From 27b711b9aae7d216b5c6c4105d68700796eecf65 Mon Sep 17 00:00:00 2001
From: Mikhail Nekrashevich <mn.nekrashevich@yandex.ru>
Date: Sat, 8 Aug 2026 23:49:35 +0300
Subject: [PATCH] Escape C1 control characters with Unicode escapes

Assisted-by: Codex:GPT-5
Signed-off-by: Mikhail Nekrashevich <mn.nekrashevich@yandex.ru>
---
 lib/elixir/lib/code/identifier.ex     | 7 ++++++-
 lib/elixir/test/elixir/macro_test.exs | 8 ++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/lib/elixir/lib/code/identifier.ex b/lib/elixir/lib/code/identifier.ex
index 5030d2af0d..aa2f2cf415 100644
--- a/lib/elixir/lib/code/identifier.ex
+++ b/lib/elixir/lib/code/identifier.ex
@@ -150,11 +150,16 @@ defmodule Code.Identifier do
     <<acc::binary, char::utf8>>
   end
 
-  defp escape_char(char, acc) when char < 0x100 do
+  defp escape_char(char, acc) when char < 0x80 do
     <<a::4, b::4>> = <<char::8>>
     <<acc::binary, ?\\, ?x, to_hex(a), to_hex(b)>>
   end
 
+  defp escape_char(char, acc) when char < 0x100 do
+    <<a::4, b::4>> = <<char::8>>
+    <<acc::binary, ?\\, ?u, ?0, ?0, to_hex(a), to_hex(b)>>
+  end
+
   defp escape_char(char, acc) when char < 0x10000 do
     <<a::4, b::4, c::4, d::4>> = <<char::16>>
     <<acc::binary, ?\\, ?x, ?{, to_hex(a), to_hex(b), to_hex(c), to_hex(d), ?}>>
diff --git a/lib/elixir/test/elixir/macro_test.exs b/lib/elixir/test/elixir/macro_test.exs
index 08d98fabe7..3f4f3f9cef 100644
--- a/lib/elixir/test/elixir/macro_test.exs
+++ b/lib/elixir/test/elixir/macro_test.exs
@@ -1081,6 +1081,14 @@ defmodule MacroTest do
       end
     end
 
+    test "escapes C1 control characters without changing their encoding" do
+      for codepoint <- 0x80..0x9F do
+        string = <<codepoint::utf8>>
+        source = Macro.to_string(string)
+        assert {^string, []} = Code.eval_string(source)
+      end
+    end
+
     test "converts invalid AST with inspect" do
       assert Macro.to_string(1..3) == "1..3"
     end