Use wl_container_of() instead of casts

This slightly improves type safety.

The culprits were found with:

    git grep -E '\([a-z0-9_ ]+ \*\)\W?[a-z]'

backend/drm/backend.c

@@ -16,7 +16,8 @@
 struct wlr_drm_backend *get_drm_backend_from_backend(
 		struct wlr_backend *wlr_backend) {
 	assert(wlr_backend_is_drm(wlr_backend));
-	return (struct wlr_drm_backend *)wlr_backend;
+	struct wlr_drm_backend *backend = wl_container_of(wlr_backend, backend, backend);
+	return backend;
 }
 
 static bool backend_start(struct wlr_backend *backend) {

backend/drm/drm.c

@@ -331,7 +331,8 @@ void finish_drm_resources(struct wlr_drm_backend *drm) {
 static struct wlr_drm_connector *get_drm_connector_from_output(
 		struct wlr_output *wlr_output) {
 	assert(wlr_output_is_drm(wlr_output));
-	return (struct wlr_drm_connector *)wlr_output;
+	struct wlr_drm_connector *conn = wl_container_of(wlr_output, conn, output);
+	return conn;
 }
 
 static void layer_handle_addon_destroy(struct wlr_addon *addon) {

backend/headless/backend.c

@@ -7,7 +7,8 @@
 struct wlr_headless_backend *headless_backend_from_backend(
 		struct wlr_backend *wlr_backend) {
 	assert(wlr_backend_is_headless(wlr_backend));
-	return (struct wlr_headless_backend *)wlr_backend;
+	struct wlr_headless_backend *backend = wl_container_of(wlr_backend, backend, backend);
+	return backend;
 }
 
 static bool backend_start(struct wlr_backend *wlr_backend) {

backend/headless/output.c

@@ -17,7 +17,8 @@ static size_t last_output_num = 0;
 static struct wlr_headless_output *headless_output_from_output(
 		struct wlr_output *wlr_output) {
 	assert(wlr_output_is_headless(wlr_output));
-	return (struct wlr_headless_output *)wlr_output;
+	struct wlr_headless_output *output = wl_container_of(wlr_output, output, wlr_output);
+	return output;
 }
 
 static bool output_set_custom_mode(struct wlr_headless_output *output,

backend/libinput/backend.c

@@ -11,7 +11,8 @@
 static struct wlr_libinput_backend *get_libinput_backend_from_backend(
 		struct wlr_backend *wlr_backend) {
 	assert(wlr_backend_is_libinput(wlr_backend));
-	return (struct wlr_libinput_backend *)wlr_backend;
+	struct wlr_libinput_backend *backend = wl_container_of(wlr_backend, backend, backend);
+	return backend;
 }
 
 static int libinput_open_restricted(const char *path,

backend/multi/backend.c

@@ -21,7 +21,8 @@ struct subbackend_state {
 static struct wlr_multi_backend *multi_backend_from_backend(
 		struct wlr_backend *wlr_backend) {
 	assert(wlr_backend_is_multi(wlr_backend));
-	return (struct wlr_multi_backend *)wlr_backend;
+	struct wlr_multi_backend *backend = wl_container_of(wlr_backend, backend, backend);
+	return backend;
 }
 
 static bool multi_backend_start(struct wlr_backend *wlr_backend) {

backend/wayland/backend.c

@@ -46,9 +46,10 @@ struct wlr_wl_linux_dmabuf_v1_table_entry {
 	uint64_t modifier;
 };
 
-struct wlr_wl_backend *get_wl_backend_from_backend(struct wlr_backend *backend) {
-	assert(wlr_backend_is_wl(backend));
-	return (struct wlr_wl_backend *)backend;
+struct wlr_wl_backend *get_wl_backend_from_backend(struct wlr_backend *wlr_backend) {
+	assert(wlr_backend_is_wl(wlr_backend));
+	struct wlr_wl_backend *backend = wl_container_of(wlr_backend, backend, backend);
+	return backend;
 }
 
 static int dispatch_events(int fd, uint32_t mask, void *data) {

backend/wayland/output.c

@@ -39,7 +39,8 @@ static size_t last_output_num = 0;
 static struct wlr_wl_output *get_wl_output_from_output(
 		struct wlr_output *wlr_output) {
 	assert(wlr_output_is_wl(wlr_output));
-	return (struct wlr_wl_output *)wlr_output;
+	struct wlr_wl_output *output = wl_container_of(wlr_output, output, wlr_output);
+	return output;
 }
 
