Add usage checks for loaded mapgen instances.

This will find mapgen instances that were loaded from JSON, but have no usage within the game (e.g. because the belong to undefined overmap terrain).

Register some mapgen ids that are only used within the C++ code as used

src/mapgen.cpp

@@ -15,6 +15,7 @@
 #include <cmath>
 
 #include "clzones.h"
+#include "generic_factory.h"
 #include "computer.h"
 #include "coordinate_conversions.h"
 #include "coordinates.h"
@@ -254,6 +255,25 @@ class mapgen_factory
     private:
         std::map<std::string, mapgen_basic_container> mapgens_;
 
+        static std::set<std::string> get_usages() {
+            std::set<std::string> result;
+            for( const oter_t &elem : overmap_terrains::get_all() ) {
+                result.insert( elem.get_mapgen_id() );
+                result.insert( elem.id.str() );
+            }
+            // Why do I have to repeat the MapExtras here? Wouldn't "MapExtras::factory" be enough?
+            for( const map_extra &elem : MapExtras::mapExtraFactory().get_all() ) {
+                if( elem.generator_method == map_extra_method::mapgen ) {
+                    result.insert( elem.generator_id );
+                }
+            }
+            // Used in C++ code only, see calls to `oter_mapgen.pick()` below
+            result.insert( "lab_1side" );
+            result.insert( "lab_4side" );
+            result.insert( "lab_finale_1level" );
+            return result;
+        }
+
     public:
         void reset() {
             mapgens_.clear();
@@ -264,8 +284,14 @@ class mapgen_factory
             }
         }
         void check_consistency() {
+            // Cache all strings that may get looked up here so we don't have to go through
+            // all the sources for them upon each loop.
+            const std::set<std::string> usages = get_usages();
             for( std::pair<const std::string, mapgen_basic_container> &omw : mapgens_ ) {
                 omw.second.check_consistency( omw.first );
+                if( usages.count( omw.first ) == 0 ) {
+                    debugmsg( "Mapgen %s is not used by anything!", omw.first );
+                }
             }
         }
         bool has( const std::string &key ) const {
@@ -3517,6 +3543,7 @@ void map::draw_lab( mapgendata &dat )
 
             //A lab area with only one entrance
             if( boarders == 1 ) {
+                // If you remove the usage of "lab_1side" here, remove it from mapgen_factory::get_usages above as well.
                 if( const auto ptr = oter_mapgen.pick( "lab_1side" ) ) {
                     ptr->generate( dat );
                     if( tw == 2 ) {
@@ -3535,6 +3562,7 @@ void map::draw_lab( mapgendata &dat )
                 maybe_insert_stairs( terrain_type, t_stairs_down );
             } else {
                 const int hardcoded_4side_map_weight = 1500; // weight of all hardcoded maps.
+                // If you remove the usage of "lab_4side" here, remove it from mapgen_factory::get_usages above as well.
                 if( const auto ptr = oter_mapgen.pick( "lab_4side", hardcoded_4side_map_weight ) ) {
                     ptr->generate( dat );
                     // If the map template hasn't handled borders, handle them in code.
@@ -4080,6 +4108,7 @@ void map::draw_lab( mapgendata &dat )
         lw = is_ot_match( "lab", dat.west(), ot_match_type::contains ) ? 0 : 2;
 
         const int hardcoded_finale_map_weight = 500; // weight of all hardcoded maps.
+        // If you remove the usage of "lab_finale_1level" here, remove it from mapgen_factory::get_usages above as well.
         if( const auto ptr = oter_mapgen.pick( "lab_finale_1level", hardcoded_finale_map_weight ) ) {
             ptr->generate( dat );