When fn is followed by a newline, use multi-clause formatting style (#7737)

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: Sihui

lib/elixir/lib/code/formatter.ex

@@ -522,7 +522,7 @@ defmodule Code.Formatter do
   end
 
   defp quoted_to_algebra({:fn, meta, [_ | _] = clauses}, _context, state) do
-    anon_fun_to_algebra(clauses, line(meta), end_line(meta), state)
+    anon_fun_to_algebra(clauses, line(meta), end_line(meta), state, eol?(meta))
   end
 
   defp quoted_to_algebra({fun, meta, args}, context, state) when is_atom(fun) and is_list(args) do
@@ -1598,7 +1598,13 @@ defmodule Code.Formatter do
   ## Anonymous functions
 
   # fn -> block end
-  defp anon_fun_to_algebra([{:->, meta, [[], body]}] = clauses, _min_line, max_line, state) do
+  defp anon_fun_to_algebra(
+         [{:->, meta, [[], body]}] = clauses,
+         _min_line,
+         max_line,
+         state,
+         _multi_clauses_style
+       ) do
     min_line = line(meta)
     {body_doc, state} = block_to_algebra(body, min_line, max_line, state)
 
@@ -1617,7 +1623,13 @@ defmodule Code.Formatter do
   # fn x ->
   #   y
   # end
-  defp anon_fun_to_algebra([{:->, meta, [args, body]}] = clauses, _min_line, max_line, state) do
+  defp anon_fun_to_algebra(
+         [{:->, meta, [args, body]}] = clauses,
+         _min_line,
+         max_line,
+         state,
+         false = _multi_clauses_style
+       ) do
     min_line = line(meta)
     {args_doc, state} = clause_args_to_algebra(args, min_line, state)
     {body_doc, state} = block_to_algebra(body, min_line, max_line, state)
@@ -1647,7 +1659,7 @@ defmodule Code.Formatter do
   #   args2 ->
   #     block2
   # end
-  defp anon_fun_to_algebra(clauses, min_line, max_line, state) do
+  defp anon_fun_to_algebra(clauses, min_line, max_line, state, _multi_clauses_style) do
     {clauses_doc, state} = clauses_to_algebra(clauses, min_line, max_line, state)
     {"fn" |> line(clauses_doc) |> nest(2) |> line("end") |> force_unfit(), state}
   end
@@ -2139,6 +2151,10 @@ defmodule Code.Formatter do
     false
   end
 
+  defp eol?(meta) do
+    Keyword.get(meta, :eol, false)
+  end
+
   defp line(meta) do
     Keyword.get(meta, :line, @max_line)
   end

lib/elixir/src/elixir_parser.yrl

@@ -305,7 +305,7 @@ eoe -> ';' : '$1'.
 eoe -> eol ';' : '$1'.
 
 fn_eoe -> 'fn' : '$1'.
-fn_eoe -> 'fn' eoe : '$1'.
+fn_eoe -> 'fn' eoe : next_is_eol('$1').
 
 do_eoe -> 'do' : '$1'.
 do_eoe -> 'do' eoe : '$1'.
@@ -659,7 +659,7 @@ meta_from_token_with_end_line(Begin, End) ->
   case ?formatter_metadata() of
     true ->
       [{end_line, line_from_location(?location(End))}
-       | meta_from_token(Begin)];
+       | eol_op(?location(Begin)) ++ meta_from_token(Begin)];
     false ->
       meta_from_token(Begin)
   end.

lib/elixir/test/elixir/code_formatter/comments_test.exs

@@ -449,14 +449,15 @@ defmodule Code.Formatter.CommentsTest do
 
       assert_format bad, ~S"""
       # fn
-      # before head
-      # middle head
-      # after head
-      fn hello ->
-        # before body
-        # middle body
-        world
-        # after body
+      fn
+        # before head
+        # middle head
+        hello ->
+          # after head
+          # before body
+          # middle body
+          world
+          # after body
       end
       """
     end

lib/elixir/test/elixir/code_formatter/general_test.exs

@@ -187,6 +187,22 @@ defmodule Code.Formatter.GeneralTest do
       assert_same code, @short_length
     end
 
+    test "with a single clause, followed by a newline, and can fit in one line" do
+      assert_same """
+      fn
+        hello -> world
+      end
+      """
+    end
+
+    test "with a single clause, followed by a newline, and can not fit in one line" do
+      assert_same """
+      SomeModule.long_function_name_that_approaches_max_columns(argument, acc, fn
+        %SomeStruct{key: key}, acc -> more_code(key, acc)
+      end)
+      """
+    end
+
     test "with multiple clauses" do
       code = """
       fn