 static void surface_frame_callback(void *data, struct wl_callback *cb,

backend/x11/backend.c

@@ -154,7 +154,8 @@ static int x11_event(int fd, uint32_t mask, void *data) {
 struct wlr_x11_backend *get_x11_backend_from_backend(
 		struct wlr_backend *wlr_backend) {
 	assert(wlr_backend_is_x11(wlr_backend));
-	return (struct wlr_x11_backend *)wlr_backend;
+	struct wlr_x11_backend *backend = wl_container_of(wlr_backend, backend, backend);
+	return backend;
 }
 
 static bool backend_start(struct wlr_backend *backend) {

backend/x11/output.c

@@ -54,7 +54,8 @@ static void parse_xcb_setup(struct wlr_output *output,
 static struct wlr_x11_output *get_x11_output_from_output(
 		struct wlr_output *wlr_output) {
 	assert(wlr_output_is_x11(wlr_output));
-	return (struct wlr_x11_output *)wlr_output;
+	struct wlr_x11_output *output = wl_container_of(wlr_output, output, wlr_output);
+	return output;
 }
 
 static bool output_set_custom_mode(struct wlr_output *wlr_output,

render/allocator/drm_dumb.c

@@ -25,7 +25,8 @@ static const struct wlr_buffer_impl buffer_impl;
 static struct wlr_drm_dumb_buffer *drm_dumb_buffer_from_buffer(
 		struct wlr_buffer *wlr_buf) {
 	assert(wlr_buf->impl == &buffer_impl);
-	return (struct wlr_drm_dumb_buffer *)wlr_buf;
+	struct wlr_drm_dumb_buffer *buf = wl_container_of(wlr_buf, buf, base);
+	return buf;
 }
 
 static struct wlr_drm_dumb_buffer *create_buffer(
@@ -163,7 +164,8 @@ static const struct wlr_allocator_interface allocator_impl;
 static struct wlr_drm_dumb_allocator *drm_dumb_alloc_from_alloc(
 		struct wlr_allocator *wlr_alloc) {
 	assert(wlr_alloc->impl == &allocator_impl);
-	return (struct wlr_drm_dumb_allocator *)wlr_alloc;
+	struct wlr_drm_dumb_allocator *alloc = wl_container_of(wlr_alloc, alloc, base);
+	return alloc;
 }
 
 static struct wlr_buffer *allocator_create_buffer(

render/allocator/gbm.c

@@ -17,9 +17,10 @@
 static const struct wlr_buffer_impl buffer_impl;
 
 static struct wlr_gbm_buffer *get_gbm_buffer_from_buffer(
-		struct wlr_buffer *buffer) {
-	assert(buffer->impl == &buffer_impl);
-	return (struct wlr_gbm_buffer *)buffer;
+		struct wlr_buffer *wlr_buffer) {
+	assert(wlr_buffer->impl == &buffer_impl);
+	struct wlr_gbm_buffer *buffer = wl_container_of(wlr_buffer, buffer, base);
+	return buffer;
 }
 
 static bool export_gbm_bo(struct gbm_bo *bo,
@@ -179,9 +180,10 @@ static const struct wlr_buffer_impl buffer_impl = {
 static const struct wlr_allocator_interface allocator_impl;
 
 static struct wlr_gbm_allocator *get_gbm_alloc_from_alloc(
-		struct wlr_allocator *alloc) {
-	assert(alloc->impl == &allocator_impl);
-	return (struct wlr_gbm_allocator *)alloc;
+		struct wlr_allocator *wlr_alloc) {
+	assert(wlr_alloc->impl == &allocator_impl);
+	struct wlr_gbm_allocator *alloc = wl_container_of(wlr_alloc, alloc, base);
+	return alloc;
 }
 
 struct wlr_allocator *wlr_gbm_allocator_create(int fd) {

render/allocator/shm.c

@@ -17,7 +17,8 @@ static const struct wlr_buffer_impl buffer_impl;
 static struct wlr_shm_buffer *shm_buffer_from_buffer(
 		struct wlr_buffer *wlr_buffer) {
 	assert(wlr_buffer->impl == &buffer_impl);
-	return (struct wlr_shm_buffer *)wlr_buffer;
+	struct wlr_shm_buffer *buffer = wl_container_of(wlr_buffer, buffer, base);
+	return buffer;
 }
 
