Implement Resource UIDs

* Most resource types now have unique identifiers.
* Applies to text, binary and imported resources.
* File formats reference both by text and UID (when available). UID always has priority.
* Resource UIDs are 64 bits for better compatibility with the engine.
* Can be represented and used textually, example `uuid://dapwmgsmnl28u`.
* A special binary cache file is used and exported, containing the mappings.

Example of how it looks:

```GDScript
[gd_scene load_steps=2 format=3 uid="uid://dw86wq31afig2"]

[ext_resource type="PackedScene" uid="uid://bt36ojelx8q6c" path="res://subscene.scn" id="1_t56hs"]
```

GDScript, shaders and other special resource files can't currently provide UIDs, but this should be doable with special keywords on the files.
This will be reserved for future PRs.

14 files changed, 540 insertions, 94 deletions

diff --git a/core/io/resource.h b/core/io/resource.h
index 0f88738f76..e864b371ad 100644
--- a/core/io/resource.h
+++ b/core/io/resource.h
@@ -31,6 +31,7 @@
 #ifndef RESOURCE_H
 #define RESOURCE_H
 
+#include "core/io/resource_uid.h"
 #include "core/object/class_db.h"
 #include "core/object/ref_counted.h"
 #include "core/templates/safe_refcount.h"
diff --git a/core/io/resource_format_binary.cpp b/core/io/resource_format_binary.cpp
index e889586afc..38ad541a40 100644
--- a/core/io/resource_format_binary.cpp
+++ b/core/io/resource_format_binary.cpp
@@ -816,13 +816,18 @@ String ResourceLoaderBinary::get_unicode_string() {
 }
 
 void ResourceLoaderBinary::get_dependencies(FileAccess *p_f, List<String> *p_dependencies, bool p_add_types) {
-	open(p_f);
+	open(p_f, false, true);
 	if (error) {
 		return;
 	}
 
 	for (int i = 0; i < external_resources.size(); i++) {
-		String dep = external_resources[i].path;
+		String dep;
+		if (external_resources[i].uid != ResourceUID::INVALID_ID) {
+			dep = ResourceUID::get_singleton()->id_to_text(external_resources[i].uid);
+		} else {
+			dep = external_resources[i].path;
+		}
 
 		if (p_add_types && external_resources[i].type != String()) {
 			dep += "::" + external_resources[i].type;
@@ -832,7 +837,7 @@ void ResourceLoaderBinary::get_dependencies(FileAccess *p_f, List<String> *p_dep
 	}
 }
 
-void ResourceLoaderBinary::open(FileAccess *p_f) {
+void ResourceLoaderBinary::open(FileAccess *p_f, bool p_no_resources, bool p_keep_uuid_paths) {
 	error = OK;
 
 	f = p_f;
@@ -891,10 +896,24 @@ void ResourceLoaderBinary::open(FileAccess *p_f) {
 	if (flags & ResourceFormatSaverBinaryInstance::FORMAT_FLAG_NAMED_SCENE_IDS) {
 		using_named_scene_ids = true;
 	}
-	for (int i = 0; i < 13; i++) {
+	if (flags & ResourceFormatSaverBinaryInstance::FORMAT_FLAG_UIDS) {
+		using_uids = true;
+	}
+
+	if (using_uids) {
+		uid = f->get_64();
+	} else {
+		uid = ResourceUID::INVALID_ID;
+	}
+
+	for (int i = 0; i < 5; i++) {
 		f->get_32(); //skip a few reserved fields
 	}
 
+	if (p_no_resources) {
+		return;
+	}
+
 	uint32_t string_table_size = f->get_32();
 	string_map.resize(string_table_size);
 	for (uint32_t i = 0; i < string_table_size; i++) {
@@ -908,8 +927,18 @@ void ResourceLoaderBinary::open(FileAccess *p_f) {
 	for (uint32_t i = 0; i < ext_resources_size; i++) {
 		ExtResource er;
 		er.type = get_unicode_string();
-
 		er.path = get_unicode_string();
+		if (using_uids) {
+			er.uid = f->get_64();
+			if (!p_keep_uuid_paths && er.uid != ResourceUID::INVALID_ID) {
+				if (ResourceUID::get_singleton()->has_id(er.uid)) {
+					// If a UID is found and the path is valid, it will be used, otherwise, it falls back to the path.
+					er.path = ResourceUID::get_singleton()->get_id_path(er.uid);
+				} else {
+					WARN_PRINT(String(res_path + ": In external resouce #" + itos(i) + ", invalid UUID: " + ResourceUID::get_singleton()->id_to_text(er.uid) + " - using text path instead: " + er.path).utf8().get_data());
+				}
+			}
+		}
 
 		external_resources.push_back(er);
 	}
@@ -1173,8 +1202,15 @@ Error ResourceFormatLoaderBinary::rename_dependencies(const String &p_path, cons
 	uint64_t importmd_ofs = f->get_64();
 	fw->store_64(0); //metadata offset
 
-	for (int i = 0; i < 14; i++) {
-		fw->store_32(0);
+	uint32_t flags = f->get_32();
+	bool using_uids = (flags & ResourceFormatSaverBinaryInstance::FORMAT_FLAG_UIDS);
+	uint64_t uid_data = f->get_64();
+
+	fw->store_32(flags);
+	fw->store_64(uid_data);
+
+	for (int i = 0; i < 5; i++) {
+		f->store_32(0); // reserved
 		f->get_32();
 	}
 
