Move DebugChar into its own file

Change-Id: Ia40549e5d0b78ece8dd0722c3a5a032b9915f24b

src/libutil/escape-char.cc

@@ -0,0 +1,22 @@
+#include <boost/io/ios_state.hpp>
+#include <iomanip>
+#include <iostream>
+
+#include "escape-char.hh"
+
+namespace nix {
+
+std::ostream & operator<<(std::ostream & s, MaybeHexEscapedChar c)
+{
+    boost::io::ios_flags_saver _ifs(s);
+
+    if (isprint(c.c)) {
+        s << static_cast<char>(c.c);
+    } else {
+        s << "\\x" << std::hex << std::setfill('0') << std::setw(2)
+          << (static_cast<unsigned int>(c.c) & 0xff);
+    }
+    return s;
+}
+
+} // namespace nix

src/libutil/escape-char.hh

@@ -0,0 +1,22 @@
+#pragma once
+#include <ostream>
+
+namespace nix {
+
+/**
+ * A struct that prints a debug representation of a character, like `\x1f` for
+ * non-printable characters, or the character itself for other characters.
+ *
+ * Note that these are suitable for human readable output, but further care is
+ * necessary to include them in C++ strings to avoid running into adjacent
+ * hex-like characters. (`"puppy\x1bdoggy"` parses as `"puppy" "\x1bd" "oggy"`
+ * and errors because 0x1bd is too big for a `char`.)
+ */
+struct MaybeHexEscapedChar
+{
+    char c;
+};
+
+std::ostream & operator<<(std::ostream & s, MaybeHexEscapedChar c);
+
+} // namespace nix

src/libutil/meson.build

@@ -8,6 +8,7 @@ libutil_sources = files(
   'config.cc',
   'english.cc',
   'error.cc',
+  'escape-char.cc',
   'exit.cc',
   'experimental-features.cc',
   'filesystem.cc',
@@ -49,6 +50,7 @@ libutil_headers = files(
   'config.hh',
   'english.hh',
   'error.hh',
+  'escape-char.hh',
   'exit.hh',
   'experimental-features.hh',
   'experimental-features-json.hh',