 static void buffer_destroy(struct wlr_buffer *wlr_buffer) {

render/gles2/renderer.c

@@ -45,7 +45,8 @@ bool wlr_renderer_is_gles2(struct wlr_renderer *wlr_renderer) {
 struct wlr_gles2_renderer *gles2_get_renderer(
 		struct wlr_renderer *wlr_renderer) {
 	assert(wlr_renderer_is_gles2(wlr_renderer));
-	return (struct wlr_gles2_renderer *)wlr_renderer;
+	struct wlr_gles2_renderer *renderer = wl_container_of(wlr_renderer, renderer, wlr_renderer);
+	return renderer;
 }
 
 static struct wlr_gles2_renderer *gles2_get_renderer_in_context(
@@ -634,7 +635,7 @@ static int gles2_get_render_time(struct wlr_render_timer *wlr_timer) {
 }
 
 static void gles2_render_timer_destroy(struct wlr_render_timer *wlr_timer) {
-	struct wlr_gles2_render_timer *timer = (struct wlr_gles2_render_timer *)wlr_timer;
+	struct wlr_gles2_render_timer *timer = wl_container_of(wlr_timer, timer, base);
 	struct wlr_gles2_renderer *renderer = timer->renderer;
 
 	struct wlr_egl_context prev_ctx;

render/gles2/texture.c

@@ -25,7 +25,8 @@ bool wlr_texture_is_gles2(struct wlr_texture *wlr_texture) {
 struct wlr_gles2_texture *gles2_get_texture(
 		struct wlr_texture *wlr_texture) {
 	assert(wlr_texture_is_gles2(wlr_texture));
-	return (struct wlr_gles2_texture *)wlr_texture;
+	struct wlr_gles2_texture *texture = wl_container_of(wlr_texture, texture, wlr_texture);
+	return texture;
 }
 
 static bool gles2_texture_update_from_buffer(struct wlr_texture *wlr_texture,

render/pixman/renderer.c

@@ -20,7 +20,8 @@ bool wlr_renderer_is_pixman(struct wlr_renderer *wlr_renderer) {
 static struct wlr_pixman_renderer *get_renderer(
 		struct wlr_renderer *wlr_renderer) {
 	assert(wlr_renderer_is_pixman(wlr_renderer));
-	return (struct wlr_pixman_renderer *)wlr_renderer;
+	struct wlr_pixman_renderer *renderer = wl_container_of(wlr_renderer, renderer, wlr_renderer);
+	return renderer;
 }
 
 bool begin_pixman_data_ptr_access(struct wlr_buffer *wlr_buffer, pixman_image_t **image_ptr,
@@ -76,7 +77,8 @@ bool wlr_texture_is_pixman(struct wlr_texture *texture) {
 static struct wlr_pixman_texture *get_texture(
 		struct wlr_texture *wlr_texture) {
 	assert(wlr_texture_is_pixman(wlr_texture));
-	return (struct wlr_pixman_texture *)wlr_texture;
+	struct wlr_pixman_texture *texture = wl_container_of(wlr_texture, texture, wlr_texture);
+	return texture;
 }
 
 static void texture_destroy(struct wlr_texture *wlr_texture) {

render/vulkan/renderer.c

@@ -52,7 +52,8 @@ bool wlr_renderer_is_vk(struct wlr_renderer *wlr_renderer) {
 
 struct wlr_vk_renderer *vulkan_get_renderer(struct wlr_renderer *wlr_renderer) {
 	assert(wlr_renderer_is_vk(wlr_renderer));
-	return (struct wlr_vk_renderer *)wlr_renderer;
+	struct wlr_vk_renderer *renderer = wl_container_of(wlr_renderer, renderer, wlr_renderer);
+	return renderer;
 }
 
 static struct wlr_vk_render_format_setup *find_or_create_render_setup(

render/vulkan/texture.c

@@ -22,7 +22,8 @@ bool wlr_texture_is_vk(struct wlr_texture *wlr_texture) {
 
 struct wlr_vk_texture *vulkan_get_texture(struct wlr_texture *wlr_texture) {
 	assert(wlr_texture_is_vk(wlr_texture));
-	return (struct wlr_vk_texture *)wlr_texture;
+	struct wlr_vk_texture *texture = wl_container_of(wlr_texture, texture, wlr_texture);
+	return texture;
 }
 
 static VkImageAspectFlagBits mem_plane_aspect(unsigned i) {

types/buffer/client.c

@@ -7,11 +7,12 @@
 
 static const struct wlr_buffer_impl client_buffer_impl;
 