@@ -1195,6 +1231,16 @@ Error ResourceFormatLoaderBinary::rename_dependencies(const String &p_path, cons
 		String type = get_ustring(f);
 		String path = get_ustring(f);
 
+		if (using_uids) {
+			ResourceUID::ID uid = f->get_64();
+			if (uid != ResourceUID::INVALID_ID) {
+				if (ResourceUID::get_singleton()->has_id(uid)) {
+					// If a UID is found and the path is valid, it will be used, otherwise, it falls back to the path.
+					path = ResourceUID::get_singleton()->get_id_path(uid);
+				}
+			}
+		}
+
 		bool relative = false;
 		if (!path.begins_with("res://")) {
 			path = local_path.plus_file(path).simplify_path();
@@ -1206,6 +1252,8 @@ Error ResourceFormatLoaderBinary::rename_dependencies(const String &p_path, cons
 			path = np;
 		}
 
+		String full_path = path;
+
 		if (relative) {
 			//restore relative
 			path = local_path.path_to_file(path);
@@ -1213,6 +1261,11 @@ Error ResourceFormatLoaderBinary::rename_dependencies(const String &p_path, cons
 
 		save_ustring(fw, type);
 		save_ustring(fw, path);
+
+		if (using_uids) {
+			ResourceUID::ID uid = ResourceSaver::get_resource_id_for_path(full_path);
+			f->store_64(uid);
+		}
 	}
 
 	int64_t size_diff = (int64_t)fw->get_position() - (int64_t)f->get_position();
@@ -1268,6 +1321,28 @@ String ResourceFormatLoaderBinary::get_resource_type(const String &p_path) const
 	return ClassDB::get_compatibility_remapped_class(r);
 }
 
+ResourceUID::ID ResourceFormatLoaderBinary::get_resource_uid(const String &p_path) const {
+	String ext = p_path.get_extension().to_lower();
+	if (!ClassDB::is_resource_extension(ext)) {
+		return ResourceUID::INVALID_ID;
+	}
+
+	FileAccess *f = FileAccess::open(p_path, FileAccess::READ);
+	if (!f) {
+		return ResourceUID::INVALID_ID; //could not read
+	}
+
+	ResourceLoaderBinary loader;
+	loader.local_path = ProjectSettings::get_singleton()->localize_path(p_path);
+	loader.res_path = loader.local_path;
+	//loader.set_local_path( Globals::get_singleton()->localize_path(p_path) );
+	loader.open(f, true);
+	if (loader.error != OK) {
+		return ResourceUID::INVALID_ID; //could not read
+	}
+	return loader.uid;
+}
+
 ///////////////////////////////////////////////////////////
 ///////////////////////////////////////////////////////////
 ///////////////////////////////////////////////////////////
@@ -1824,8 +1899,10 @@ Error ResourceFormatSaverBinaryInstance::save(const String &p_path, const RES &p
 
 	save_unicode_string(f, p_resource->get_class());
 	f->store_64(0); //offset to import metadata
-	f->store_32(FORMAT_FLAG_NAMED_SCENE_IDS);
-	for (int i = 0; i < 13; i++) {
+	f->store_32(FORMAT_FLAG_NAMED_SCENE_IDS | FORMAT_FLAG_UIDS);
+	ResourceUID::ID uid = ResourceSaver::get_resource_id_for_path(p_path, true);
+	f->store_64(uid);
+	for (int i = 0; i < 5; i++) {
 		f->store_32(0); // reserved
 	}
 
@@ -1891,6 +1968,8 @@ Error ResourceFormatSaverBinaryInstance::save(const String &p_path, const RES &p
 		String path = save_order[i]->get_path();
 		path = relative_paths ? local_path.path_to_file(path) : path;
 		save_unicode_string(f, path);
+		ResourceUID::ID ruid = ResourceSaver::get_resource_id_for_path(save_order[i]->get_path(), false);
+		f->store_64(ruid);
 	}
 	// save internal resource table
 	f->store_32(saved_resources.size()); //amount of internal resources
diff --git a/core/io/resource_format_binary.h b/core/io/resource_format_binary.h
index e3dcf44492..ac964d2053 100644
--- a/core/io/resource_format_binary.h
+++ b/core/io/resource_format_binary.h
@@ -47,6 +47,8 @@ class ResourceLoaderBinary {
 
 	uint64_t importmd_ofs = 0;
 
+	ResourceUID::ID uid = ResourceUID::INVALID_ID;
+
 	Vector<char> str_buf;
 	List<RES> resource_cache;
 
@@ -57,10 +59,12 @@ class ResourceLoaderBinary {
 	struct ExtResource {
 		String path;
 		String type;
+		ResourceUID::ID uid = ResourceUID::INVALID_ID;
 		RES cache;
 	};
 
 	bool using_named_scene_ids = false;
+	bool using_uids = false;
 	bool use_sub_threads = false;
 	float *progress = nullptr;
 	Vector<ExtResource> external_resources;
@@ -94,7 +98,7 @@ public:
 	void set_translation_remapped(bool p_remapped);
 
 	void set_remaps(const Map<String, String> &p_remaps) { remaps = p_remaps; }
-	void open(FileAccess *p_f);
+	void open(FileAccess *p_f, bool p_no_resources = false, bool p_keep_uuid_paths = false);
 	String recognize(FileAccess *p_f);
 	void get_dependencies(FileAccess *p_f, List<String> *p_dependencies, bool p_add_types);
 
