnostics) = req, state = %__MODULE__{}) do
+    {:async, fn -> CodeAction.code_actions(uri, diagnostics) end, state}
+  end
+
   defp handle_request(%{"method" => "$/" <> _}, state = %__MODULE__{}) do
     # "$/" requests that the server doesn't support must return method_not_found
     {:error, :method_not_found, nil, state}
@@ -820,7 +827,8 @@ defmodule ElixirLS.LanguageServer.Server do
       "workspace" => %{
         "workspaceFolders" => %{"supported" => false, "changeNotifications" => false}
       },
-      "foldingRangeProvider" => true
+      "foldingRangeProvider" => true,
+      "codeActionProvider" => true
     }
   end
 

apps/language_server/test/server_test.exs

@@ -1482,6 +1482,70 @@ defmodule ElixirLS.LanguageServer.ServerTest do
     end
   end
 
+  describe "textDocument/codeAction" do
+    test "return code actions on unused variables", %{server: server} do
+      uri = "file:///file.ex"
+      fake_initialize(server)
+
+      Server.receive_packet(server, did_open(uri, "elixir", 1, ""))
+
+      Server.receive_packet(
+        server,
+        code_action_req(1, uri, [
+          %{
+            "message" =>
+              "variable \"foo\" is unused (if the variable is not meant to be used, prefix it with an underscore)",
+            "range" => %{
+              "end" => %{"character" => 13, "line" => 19},
+              "start" => %{"character" => 4, "line" => 19}
+            },
+            "severity" => 1,
+            "source" => "Elixir"
+          }
+        ])
+      )
+
+      resp = assert_receive(%{"id" => 1}, 5000)
+
+      assert response(1, [
+               %{
+                 "edit" => %{
+                   "changes" => %{
+                     "file:///file.ex" => [
+                       %{
+                         "newText" => "_",
+                         "range" => %{
+                           "end" => %{"character" => 4, "line" => 19},
+                           "start" => %{"character" => 4, "line" => 19}
+                         }
+                       }
+                     ]
+                   }
+                 },
+                 "kind" => "quickfix",
+                 "title" => "Add '_' to unused variable"
+               },
+               %{
+                 "edit" => %{
+                   "changes" => %{
+                     "file:///file.ex" => [
+                       %{
+                         "newText" => "",
+                         "range" => %{
+                           "end" => %{"character" => 13, "line" => 19},
+                           "start" => %{"character" => 4, "line" => 19}
+                         }
+                       }
+                     ]
+                   }
+                 },
+                 "kind" => "quickfix",
+                 "title" => "Remove unused variable"
+               }
+             ]) == resp
+    end
+  end
+
   defp with_new_server(func) do
     server = start_supervised!({Server, nil})
     packet_capture = start_supervised!({PacketCapture, self()})