Game Lua Kernel: simplify a horrendous loop

This can't be done in a simple loop over all_children_range since splice_children modifies
the source config. This adds a new getter method for a view over all tag names.

src/config.cpp

@@ -583,7 +583,7 @@ void config::clear_children_impl(config_key_type key)
 	children_.erase(i);
 }
 
-void config::splice_children(config& src, const std::string& key)
+void config::splice_children(config& src, config_key_type key)
 {
 	child_map::iterator i_src = src.children_.find(key);
 	if(i_src == src.children_.end()) {

src/config.hpp

@@ -33,6 +33,12 @@
 #include "utils/const_clone.hpp"
 #include "utils/optional_reference.hpp"
 
+#ifdef __cpp_lib_ranges
+#include <ranges>
+#else
+#include <boost/range/adaptor/map.hpp>
+#endif
+
 #include <functional>
 #include <iosfwd>
 #include <map>
@@ -609,7 +615,7 @@ public:
 	/**
 	 * Moves all the children with tag @a key from @a src to this.
 	 */
-	void splice_children(config &src, const std::string &key);
+	void splice_children(config& src, config_key_type key);
 
 	void remove_child(config_key_type key, std::size_t index);
 	/**
@@ -875,6 +881,16 @@ public:
 	 */
 	bool validate_wml() const;
 
+	/** A non-owning view over all child tag names. */
+	auto child_name_view() const
+	{
+#ifdef __cpp_lib_ranges
+		return children_ | std::views::keys;
+#else
+		return children_ | boost::adaptors::map_keys;
+#endif
+	}
+
 private:
 	/**
 	 * Removes the child at position @a pos of @a l.

src/scripting/game_lua_kernel.cpp

@@ -5708,11 +5708,11 @@ void game_lua_kernel::set_game_display(game_display * gd) {
  * elsewhere (in the C++ code).
  * Any child tags not in this list will be passed to Lua's on_load event.
  */
-static bool is_handled_file_tag(const std::string& s)
+static bool is_handled_file_tag(std::string_view s)
 {
 	// Make sure this is sorted, since we binary_search!
 	using namespace std::literals::string_view_literals;
-	static const std::array handled_file_tags {
+	static constexpr std::array handled_file_tags {
 		"color_palette"sv,
 		"color_range"sv,
 		"display"sv,
@@ -5787,23 +5787,24 @@ void game_lua_kernel::save_game(config &cfg)
 	luaW_toconfig(L, -1, v);
 	lua_pop(L, 1);
 
-	for (;;)
-	{
-		config::all_children_iterator i = v.ordered_begin();
-		if (i == v.ordered_end()) break;
-		if (is_handled_file_tag(i->key))
-		{
+	// Make a copy of the source tag names. Since splice is a destructive operation,
+	// we can't guarantee that the view will remain valid during iteration.
+	const auto temp = v.child_name_view();
+	const std::vector<std::string> src_tags(temp.begin(), temp.end());
+
+	for(const auto& key : src_tags) {
+		if(is_handled_file_tag(key)) {
 			/*
 			 * It seems the only tags appearing in the config v variable here
 			 * are the core-lua-handled (currently [item] and [objectives])
 			 * and the extra UMC ones.
 			 */
-			const std::string m = "Tag is already used: [" + i->key + "]";
+			const std::string m = "Tag is already used: [" + key + "]";
 			log_error(m.c_str());
-			v.erase(i);
 			continue;
+		} else {
+			cfg.splice_children(v, key);
 		}
-		cfg.splice_children(v, i->key);
 	}
 }