@@ -109,6 +113,7 @@ public:
 	virtual void get_recognized_extensions(List<String> *p_extensions) const;
 	virtual bool handles_type(const String &p_type) const;
 	virtual String get_resource_type(const String &p_path) const;
+	virtual ResourceUID::ID get_resource_uid(const String &p_path) const;
 	virtual void get_dependencies(const String &p_path, List<String> *p_dependencies, bool p_add_types = false);
 	virtual Error rename_dependencies(const String &p_path, const Map<String, String> &p_map);
 };
@@ -157,7 +162,8 @@ class ResourceFormatSaverBinaryInstance {
 
 public:
 	enum {
-		FORMAT_FLAG_NAMED_SCENE_IDS = 1
+		FORMAT_FLAG_NAMED_SCENE_IDS = 1,
+		FORMAT_FLAG_UIDS = 2,
 	};
 	Error save(const String &p_path, const RES &p_resource, uint32_t p_flags = 0);
 	static void write_variant(FileAccess *f, const Variant &p_property, Map<RES, int> &resource_map, Map<RES, int> &external_resources, Map<StringName, int> &string_map, const PropertyInfo &p_hint = PropertyInfo());
diff --git a/core/io/resource_importer.cpp b/core/io/resource_importer.cpp
index f612b84404..6819511eb9 100644
--- a/core/io/resource_importer.cpp
+++ b/core/io/resource_importer.cpp
@@ -93,6 +93,8 @@ Error ResourceFormatImporter::_get_path_and_type(const String &p_path, PathAndTy
 				r_path_and_type.type = ClassDB::get_compatibility_remapped_class(value);
 			} else if (assign == "importer") {
 				r_path_and_type.importer = value;
+			} else if (assign == "uid") {
+				r_path_and_type.uid = ResourceUID::get_singleton()->text_to_id(value);
 			} else if (assign == "group_file") {
 				r_path_and_type.group_file = value;
 			} else if (assign == "metadata") {
@@ -336,6 +338,17 @@ String ResourceFormatImporter::get_resource_type(const String &p_path) const {
 	return pat.type;
 }
 
+ResourceUID::ID ResourceFormatImporter::get_resource_uid(const String &p_path) const {
+	PathAndType pat;
+	Error err = _get_path_and_type(p_path, pat);
+
+	if (err != OK) {
+		return ResourceUID::INVALID_ID;
+	}
+
+	return pat.uid;
+}
+
 Variant ResourceFormatImporter::get_resource_metadata(const String &p_path) const {
 	PathAndType pat;
 	Error err = _get_path_and_type(p_path, pat);
diff --git a/core/io/resource_importer.h b/core/io/resource_importer.h
index 62cb608a42..a1cacbd306 100644
--- a/core/io/resource_importer.h
+++ b/core/io/resource_importer.h
@@ -42,6 +42,7 @@ class ResourceFormatImporter : public ResourceFormatLoader {
 		String importer;
 		String group_file;
 		Variant metadata;
+		uint64_t uid = ResourceUID::INVALID_ID;
 	};
 
 	Error _get_path_and_type(const String &p_path, PathAndType &r_path_and_type, bool *r_valid = nullptr) const;
@@ -63,6 +64,8 @@ public:
 	virtual bool recognize_path(const String &p_path, const String &p_for_type = String()) const;
 	virtual bool handles_type(const String &p_type) const;
 	virtual String get_resource_type(const String &p_path) const;
+	virtual ResourceUID::ID get_resource_uid(const String &p_path) const;
+
 	virtual Variant get_resource_metadata(const String &p_path) const;
 	virtual bool is_import_valid(const String &p_path) const;
 	virtual void get_dependencies(const String &p_path, List<String> *p_dependencies, bool p_add_types = false);
diff --git a/core/io/resource_loader.cpp b/core/io/resource_loader.cpp
index c5dfe1f2b0..b685f973e8 100644
--- a/core/io/resource_loader.cpp
+++ b/core/io/resource_loader.cpp
@@ -84,6 +84,14 @@ String ResourceFormatLoader::get_resource_type(const String &p_path) const {
 	return "";
 }
 
+ResourceUID::ID ResourceFormatLoader::get_resource_uid(const String &p_path) const {
+	if (get_script_instance() && get_script_instance()->has_method("_get_resource_uid")) {
+		return get_script_instance()->call("_get_resource_uid", p_path);
+	}
+
+	return ResourceUID::INVALID_ID;
+}
+
 void ResourceFormatLoader::get_recognized_extensions_for_type(const String &p_type, List<String> *p_extensions) const {
 	if (p_type == "" || handles_type(p_type)) {
 		get_recognized_extensions(p_extensions);
@@ -270,13 +278,18 @@ void ResourceLoader::_thread_load_function(void *p_userdata) {
 	thread_load_mutex->unlock();
 }
 
-Error ResourceLoader::load_threaded_request(const String &p_path, const String &p_type_hint, bool p_use_sub_threads, ResourceFormatLoader::CacheMode p_cache_mode, const String &p_source_resource) {
-	String local_path;
-	if (p_path.is_rel_path()) {
-		local_path = "res://" + p_path;
+static String _validate_local_path(const String &p_path) {
+	ResourceUID::ID uid = ResourceUID::get_singleton()->text_to_id(p_path);
+	if (uid != ResourceUID::INVALID_ID) {
+		return ResourceUID::get_singleton()->get_id_path(uid);
+	} else if (p_path.is_rel_path()) {
+		return "res://" + p_path;
 	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(p_path);
+		return ProjectSettings::get_singleton()->localize_path(p_path);
 	}
+}
+Error ResourceLoader::load_threaded_request(const String &p_path, const String &p_type_hint, bool p_use_sub_threads, ResourceFormatLoader::CacheMode p_cache_mode, const String &p_source_resource) {
+	String local_path = _validate_local_path(p_path);
 
 	thread_load_mutex->lock();
 
