libexpr: Use C++20 heterogeneous lookup for RegexCache

src/libexpr/primops.cc

@@ -4311,9 +4311,7 @@ struct RegexCache
 {
     struct State
     {
-        // TODO use C++20 transparent comparison when available
-        std::unordered_map<std::string_view, std::regex> cache;
-        std::list<std::string> keys;
+        std::unordered_map<std::string, std::regex, StringViewHash, std::equal_to<>> cache;
     };
 
     Sync<State> state_;
@@ -4324,8 +4322,14 @@ struct RegexCache
         auto it = state->cache.find(re);
         if (it != state->cache.end())
             return it->second;
-        state->keys.emplace_back(re);
-        return state->cache.emplace(state->keys.back(), std::regex(state->keys.back(), std::regex::extended)).first->second;
+        /* No std::regex constructor overload from std::string_view, but can be constructed
+           from a pointer + size or an iterator range. */
+        return state->cache
+            .emplace(
+                std::piecewise_construct,
+                std::forward_as_tuple(re),
+                std::forward_as_tuple(/*s=*/re.data(), /*count=*/re.size(), std::regex::extended))
+            .first->second;
     }
 };
 

src/libutil/include/nix/util/strings.hh

@@ -97,4 +97,39 @@ extern template std::string dropEmptyInitThenConcatStringsSep(std::string_view,
  * Arguments that need to be passed to ssh with spaces in them.
  */
 std::list<std::string> shellSplitString(std::string_view s);
+
+/**
+ * Hash implementation that can be used for zero-copy heterogenous lookup from
+ * P1690R1[1] in unordered containers.
+ *
+ * [1]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1690r1.html
+ */
+struct StringViewHash
+{
+private:
+    using HashType = std::hash<std::string_view>;
+
+public:
+    using is_transparent = void;
+
+    auto operator()(const char * str) const
+    {
+        /* This has a slight overhead due to an implicit strlen, but there isn't
+           a good way around it because the hash value of all overloads must be
+           consistent. Delegating to string_view is the solution initially proposed
+           in P0919R3. */
+        return HashType{}(std::string_view{str});
+    }
+
+    auto operator()(std::string_view str) const
+    {
+        return HashType{}(str);
+    }
+
+    auto operator()(const std::string & str) const
+    {
+        return HashType{}(std::string_view{str});
+    }
+};
+
 }