7 files changed, 54 insertions, 32 deletions
diff --git a/src/libutil/escape-char.cc b/src/libutil/escape-char.cc
new file mode 100644
index 000000000..132260286
--- /dev/null
+++ b/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
diff --git a/src/libutil/escape-char.hh b/src/libutil/escape-char.hh
new file mode 100644
index 000000000..c7bae7ec0
--- /dev/null
+++ b/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
diff --git a/src/libutil/meson.build b/src/libutil/meson.build
index 8e4b5211d..cdfda3cf5 100644
--- a/src/libutil/meson.build
+++ b/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',
diff --git a/tests/functional/repl_characterization/test-session.cc b/tests/functional/repl_characterization/test-session.cc
index c35030fc7..50e27e58c 100644
--- a/tests/functional/repl_characterization/test-session.cc
+++ b/tests/functional/repl_characterization/test-session.cc
@@ -3,7 +3,7 @@
 
 #include "test-session.hh"
 #include "util.hh"
-#include "tests/debug-char.hh"
+#include "escape-char.hh"
 
 namespace nix {
 
@@ -60,7 +60,7 @@ std::ostream & operator<<(std::ostream & os, ReplOutputParser::State s)
 void ReplOutputParser::transition(State new_state, char responsible_char, bool wasPrompt)
 {
     if constexpr (DEBUG_REPL_PARSER) {
-        std::cerr << "transition " << new_state << " for " << DebugChar{responsible_char}
+        std::cerr << "transition " << new_state << " for " << MaybeHexEscapedChar{responsible_char}
                   << (wasPrompt ? " [prompt]" : "") << "\n";
     }
     state = new_state;
@@ -118,7 +118,7 @@ bool TestSession::waitForPrompt()
             });
 
             if constexpr (DEBUG_REPL_PARSER) {
-                std::cerr << "raw " << DebugChar{buf[i]} << (wasEaten ? " [eaten]" : "") << "\n";
+                std::cerr << "raw " << MaybeHexEscapedChar{buf[i]} << (wasEaten ? " [eaten]" : "") << "\n";
             }
         }
 
diff --git a/tests/unit/libutil-support/tests/cli-literate-parser.cc b/tests/unit/libutil-support/tests/cli-literate-parser.cc
index b3830e32c..08ebddebc 100644
--- a/tests/unit/libutil-support/tests/cli-literate-parser.cc
+++ b/tests/unit/libutil-support/tests/cli-literate-parser.cc
@@ -1,6 +1,6 @@
 #include "cli-literate-parser.hh"
 #include "libexpr/print.hh"
-#include "debug-char.hh"
+#include "escape-char.hh"
 #include "types.hh"
 #include "util.hh"
 #include <ranges>
@@ -77,7 +77,7 @@ CLILiterateParser::CLILiterateParser(std::string prompt, size_t indent)
 void CLILiterateParser::feed(char c)
 {
     if constexpr (DEBUG_PARSER) {
-        std::cout << stateDebug(state_) << " " << DebugChar{c} << "\n";
+        std::cout << stateDebug(state_) << " " << MaybeHexEscapedChar{c} << "\n";
     }
 
     if (c == '\n') {
diff --git a/tests/unit/libutil-support/tests/debug-char.hh b/tests/unit/libutil-support/tests/debug-char.hh
deleted file mode 100644
index 765d8553f..000000000
--- a/tests/unit/libutil-support/tests/debug-char.hh
+++ /dev/null
@@ -1,24 +0,0 @@
-///@file
-#include <ostream>
-#include <boost/io/ios_state.hpp>
-
-namespace nix {
-
-struct DebugChar
-{
-    char c;
-};
-
-inline std::ostream & operator<<(std::ostream & s, DebugChar c)
-{
-    boost::io::ios_flags_saver _ifs(s);
-
-    if (isprint(c.c)) {
-        s << static_cast<char>(c.c);
-    } else {
-        s << std::hex << "0x" << (static_cast<unsigned int>(c.c) & 0xff);
-    }
-    return s;
-}
-
-}
diff --git a/tests/unit/libutil-support/tests/terminal-code-eater.cc b/tests/unit/libutil-support/tests/terminal-code-eater.cc
index 51e1d565e..ad05ed1f6 100644
--- a/tests/unit/libutil-support/tests/terminal-code-eater.cc
+++ b/tests/unit/libutil-support/tests/terminal-code-eater.cc
@@ -1,5 +1,5 @@
 #include "terminal-code-eater.hh"
-#include "debug-char.hh"
+#include "escape-char.hh"
 #include <assert.h>
 #include <cstdint>
 #include <iostream>
@@ -14,7 +14,7 @@ void TerminalCodeEater::feed(char c, std::function<void(char)> on_char)
     auto isIntermediateChar = [](char v) -> bool { return v >= 0x20 && v <= 0x2f; };
     auto isFinalChar = [](char v) -> bool { return v >= 0x40 && v <= 0x7e; };
     if constexpr (DEBUG_EATER) {
-        std::cerr << "eater" << DebugChar{c} << "\n";
+        std::cerr << "eater" << MaybeHexEscapedChar{c} << "\n";
     }
 
     switch (state) {
@@ -28,7 +28,7 @@ void TerminalCodeEater::feed(char c, std::function<void(char)> on_char)
             return;
         }
         if constexpr (DEBUG_EATER) {
-            std::cerr << "eater uneat" << DebugChar{c} << "\n";
+            std::cerr << "eater uneat" << MaybeHexEscapedChar{c} << "\n";
         }
         on_char(c);
         break;