@@ -399,12 +412,7 @@ float ResourceLoader::_dependency_get_progress(const String &p_path) {
 }
 
 ResourceLoader::ThreadLoadStatus ResourceLoader::load_threaded_get_status(const String &p_path, float *r_progress) {
-	String local_path;
-	if (p_path.is_rel_path()) {
-		local_path = "res://" + p_path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(p_path);
-	}
+	String local_path = _validate_local_path(p_path);
 
 	thread_load_mutex->lock();
 	if (!thread_load_tasks.has(local_path)) {
@@ -424,12 +432,7 @@ ResourceLoader::ThreadLoadStatus ResourceLoader::load_threaded_get_status(const
 }
 
 RES ResourceLoader::load_threaded_get(const String &p_path, Error *r_error) {
-	String local_path;
-	if (p_path.is_rel_path()) {
-		local_path = "res://" + p_path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(p_path);
-	}
+	String local_path = _validate_local_path(p_path);
 
 	thread_load_mutex->lock();
 	if (!thread_load_tasks.has(local_path)) {
@@ -510,12 +513,7 @@ RES ResourceLoader::load(const String &p_path, const String &p_type_hint, Resour
 		*r_error = ERR_CANT_OPEN;
 	}
 
-	String local_path;
-	if (p_path.is_rel_path()) {
-		local_path = "res://" + p_path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(p_path);
-	}
+	String local_path = _validate_local_path(p_path);
 
 	if (p_cache_mode != ResourceFormatLoader::CACHE_MODE_IGNORE) {
 		thread_load_mutex->lock();
@@ -612,12 +610,7 @@ RES ResourceLoader::load(const String &p_path, const String &p_type_hint, Resour
 }
 
 bool ResourceLoader::exists(const String &p_path, const String &p_type_hint) {
-	String local_path;
-	if (p_path.is_rel_path()) {
-		local_path = "res://" + p_path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(p_path);
-	}
+	String local_path = _validate_local_path(p_path);
 
 	if (ResourceCache::has(local_path)) {
 		return true; // If cached, it probably exists
@@ -677,14 +670,7 @@ void ResourceLoader::remove_resource_format_loader(Ref<ResourceFormatLoader> p_f
 }
 
 int ResourceLoader::get_import_order(const String &p_path) {
-	String path = _path_remap(p_path);
-
-	String local_path;
-	if (path.is_rel_path()) {
-		local_path = "res://" + path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(path);
-	}
+	String local_path = _path_remap(_validate_local_path(p_path));
 
 	for (int i = 0; i < loader_count; i++) {
 		if (!loader[i]->recognize_path(local_path)) {
@@ -702,14 +688,7 @@ int ResourceLoader::get_import_order(const String &p_path) {
 }
 
 String ResourceLoader::get_import_group_file(const String &p_path) {
-	String path = _path_remap(p_path);
-
-	String local_path;
-	if (path.is_rel_path()) {
-		local_path = "res://" + path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(path);
-	}
+	String local_path = _path_remap(_validate_local_path(p_path));
 
 	for (int i = 0; i < loader_count; i++) {
 		if (!loader[i]->recognize_path(local_path)) {
@@ -727,14 +706,7 @@ String ResourceLoader::get_import_group_file(const String &p_path) {
 }
 
 bool ResourceLoader::is_import_valid(const String &p_path) {
-	String path = _path_remap(p_path);
-
-	String local_path;
-	if (path.is_rel_path()) {
-		local_path = "res://" + path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(path);
-	}
+	String local_path = _path_remap(_validate_local_path(p_path));
 
 	for (int i = 0; i < loader_count; i++) {
 		if (!loader[i]->recognize_path(local_path)) {
@@ -752,14 +724,7 @@ bool ResourceLoader::is_import_valid(const String &p_path) {
 }
 
 bool ResourceLoader::is_imported(const String &p_path) {
-	String path = _path_remap(p_path);
-
-	String local_path;
-	if (path.is_rel_path()) {
-		local_path = "res://" + path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(path);
-	}
+	String local_path = _path_remap(_validate_local_path(p_path));
 
 	for (int i = 0; i < loader_count; i++) {
 		if (!loader[i]->recognize_path(local_path)) {
@@ -777,14 +742,7 @@ bool ResourceLoader::is_imported(const String &p_path) {
 }
 
 void ResourceLoader::get_dependencies(const String &p_path, List<String> *p_dependencies, bool p_add_types) {
-	String path = _path_remap(p_path);
-
-	String local_path;
-	if (path.is_rel_path()) {
-		local_path = "res://" + path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(path);
-	}
+	String local_path = _path_remap(_validate_local_path(p_path));
 
 	for (int i = 0; i < loader_count; i++) {
 		if (!loader[i]->recognize_path(local_path)) {
@@ -800,14 +758,7 @@ void ResourceLoader::get_dependencies(const String &p_path, List<String> *p_depe
 }
 
 Error ResourceLoader::rename_dependencies(const String &p_path, const Map<String, String> &p_map) {
-	String path = _path_remap(p_path);
-
-	String local_path;
-	if (path.is_rel_path()) {
-		local_path = "res://" + path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(path);
-	}
+	String local_path = _path_remap(_validate_local_path(p_path));
 
 	for (int i = 0; i < loader_count; i++) {
 		if (!loader[i]->recognize_path(local_path)) {
@@ -825,12 +776,7 @@ Error ResourceLoader::rename_dependencies(const String &p_path, const Map<String
 }
 