-struct wlr_client_buffer *wlr_client_buffer_get(struct wlr_buffer *buffer) {
-	if (buffer->impl != &client_buffer_impl) {
+struct wlr_client_buffer *wlr_client_buffer_get(struct wlr_buffer *wlr_buffer) {
+	if (wlr_buffer->impl != &client_buffer_impl) {
 		return NULL;
 	}
-	return (struct wlr_client_buffer *)buffer;
+	struct wlr_client_buffer *buffer = wl_container_of(wlr_buffer, buffer, base);
+	return buffer;
 }
 
 static struct wlr_client_buffer *client_buffer_from_buffer(

types/buffer/dmabuf.c

@@ -7,9 +7,10 @@
 static const struct wlr_buffer_impl dmabuf_buffer_impl;
 
 static struct wlr_dmabuf_buffer *dmabuf_buffer_from_buffer(
-		struct wlr_buffer *buffer) {
-	assert(buffer->impl == &dmabuf_buffer_impl);
-	return (struct wlr_dmabuf_buffer *)buffer;
+		struct wlr_buffer *wlr_buffer) {
+	assert(wlr_buffer->impl == &dmabuf_buffer_impl);
+	struct wlr_dmabuf_buffer *buffer = wl_container_of(wlr_buffer, buffer, base);
+	return buffer;
 }
 
 static void dmabuf_buffer_destroy(struct wlr_buffer *wlr_buffer) {

types/buffer/readonly_data.c

@@ -8,9 +8,10 @@
 static const struct wlr_buffer_impl readonly_data_buffer_impl;
 
 static struct wlr_readonly_data_buffer *readonly_data_buffer_from_buffer(
-		struct wlr_buffer *buffer) {
-	assert(buffer->impl == &readonly_data_buffer_impl);
-	return (struct wlr_readonly_data_buffer *)buffer;
+		struct wlr_buffer *wlr_buffer) {
+	assert(wlr_buffer->impl == &readonly_data_buffer_impl);
+	struct wlr_readonly_data_buffer *buffer = wl_container_of(wlr_buffer, buffer, base);
+	return buffer;
 }
 
 static void readonly_data_buffer_destroy(struct wlr_buffer *wlr_buffer) {

types/data_device/wlr_data_source.c

@@ -91,7 +91,8 @@ static void client_data_source_accept(struct wlr_data_source *wlr_source,
 static struct wlr_client_data_source *client_data_source_from_wlr_data_source(
 		struct wlr_data_source *wlr_source) {
 	assert(wlr_source->impl->accept == client_data_source_accept);
-	return (struct wlr_client_data_source *)wlr_source;
+	struct wlr_client_data_source *source = wl_container_of(wlr_source, source, source);
+	return source;
 }
 
 static void client_data_source_accept(struct wlr_data_source *wlr_source,

types/scene/wlr_scene.c

@@ -52,7 +52,8 @@ struct wlr_scene *scene_node_get_root(struct wlr_scene_node *node) {
 	while (tree->node.parent != NULL) {
 		tree = tree->node.parent;
 	}
-	return (struct wlr_scene *)tree;
+	struct wlr_scene *scene = wl_container_of(tree, scene, tree);
+	return scene;
 }
 
 static void scene_node_init(struct wlr_scene_node *node,

types/wlr_data_control_v1.c

@@ -120,7 +120,8 @@ static const struct wlr_data_source_impl client_source_impl;
 static struct client_data_source *
 		client_data_source_from_source(struct wlr_data_source *wlr_source) {
 	assert(wlr_source->impl == &client_source_impl);
-	return (struct client_data_source *)wlr_source;
+	struct client_data_source *source = wl_container_of(wlr_source, source, source);
+	return source;
 }
 
 static void client_source_send(struct wlr_data_source *wlr_source,
@@ -166,7 +167,8 @@ static struct client_primary_selection_source *
 		client_primary_selection_source_from_source(
 			struct wlr_primary_selection_source *wlr_source) {
 	assert(wlr_source->impl == &client_primary_selection_source_impl);
-	return (struct client_primary_selection_source *)wlr_source;
+	struct client_primary_selection_source *source = wl_container_of(wlr_source, source, source);
+	return source;
 }
 
 static void client_primary_selection_source_send(

types/wlr_drm.c

@@ -27,9 +27,10 @@ static const struct wl_buffer_interface wl_buffer_impl = {
 static const struct wlr_buffer_impl buffer_impl;
 
