Snippet variants with n-1 placeholders to use after pipe

apps/language_server/lib/language_server/providers/completion.ex

@@ -56,6 +56,13 @@ defmodule ElixirLS.LanguageServer.Providers.Completion do
     {"ExUnit.Case", "describe"} => "describe \"$1\" do\n\t$0\nend"
   }
 
+  # Alternative snippets versions to be preferred when preceded by a pipe
+  @pipe_func_snippets %{
+    {"Kernel.SpecialForms", "case"} => "case do\n\t$1 ->\n\t\t$0\nend",
+    {"Kernel", "if"} => "if do\n\t$0\nend",
+    {"Kernel", "unless"} => "unless do\n\t$0\nend"
+  }
+
   @use_name_only MapSet.new([
                    {"Kernel", "not"},
                    {"Kernel", "use"},
@@ -493,7 +500,7 @@ defmodule ElixirLS.LanguageServer.Providers.Completion do
         completion
       end
 
-    if snippet = Map.get(@func_snippets, {origin, name}) do
+    if snippet = snippet_for({origin, name}, context) do
       %{completion | insert_text: snippet, kind: :snippet, label: name}
     else
       completion
@@ -504,6 +511,15 @@ defmodule ElixirLS.LanguageServer.Providers.Completion do
     nil
   end
 
+  defp snippet_for(key, %{pipe_before?: true}) do
+    # Get pipe-friendly version of snippet if available, otherwise fallback to standard
+    Map.get(@pipe_func_snippets, key) || Map.get(@func_snippets, key)
+  end
+
+  defp snippet_for(key, _context) do
+    Map.get(@func_snippets, key)
+  end
+
   defp def_snippet(def_str, name, args, arity, opts) do
     if Keyword.get(opts, :snippets_supported, false) do
       "#{def_str}#{function_snippet(name, args, arity, opts)} do\n\t$0\nend"

apps/language_server/test/providers/completion_test.exs

@@ -940,4 +940,68 @@ defmodule ElixirLS.LanguageServer.Providers.CompletionTest do
       refute Enum.any?(items, fn i -> i["label"] == "make_ref/0" end)
     end
   end
+
+  describe "use the (arity - 1) version of snippets after pipe" do
+    test "case/2 snippet skips the condition argument" do
+      text = """
+      defmodule MyModule do
+        def hello do
+          [1, 2]
+          |> Enum.random()
+          |> ca
+          #    ^
+        end
+      end
+      """
+
+      {line, char} = {4, 9}
+      TestUtils.assert_has_cursor_char(text, line, char)
+      assert {:ok, %{"items" => items}} = Completion.completion(text, line, char, @supports)
+      assert %{"insertText" => insert_text} = Enum.find(items, &match?(%{"label" => "case"}, &1))
+      assert insert_text =~ "case do\n\t"
+    end
+
+    test "unless/2 snippet skips the condition argument" do
+      text = """
+      defmodule MyModule do
+        def hello do
+          [1, 2]
+          |> Enum.random()
+          |> unl
+          #     ^
+        end
+      end
+      """
+
+      {line, char} = {4, 10}
+      TestUtils.assert_has_cursor_char(text, line, char)
+      assert {:ok, %{"items" => items}} = Completion.completion(text, line, char, @supports)
+
+      assert %{"insertText" => insert_text} =
+               Enum.find(items, &match?(%{"label" => "unless"}, &1))
+
+      assert insert_text =~ "unless do\n\t"
+    end
+
+    test "if/2 snippet skips the condition argument" do
+      text = """
+      defmodule MyModule do
+        def hello do
+          [1, 2]
+          |> Enum.random()
+          |> if
+          #    ^
+        end
+      end
+      """
+
+      {line, char} = {4, 9}
+      TestUtils.assert_has_cursor_char(text, line, char)
+      assert {:ok, %{"items" => items}} = Completion.completion(text, line, char, @supports)
+
+      assert %{"insertText" => insert_text} = Enum.find(items, &match?(%{"label" => "if"}, &1))
+
+      assert insert_text =~ "if do\n\t"
+    end
+  end
 end