 String ResourceLoader::get_resource_type(const String &p_path) {
-	String local_path;
-	if (p_path.is_rel_path()) {
-		local_path = "res://" + p_path;
-	} else {
-		local_path = ProjectSettings::get_singleton()->localize_path(p_path);
-	}
+	String local_path = _validate_local_path(p_path);
 
 	for (int i = 0; i < loader_count; i++) {
 		String result = loader[i]->get_resource_type(local_path);
@@ -842,6 +788,19 @@ String ResourceLoader::get_resource_type(const String &p_path) {
 	return "";
 }
 
+ResourceUID::ID ResourceLoader::get_resource_uid(const String &p_path) {
+	String local_path = _validate_local_path(p_path);
+
+	for (int i = 0; i < loader_count; i++) {
+		ResourceUID::ID id = loader[i]->get_resource_uid(local_path);
+		if (id != ResourceUID::INVALID_ID) {
+			return id;
+		}
+	}
+
+	return ResourceUID::INVALID_ID;
+}
+
 String ResourceLoader::_path_remap(const String &p_path, bool *r_translation_remapped) {
 	String new_path = p_path;
 
diff --git a/core/io/resource_loader.h b/core/io/resource_loader.h
index c656b9a69c..b9e234fdd2 100644
--- a/core/io/resource_loader.h
+++ b/core/io/resource_loader.h
@@ -56,6 +56,7 @@ public:
 	virtual bool recognize_path(const String &p_path, const String &p_for_type = String()) const;
 	virtual bool handles_type(const String &p_type) const;
 	virtual String get_resource_type(const String &p_path) const;
+	virtual ResourceUID::ID get_resource_uid(const String &p_path) const;
 	virtual void get_dependencies(const String &p_path, List<String> *p_dependencies, bool p_add_types = false);
 	virtual Error rename_dependencies(const String &p_path, const Map<String, String> &p_map);
 	virtual bool is_import_valid(const String &p_path) const { return true; }
@@ -157,6 +158,7 @@ public:
 	static void add_resource_format_loader(Ref<ResourceFormatLoader> p_format_loader, bool p_at_front = false);
 	static void remove_resource_format_loader(Ref<ResourceFormatLoader> p_format_loader);
 	static String get_resource_type(const String &p_path);
+	static ResourceUID::ID get_resource_uid(const String &p_path);
 	static void get_dependencies(const String &p_path, List<String> *p_dependencies, bool p_add_types = false);
 	static Error rename_dependencies(const String &p_path, const Map<String, String> &p_map);
 	static bool is_import_valid(const String &p_path);
diff --git a/core/io/resource_saver.cpp b/core/io/resource_saver.cpp
index 80cb85fba3..bdfc7538a0 100644
--- a/core/io/resource_saver.cpp
+++ b/core/io/resource_saver.cpp
@@ -39,6 +39,7 @@ Ref<ResourceFormatSaver> ResourceSaver::saver[MAX_SAVERS];
 int ResourceSaver::saver_count = 0;
 bool ResourceSaver::timestamp_on_save = false;
 ResourceSavedCallback ResourceSaver::save_callback = nullptr;
+ResourceSaverGetResourceIDForPath ResourceSaver::save_get_id_for_path = nullptr;
 
 Error ResourceFormatSaver::save(const String &p_path, const RES &p_resource, uint32_t p_flags) {
 	if (get_script_instance() && get_script_instance()->has_method("_save")) {
@@ -259,3 +260,14 @@ void ResourceSaver::remove_custom_savers() {
 		remove_resource_format_saver(custom_savers[i]);
 	}
 }
+
+ResourceUID::ID ResourceSaver::get_resource_id_for_path(const String &p_path, bool p_generate) {
+	if (save_get_id_for_path) {
+		return save_get_id_for_path(p_path, p_generate);
+	}
+	return ResourceUID::INVALID_ID;
+}
+
+void ResourceSaver::set_get_resource_id_for_path(ResourceSaverGetResourceIDForPath p_callback) {
+	save_get_id_for_path = p_callback;
+}
diff --git a/core/io/resource_saver.h b/core/io/resource_saver.h
index 07154aac4d..2fc8d32126 100644
--- a/core/io/resource_saver.h
+++ b/core/io/resource_saver.h
@@ -48,6 +48,7 @@ public:
 };
 
 typedef void (*ResourceSavedCallback)(Ref<Resource> p_resource, const String &p_path);
+typedef ResourceUID::ID (*ResourceSaverGetResourceIDForPath)(const String &p_path, bool p_generate);
 
 class ResourceSaver {
 	enum {
@@ -58,6 +59,7 @@ class ResourceSaver {
 	static int saver_count;
 	static bool timestamp_on_save;
 	static ResourceSavedCallback save_callback;
+	static ResourceSaverGetResourceIDForPath save_get_id_for_path;
 
 	static Ref<ResourceFormatSaver> _find_custom_resource_format_saver(String path);
 
@@ -80,7 +82,10 @@ public:
 	static void set_timestamp_on_save(bool p_timestamp) { timestamp_on_save = p_timestamp; }
 	static bool get_timestamp_on_save() { return timestamp_on_save; }
 