 static struct wlr_drm_buffer *drm_buffer_from_buffer(
-		struct wlr_buffer *buffer) {
-	assert(buffer->impl == &buffer_impl);
-	return (struct wlr_drm_buffer *)buffer;
+		struct wlr_buffer *wlr_buffer) {
+	assert(wlr_buffer->impl == &buffer_impl);
+	struct wlr_drm_buffer *buffer = wl_container_of(wlr_buffer, buffer, base);
+	return buffer;
 }
 
 static void buffer_destroy(struct wlr_buffer *wlr_buffer) {

types/wlr_keyboard_group.c

@@ -64,7 +64,8 @@ struct wlr_keyboard_group *wlr_keyboard_group_from_wlr_keyboard(
 	if (keyboard->impl != &impl) {
 		return NULL;
 	}
-	return (struct wlr_keyboard_group *)keyboard;
+	struct wlr_keyboard_group *group = wl_container_of(keyboard, group, keyboard);
+	return group;
 }
 
 static bool process_key(struct keyboard_group_device *group_device,

types/wlr_linux_dmabuf_v1.c

@@ -90,9 +90,10 @@ struct wlr_dmabuf_v1_buffer *wlr_dmabuf_v1_buffer_from_buffer_resource(
 static const struct wlr_buffer_impl buffer_impl;
 
 static struct wlr_dmabuf_v1_buffer *dmabuf_v1_buffer_from_buffer(
-		struct wlr_buffer *buffer) {
-	assert(buffer->impl == &buffer_impl);
-	return (struct wlr_dmabuf_v1_buffer *)buffer;
+		struct wlr_buffer *wlr_buffer) {
+	assert(wlr_buffer->impl == &buffer_impl);
+	struct wlr_dmabuf_v1_buffer *buffer = wl_container_of(wlr_buffer, buffer, base);
+	return buffer;
 }
 
 static void buffer_destroy(struct wlr_buffer *wlr_buffer) {

types/wlr_primary_selection_v1.c

@@ -103,14 +103,14 @@ struct client_data_source {
 static void client_source_send(
 		struct wlr_primary_selection_source *wlr_source,
 		const char *mime_type, int fd) {
-	struct client_data_source *source = (struct client_data_source *)wlr_source;
+	struct client_data_source *source = wl_container_of(wlr_source, source, source);
 	zwp_primary_selection_source_v1_send_send(source->resource, mime_type, fd);
 	close(fd);
 }
 
 static void client_source_destroy(
 		struct wlr_primary_selection_source *wlr_source) {
-	struct client_data_source *source = (struct client_data_source *)wlr_source;
+	struct client_data_source *source = wl_container_of(wlr_source, source, source);
 	zwp_primary_selection_source_v1_send_cancelled(source->resource);
 	// Make the source resource inert
 	wl_resource_set_user_data(source->resource, NULL);

xwayland/selection/incoming.c

@@ -257,7 +257,8 @@ bool data_source_is_xwayland(
 static struct x11_data_source *data_source_from_wlr_data_source(
 		struct wlr_data_source *wlr_source) {
 	assert(data_source_is_xwayland(wlr_source));
-	return (struct x11_data_source *)wlr_source;
+	struct x11_data_source *source = wl_container_of(wlr_source, source, base);
+	return source;
 }
 
 static void data_source_send(struct wlr_data_source *wlr_source,
@@ -299,8 +300,7 @@ bool primary_selection_source_is_xwayland(
 static void primary_selection_source_send(
 		struct wlr_primary_selection_source *wlr_source,
 		const char *mime_type, int fd) {
-	struct x11_primary_selection_source *source =
-		(struct x11_primary_selection_source *)wlr_source;
+	struct x11_primary_selection_source *source = wl_container_of(wlr_source, source, base);
 	struct wlr_xwm_selection *selection = source->selection;
 
 	source_send(selection, &wlr_source->mime_types, &source->mime_types_atoms,
@@ -309,8 +309,7 @@ static void primary_selection_source_send(
 
 static void primary_selection_source_destroy(
 		struct wlr_primary_selection_source *wlr_source) {
-	struct x11_primary_selection_source *source =
-		(struct x11_primary_selection_source *)wlr_source;
+	struct x11_primary_selection_source *source = wl_container_of(wlr_source, source, base);
 	wl_array_release(&source->mime_types_atoms);
 	free(source);
 }