From 49d026083b52dd0f79cb46df65510d219bf999c6 Mon Sep 17 00:00:00 2001
From: Sergei Zimmerman <xokdvium@proton.me>
Date: Mon, 19 May 2025 20:38:19 +0000
Subject: [PATCH] Use transparent comparators for `StringMap` and `StringPairs`

---
 src/libutil/include/nix/util/types.hh | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/libutil/include/nix/util/types.hh b/src/libutil/include/nix/util/types.hh
index 5139256ca..edb34f5e2 100644
--- a/src/libutil/include/nix/util/types.hh
+++ b/src/libutil/include/nix/util/types.hh
@@ -12,8 +12,25 @@
 namespace nix {
 
 typedef std::list<std::string> Strings;
-typedef std::map<std::string, std::string> StringMap;
-typedef std::map<std::string, std::string> StringPairs;
+
+/**
+ * Alias to ordered std::string -> std::string map container with transparent comparator.
+ *
+ * Used instead of std::map<std::string, std::string> to use C++14 N3657 [1]
+ * heterogenous lookup consistently across the whole codebase.
+ * Transparent comparators get rid of creation of unnecessary
+ * temporary variables when looking up keys by `std::string_view`
+ * or C-style `const char *` strings.
+ *
+ * [1]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3657.htm
+ */
+using StringMap = std::map<std::string, std::string, std::less<>>;
+/**
+ * Alias to an ordered map of std::string -> std::string. Uses transparent comparator.
+ *
+ * @see StringMap
+ */
+using StringPairs = StringMap;
 
 /**
  * Alias to ordered set container with transparent comparator.