+	static ResourceUID::ID get_resource_id_for_path(const String &p_path, bool p_generate = false);
+
 	static void set_save_callback(ResourceSavedCallback p_callback);
+	static void set_get_resource_id_for_path(ResourceSaverGetResourceIDForPath p_callback);
 
 	static bool add_custom_resource_format_saver(String script_path);
 	static void remove_custom_resource_format_saver(String script_path);
diff --git a/core/io/resource_uid.cpp b/core/io/resource_uid.cpp
new file mode 100644
index 0000000000..d4e8fcb6b0
--- /dev/null
+++ b/core/io/resource_uid.cpp
@@ -0,0 +1,262 @@
+/*************************************************************************/
+/*  resource_uid.cpp                                                     */
+/*************************************************************************/
+/*                       This file is part of:                           */
+/*                           GODOT ENGINE                                */
+/*                      https://godotengine.org                          */
+/*************************************************************************/
+/* Copyright (c) 2007-2021 Juan Linietsky, Ariel Manzur.                 */
+/* Copyright (c) 2014-2021 Godot Engine contributors (cf. AUTHORS.md).   */
+/*                                                                       */
+/* Permission is hereby granted, free of charge, to any person obtaining */
+/* a copy of this software and associated documentation files (the       */
+/* "Software"), to deal in the Software without restriction, including   */
+/* without limitation the rights to use, copy, modify, merge, publish,   */
+/* distribute, sublicense, and/or sell copies of the Software, and to    */
+/* permit persons to whom the Software is furnished to do so, subject to */
+/* the following conditions:                                             */
+/*                                                                       */
+/* The above copyright notice and this permission notice shall be        */
+/* included in all copies or substantial portions of the Software.       */
+/*                                                                       */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY  */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,  */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
+/*************************************************************************/
+
+#include "resource_uid.h"
+#include "core/crypto/crypto.h"
+#include "core/io/dir_access.h"
+#include "core/io/file_access.h"
+
+static constexpr uint32_t char_count = ('z' - 'a');
+static constexpr uint32_t base = char_count + ('9' - '0');
+
+const char *ResourceUID::CACHE_FILE = "res://.godot/uid_cache.bin";
+
+String ResourceUID::id_to_text(ID p_id) const {
+	if (p_id < 0) {
+		return "uid://<invalid>";
+	}
+	String txt;
+
+	while (p_id) {
+		uint32_t c = p_id % base;
+		if (c < char_count) {
+			txt = String::chr('a' + c) + txt;
+		} else {
+			txt = String::chr('0' + (c - char_count)) + txt;
+		}
+		p_id /= base;
+	}
+
+	return "uid://" + txt;
+}
+
+ResourceUID::ID ResourceUID::text_to_id(const String &p_text) const {
+	if (!p_text.begins_with("uid://") || p_text == "uid://<invalid>") {
+		return INVALID_ID;
+	}
+
+	uint32_t l = p_text.length();
+	uint64_t uid = 0;
+	for (uint32_t i = 6; i < l; i++) {
+		uid *= base;
+		uint32_t c = p_text[i];
+		if (c >= 'a' && c <= 'z') {
+			uid += c - 'a';
+		} else if (c >= '0' && c <= '9') {
+			uid += c - '0' + char_count;
+		} else {
+			return INVALID_ID;
+		}
+	}
+	return ID(uid & 0x7FFFFFFFFFFFFFFF);
+}
+
+ResourceUID::ID ResourceUID::create_id() const {
+	mutex.lock();
+	if (crypto.is_null()) {
+		crypto = Ref<Crypto>(Crypto::create());
+	}
+	mutex.unlock();
+	while (true) {
+		PackedByteArray bytes = crypto->generate_random_bytes(8);
+		ERR_FAIL_COND_V(bytes.size() != 8, INVALID_ID);
+		const uint64_t *ptr64 = (const uint64_t *)bytes.ptr();
+		ID id = int64_t((*ptr64) & 0x7FFFFFFFFFFFFFFF);
+		mutex.lock();
+		bool exists = unique_ids.has(id);
+		mutex.unlock();
+		if (!exists) {
+			return id;
+		}
+	}
+}
+
+bool ResourceUID::has_id(ID p_id) const {
+	MutexLock l(mutex);
+	return unique_ids.has(p_id);
+}
+void ResourceUID::add_id(ID p_id, const String &p_path) {
+	MutexLock l(mutex);
+	ERR_FAIL_COND(unique_ids.has(p_id));
+	Cache c;
+	c.cs = p_path.utf8();
+	unique_ids[p_id] = c;
+	changed = true;
+}
+
+void ResourceUID::set_id(ID p_id, const String &p_path) {
+	MutexLock l(mutex);
+	ERR_FAIL_COND(!unique_ids.has(p_id));
+	CharString cs = p_path.utf8();
+	if (strcmp(cs.ptr(), unique_ids[p_id].cs.ptr()) != 0) {
+		unique_ids[p_id].cs = cs;
+		unique_ids[p_id].saved_to_cache = false; //changed
+		changed = true;
+	}
+}
+
+String ResourceUID::get_id_path(ID p_id) const {
+	MutexLock l(mutex);
+	ERR_FAIL_COND_V(!unique_ids.has(p_id), String());
+	const CharString &cs = unique_ids[p_id].cs;
+	String s(cs.ptr());
+	return s;
+}
+void ResourceUID::remove_id(ID p_id) {
+	MutexLock l(mutex);
+	ERR_FAIL_COND(!unique_ids.has(p_id));
+	unique_ids.erase(p_id);
+}
+
+Error ResourceUID::save_to_cache() {
+	if (!FileAccess::exists(CACHE_FILE)) {
+		DirAccessRef d = DirAccess::create(DirAccess::ACCESS_RESOURCES);
+		d->make_dir_recursive(String(CACHE_FILE).get_base_dir()); //ensure base dir exists
+	}
+
+	FileAccessRef f = FileAccess::open(CACHE_FILE, FileAccess::WRITE);
+	if (!f) {
+		return ERR_CANT_OPEN;
+	}
+
+	MutexLock l(mutex);
+	f->store_32(unique_ids.size());
+
+	cache_entries = 0;
+
+	for (OrderedHashMap<ID, Cache>::Element E = unique_ids.front(); E; E = E.next()) {
+		f->store_64(E.key());
+		uint32_t s = E.get().cs.length();
+		f->store_32(s);
+		f->store_buffer((const uint8_t *)E.get().cs.ptr(), s);
+		E.get().saved_to_cache = true;
+		cache_entries++;
+	}
+
+	changed = false;
+	return OK;
+}
+
+Error ResourceUID::load_from_cache() {
+	FileAccessRef f = FileAccess::open(CACHE_FILE, FileAccess::READ);
+	if (!f) {
