fix(completions): dont leak <- matches from for/with (#454)

Closes #447

priv/monkey/_next_ls_private_compiler.ex

@@ -1545,6 +1545,29 @@ if Version.match?(System.version(), ">= 1.17.0-dev") do
       {{{:., meta, [module, fun]}, meta, args}, state, env}
     end
 
+    defp expand_local(meta, fun, args, state, env) when fun in [:for, :with] do
+      {params, blocks} =
+        Enum.split_while(args, fn
+          {:<-, _, _} -> true
+          _ -> false
+        end)
+
+      {_, state, penv} =
+        for p <- params, reduce: {nil, state, env} do
+          {_, state, penv} ->
+            expand_pattern(p, state, penv)
+        end
+
+      {blocks, state} =
+        for {type, block} <- blocks, reduce: {[], state} do
+          {acc, state} ->
+            {res, state, _env} = expand(block, state, penv)
+            {[{type, res} | acc], state}
+        end
+
+      {blocks, state, env}
+    end
+
     defp expand_local(meta, fun, args, state, env) do
       # A compiler may want to emit a :local_function trace in here.
       {args, state, env} = expand_list(args, state, env)

test/next_ls/completions_test.exs

@@ -735,4 +735,78 @@ defmodule NextLS.CompletionsTest do
     assert_match %{"kind" => 6, "label" => "var"} in results
     assert_match %{"kind" => 6, "label" => "vim"} in results
   end
+
+  test "<- matches dont leak from for", %{client: client, foo: foo} do
+    uri = uri(foo)
+
+    did_open(client, foo, """
+    defmodule Foo do
+      def run(items) do
+        names = 
+          for item <- items do
+            item.name
+          end
+
+        i
+      end
+    end
+    """)
+
+    request client, %{
+      method: "textDocument/completion",
+      id: 2,
+      jsonrpc: "2.0",
+      params: %{
+        textDocument: %{
+          uri: uri
+        },
+        position: %{
+          line: 7,
+          character: 5
+        }
+      }
+    }
+
+    assert_result 2, results
+
+    assert_match %{"kind" => 6, "label" => "items"} in results
+    assert_match %{"kind" => 6, "label" => "item"} not in results
+  end
+
+  test "<- matches dont leak from with", %{client: client, foo: foo} do
+    uri = uri(foo)
+
+    did_open(client, foo, """
+    defmodule Foo do
+      def run(items) do
+        names = 
+          with item <- items do
+            item.name
+          end
+
+        i
+      end
+    end
+    """)
+
+    request client, %{
+      method: "textDocument/completion",
+      id: 2,
+      jsonrpc: "2.0",
+      params: %{
+        textDocument: %{
+          uri: uri
+        },
+        position: %{
+          line: 7,
+          character: 5
+        }
+      }
+    }
+
+    assert_result 2, results
+
+    assert_match %{"kind" => 6, "label" => "items"} in results
+    assert_match %{"kind" => 6, "label" => "item"} not in results
+  end
 end