Handle comments in EEx between do and the first clause

Closes #11307.

Author: josevalim

lib/eex/lib/eex/compiler.ex

@@ -75,7 +75,8 @@ defmodule EEx.Compiler do
       :elixir_errors.erl_warn(start_line, state.file, message)
     end
 
-    {contents, line, rest} = look_ahead_middle(rest, start_line, chars)
+    {rest, line, contents} =
+      look_ahead_middle(rest, start_line, chars) || {rest, start_line, chars}
 
     {contents, rest} =
       generate_buffer(
@@ -194,24 +195,20 @@ defmodule EEx.Compiler do
 
   # Look middle expressions that immediately follow a start_expr
 
-  defp look_ahead_middle(
-         [{:text, _, _, text}, {:middle_expr, line, _, _, chars} | rest] = tokens,
-         start,
-         contents
-       ) do
+  defp look_ahead_middle([{:text, _, _, text} | rest], start, contents) do
     if only_spaces?(text) do
-      {contents ++ text ++ chars, line, rest}
+      look_ahead_middle(rest, start, contents ++ text)
     else
-      {contents, start, tokens}
+      nil
     end
   end
 
   defp look_ahead_middle([{:middle_expr, line, _column, _, chars} | rest], _start, contents) do
-    {contents ++ chars, line, rest}
+    {rest, line, contents ++ chars}
   end
 
-  defp look_ahead_middle(tokens, start, contents) do
-    {contents, start, tokens}
+  defp look_ahead_middle(_tokens, _start, _contents) do
+    nil
   end
 
   defp only_spaces?(chars) do

lib/eex/test/eex/tokenizer_test.exs

@@ -110,6 +110,28 @@ defmodule EEx.TokenizerTest do
     assert T.tokenize('foo <%# true do %>bar<%# end %>', 1, 1, @opts) == {:ok, exprs}
   end
 
+  test "EEx comments inside do-end" do
+    exprs = [
+      {:start_expr, 1, 1, '', ' if true do '},
+      {:text, 1, 31, 'bar'},
+      {:end_expr, 1, 34, [], ' end '},
+      {:eof, 1, 43}
+    ]
+
+    assert T.tokenize('<% if true do %><%# comment %>bar<% end %>', 1, 1, @opts) == {:ok, exprs}
+
+    exprs = [
+      {:start_expr, 1, 1, [], ' case true do '},
+      {:middle_expr, 1, 33, '', ' true -> '},
+      {:text, 1, 46, 'bar'},
+      {:end_expr, 1, 49, [], ' end '},
+      {:eof, 1, 58}
+    ]
+
+    assert T.tokenize('<% case true do %><%# comment %><% true -> %>bar<% end %>', 1, 1, @opts) ==
+             {:ok, exprs}
+  end
+
   test "Elixir comments" do
     exprs = [
       {:text, 1, 1, 'foo '},

lib/eex/test/eex_test.exs

@@ -198,6 +198,15 @@ defmodule EExTest do
       assert_eval("foo baz", "foo <%= if false do %>bar<% else %>baz<% end %>")
     end
 
+    test "embedded code with comments in do end" do
+      assert_eval("foo bar", "foo <%= case true do %><%# comment %><% true -> %>bar<% end %>")
+
+      assert_eval(
+        "foo\n\nbar\n",
+        "foo\n<%= case true do %>\n<%# comment %>\n<% true -> %>\nbar\n<% end %>"
+      )
+    end
+
     test "embedded code with nested do end" do
       assert_eval("foo bar", "foo <%= if true do %><%= if true do %>bar<% end %><% end %>")
     end