+		return ERR_CANT_OPEN;
+	}
+
+	MutexLock l(mutex);
+	unique_ids.clear();
+
+	uint32_t entry_count = f->get_32();
+	for (uint32_t i = 0; i < entry_count; i++) {
+		int64_t id = f->get_64();
+		int32_t len = f->get_32();
+		Cache c;
+		c.cs.resize(len + 1);
+		ERR_FAIL_COND_V(c.cs.size() != len + 1, ERR_FILE_CORRUPT); // out of memory
+		c.cs[len] = 0;
+		int32_t rl = f->get_buffer((uint8_t *)c.cs.ptrw(), len);
+		ERR_FAIL_COND_V(rl != len, ERR_FILE_CORRUPT);
+
+		c.saved_to_cache = true;
+		unique_ids[id] = c;
+	}
+
+	cache_entries = entry_count;
+	changed = false;
+	return OK;
+}
+
+Error ResourceUID::update_cache() {
+	if (!changed) {
+		return OK;
+	}
+
+	if (cache_entries == 0) {
+		return save_to_cache();
+	}
+	MutexLock l(mutex);
+
+	FileAccess *f = nullptr;
+	for (OrderedHashMap<ID, Cache>::Element E = unique_ids.front(); E; E = E.next()) {
+		if (!E.get().saved_to_cache) {
+			if (f == nullptr) {
+				f = FileAccess::open(CACHE_FILE, FileAccess::READ_WRITE); //append
+				if (!f) {
+					return ERR_CANT_OPEN;
+				}
+				f->seek_end();
+			}
+			f->store_64(E.key());
+			uint32_t s = E.get().cs.length();
+			f->store_32(s);
+			f->store_buffer((const uint8_t *)E.get().cs.ptr(), s);
+			E.get().saved_to_cache = true;
+			cache_entries++;
+		}
+	}
+
+	if (f != nullptr) {
+		f->seek(0);
+		f->store_32(cache_entries); //update amount of entries
+		f->close();
+		memdelete(f);
+	}
+
+	changed = false;
+
+	return OK;
+}
+
+void ResourceUID::clear() {
+	cache_entries = 0;
+	unique_ids.clear();
+	changed = false;
+}
+void ResourceUID::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("id_to_text", "id"), &ResourceUID::id_to_text);
+	ClassDB::bind_method(D_METHOD("text_to_id", "text_id"), &ResourceUID::text_to_id);
+
+	ClassDB::bind_method(D_METHOD("create_id"), &ResourceUID::create_id);
+
+	ClassDB::bind_method(D_METHOD("has_id", "id"), &ResourceUID::has_id);
+	ClassDB::bind_method(D_METHOD("add_id", "id", "path"), &ResourceUID::add_id);
+	ClassDB::bind_method(D_METHOD("set_id", "id", "path"), &ResourceUID::set_id);
+	ClassDB::bind_method(D_METHOD("get_id_path", "id"), &ResourceUID::get_id_path);
+	ClassDB::bind_method(D_METHOD("remove_id", "id", "path"), &ResourceUID::remove_id);
+
+	BIND_CONSTANT(INVALID_ID)
+}
+ResourceUID *ResourceUID::singleton = nullptr;
+ResourceUID::ResourceUID() {
+	ERR_FAIL_COND(singleton != nullptr);
+	singleton = this;
+}
+ResourceUID::~ResourceUID() {
+}
diff --git a/core/io/resource_uid.h b/core/io/resource_uid.h
new file mode 100644
index 0000000000..b12138425a
--- /dev/null
+++ b/core/io/resource_uid.h
@@ -0,0 +1,89 @@
+/*************************************************************************/
+/*  resource_uid.h                                                       */
+/*************************************************************************/
+/*                       This file is part of:                           */
+/*                           GODOT ENGINE                                */
+/*                      https://godotengine.org                          */
+/*************************************************************************/
+/* Copyright (c) 2007-2021 Juan Linietsky, Ariel Manzur.                 */
+/* Copyright (c) 2014-2021 Godot Engine contributors (cf. AUTHORS.md).   */
+/*                                                                       */
+/* Permission is hereby granted, free of charge, to any person obtaining */
+/* a copy of this software and associated documentation files (the       */
+/* "Software"), to deal in the Software without restriction, including   */
+/* without limitation the rights to use, copy, modify, merge, publish,   */
+/* distribute, sublicense, and/or sell copies of the Software, and to    */
+/* permit persons to whom the Software is furnished to do so, subject to */
+/* the following conditions:                                             */
+/*                                                                       */
+/* The above copyright notice and this permission notice shall be        */
+/* included in all copies or substantial portions of the Software.       */
+/*                                                                       */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY  */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,  */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
+/*************************************************************************/
+
+#ifndef RESOURCE_UUID_H
+#define RESOURCE_UUID_H
+
+#include "core/object/ref_counted.h"
+#include "core/string/string_name.h"
+#include "core/templates/ordered_hash_map.h"
+
+class Crypto;
+class ResourceUID : public Object {
+	GDCLASS(ResourceUID, Object)
+public:
+	typedef int64_t ID;
+	enum {
+		INVALID_ID = -1
+	};
+
+	static const char *CACHE_FILE;
+
+private:
+	mutable Ref<Crypto> crypto;
+	Mutex mutex;
+	struct Cache {
+		CharString cs;
+		bool saved_to_cache = false;
+	};
+
+	OrderedHashMap<ID, Cache> unique_ids; //unique IDs and utf8 paths (less memory used)
+	static ResourceUID *singleton;
+
+	uint32_t cache_entries = 0;
+	bool changed = false;
+
+protected:
+	static void _bind_methods();
+
+public:
+	String id_to_text(ID p_id) const;
+	ID text_to_id(const String &p_text) const;
+
+	ID create_id() const;
+	bool has_id(ID p_id) const;
+	void add_id(ID p_id, const String &p_path);
+	void set_id(ID p_id, const String &p_path);
+	String get_id_path(ID p_id) const;
+	void remove_id(ID p_id);
+
+	Error load_from_cache();
+	Error save_to_cache();
+	Error update_cache();
+
+	void clear();
+
+	static ResourceUID *get_singleton() { return singleton; }
+
+	ResourceUID();
+	~ResourceUID();
+};
+
+#endif // RESOURCEUUID_H
diff --git a/core/object/class_db.cpp b/core/object/class_db.cpp
index e2db5918e3..f871dd464a 100644
--- a/core/object/class_db.cpp
+++ b/core/object/class_db.cpp
@@ -1496,6 +1496,10 @@ void ClassDB::get_resource_base_extensions(List<String> *p_extensions) {
 	}
 }
 
