Clarify behaviour of Macro.to_string on invalid ASTs

Closes #13875.

Author: josevalim

lib/elixir/lib/code/formatter.ex

@@ -614,8 +614,11 @@ defmodule Code.Formatter do
   end
 
   # #PID's and #Ref's may appear on regular AST
+  # Other foreign structures, such as maps and structs,
+  # may appear from Macro.to_string, so we stick a limit,
+  # although they won't be formatted accordingly.
   defp quoted_to_algebra(unknown, _context, state) do
-    {inspect(unknown), state}
+    {inspect(unknown, printable_limit: :infinity), state}
   end
 
   ## Blocks

lib/elixir/lib/macro.ex

@@ -1125,6 +1125,10 @@ defmodule Macro do
   `Code.quoted_to_algebra/2` as a lower level function
   with more control around formatting.
 
+  If the AST contains invalid nodes, this function will
+  attempt to inspect them, to aid debugging, although
+  the elements won't be formatted accordingly.
+
   ## Examples
 
       iex> Macro.to_string(quote(do: foo.bar(1, 2, 3)))

lib/elixir/test/elixir/code_normalizer/quoted_ast_test.exs

@@ -226,17 +226,6 @@ defmodule Code.Normalizer.QuotedASTTest do
       assert quoted_to_string(quoted) <> "\n" == expected
     end
 
-    test "invalid block" do
-      assert quoted_to_string({:__block__, [], {:bar, [], []}}) ==
-               "{:__block__, [], {:bar, [], []}}"
-
-      assert quoted_to_string({:foo, [], [{:do, :ok}, :not_keyword]}) ==
-               "foo({:do, :ok}, :not_keyword)"
-
-      assert quoted_to_string({:foo, [], [[{:do, :ok}, :not_keyword]]}) ==
-               "foo([{:do, :ok}, :not_keyword])"
-    end
-
     test "not in" do
       assert quoted_to_string(quote(do: false not in [])) == "false not in []"
     end
@@ -738,6 +727,23 @@ defmodule Code.Normalizer.QuotedASTTest do
     end
   end
 
+  describe "quoted_to_algebra/2 with invalid" do
+    test "block" do
+      assert quoted_to_string({:__block__, [], {:bar, [], []}}) ==
+               "{:__block__, [], {:bar, [], []}}"
+
+      assert quoted_to_string({:foo, [], [{:do, :ok}, :not_keyword]}) ==
+               "foo({:do, :ok}, :not_keyword)"
+
+      assert quoted_to_string({:foo, [], [[{:do, :ok}, :not_keyword]]}) ==
+               "foo([{:do, :ok}, :not_keyword])"
+    end
+
+    test "ode" do
+      assert quoted_to_string(1..3) == "1..3"
+    end
+  end
+
   describe "quoted_to_algebra/2 does not escape" do
     test "sigils" do
       assert quoted_to_string(quote(do: ~s/a\nb\tc/), escape: false) == ~S"~s/a\nb\tc/"

lib/elixir/test/elixir/macro_test.exs

@@ -680,44 +680,8 @@ defmodule MacroTest do
       assert Macro.to_string(quote do: hello(world)) == "hello(world)"
     end
 
-    test "large number literals" do
-      # with quote
-      assert Macro.to_string(quote do: 576_460_752_303_423_455) == "576_460_752_303_423_455"
-      assert Macro.to_string(quote do: -576_460_752_303_423_455) == "-576_460_752_303_423_455"
-
-      # without quote
-      assert Macro.to_string(576_460_752_303_423_455) == "576_460_752_303_423_455"
-      assert Macro.to_string(-576_460_752_303_423_455) == "-576_460_752_303_423_455"
-    end
-
-    test "charlists" do
-      assert Macro.to_string(~c"foo") == ~s(~c"foo")
-    end
-
-    defmodule HTML do
-      defstruct [:string]
-
-      defimpl Inspect do
-        def inspect(%{string: string}, _) do
-          "~HTML[#{string}]"
-        end
-      end
-    end
-
-    defmacro sigil_HTML({:<<>>, _, [string]}, []) do
-      Macro.escape(%HTML{string: string})
-    end
-
-    test "sigils" do
-      assert Macro.to_string(quote(do: ~HTML[hi])) == ~S/~HTML[hi]/
-
-      assert Macro.to_string(
-               quote do
-                 ~HTML"""
-                 hi
-                 """
-               end
-             ) == ~s[~HTML"""\nhi\n"""]
+    test "converts invalid AST with inspect" do
+      assert Macro.to_string(quote do: unquote(1..3)) == "1..3"
     end
   end