+bool ClassDB::is_resource_extension(const StringName &p_extension) {
+	return resource_base_extensions.has(p_extension);
+}
+
 void ClassDB::get_extensions_for_type(const StringName &p_class, List<String> *p_extensions) {
 	const StringName *K = nullptr;
 
diff --git a/core/object/class_db.h b/core/object/class_db.h
index fd574fd2d8..3a84e9ab38 100644
--- a/core/object/class_db.h
+++ b/core/object/class_db.h
@@ -396,6 +396,7 @@ public:
 	static void add_resource_base_extension(const StringName &p_extension, const StringName &p_class);
 	static void get_resource_base_extensions(List<String> *p_extensions);
 	static void get_extensions_for_type(const StringName &p_class, List<String> *p_extensions);
+	static bool is_resource_extension(const StringName &p_extension);
 
 	static void add_compatibility_class(const StringName &p_class, const StringName &p_fallback);
 
diff --git a/core/register_core_types.cpp b/core/register_core_types.cpp
index eb37267546..c088390cf0 100644
--- a/core/register_core_types.cpp
+++ b/core/register_core_types.cpp
@@ -56,6 +56,7 @@
 #include "core/io/pck_packer.h"
 #include "core/io/resource_format_binary.h"
 #include "core/io/resource_importer.h"
+#include "core/io/resource_uid.h"
 #include "core/io/stream_peer_ssl.h"
 #include "core/io/tcp_server.h"
 #include "core/io/translation_loader_po.h"
@@ -102,6 +103,8 @@ static NativeExtensionManager *native_extension_manager = nullptr;
 extern void register_global_constants();
 extern void unregister_global_constants();
 
+static ResourceUID *resource_uid = nullptr;
+
 void register_core_types() {
 	//consistency check
 	static_assert(sizeof(Callable) <= 16);
@@ -225,6 +228,10 @@ void register_core_types() {
 
 	GDREGISTER_VIRTUAL_CLASS(NativeExtensionManager);
 
+	GDREGISTER_VIRTUAL_CLASS(ResourceUID);
+
+	resource_uid = memnew(ResourceUID);
+
 	native_extension_manager = memnew(NativeExtensionManager);
 
 	ip = IP::create();
@@ -286,6 +293,7 @@ void register_core_singletons() {
 	Engine::get_singleton()->add_singleton(Engine::Singleton("EngineDebugger", _EngineDebugger::get_singleton()));
 	Engine::get_singleton()->add_singleton(Engine::Singleton("Time", Time::get_singleton()));
 	Engine::get_singleton()->add_singleton(Engine::Singleton("NativeExtensionManager", NativeExtensionManager::get_singleton()));
+	Engine::get_singleton()->add_singleton(Engine::Singleton("ResourceUID", ResourceUID::get_singleton()));
 }
 
 void register_core_extensions() {
@@ -304,6 +312,8 @@ void unregister_core_types() {
 	native_extension_manager->deinitialize_extensions(NativeExtension::INITIALIZATION_LEVEL_CORE);
 
 	memdelete(native_extension_manager);
+
+	memdelete(resource_uid);
 	memdelete(_resource_loader);
 	memdelete(_resource_saver);
 	memdelete(_os);