This is the log output from our compositor when it's ran as a nested Wayland instance:

[wlroots-sys/wlroots/backend/wayland/backend.c:186] Creating wayland backend
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_data_device_manager v3
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_shm v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_linux_dmabuf_v1 v3
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_drm v2
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zxdg_output_manager_v1 v2
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_compositor v4
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_subcompositor v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zxdg_shell_v6 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: xdg_wm_base v2
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_shell v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwlr_layer_shell_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_tablet_manager_v2 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: gamma_control_manager v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwlr_gamma_control_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: orbital_screenshooter v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwlr_export_dmabuf_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: org_kde_kwin_server_decoration_manager v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: gtk_primary_selection_device_manager v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: org_kde_kwin_idle v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_idle_inhibit_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwlr_input_inhibit_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_input_method_manager_v2 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_text_input_manager_v3 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_virtual_keyboard_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwlr_screencopy_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zxdg_decoration_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_pointer_constraints_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wp_presentation v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_output v3
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_seat v6
[wlroots-sys/wlroots/backend/wayland/wl_seat.c:398] seat 0x7fb3a2235e10 offered pointer
[wlroots-sys/wlroots/backend/wayland/wl_seat.c:411] seat 0x7fb3a2235e10 offered keyboard
[wlroots-sys/wlroots/render/egl.c:149] Using EGL 1.4
[wlroots-sys/wlroots/render/egl.c:150] Supported EGL extensions: EGL_ANDROID_blob_cache EGL_ANDROID_native_fence_sync EGL_EXT_buffer_age EGL_EXT_create_context_robustness EGL_EXT_image_dma_buf_import EGL_EXT_image_dma_buf_import_modifiers EGL_EXT_swap_buffers_with_damage EGL_IMG_context_priority EGL_KHR_config_attribs EGL_KHR_create_context EGL_KHR_create_context_no_error EGL_KHR_fence_sync EGL_KHR_get_all_proc_addresses EGL_KHR_gl_colorspace EGL_KHR_gl_renderbuffer_image EGL_KHR_gl_texture_2D_image EGL_KHR_gl_texture_3D_image EGL_KHR_gl_texture_cubemap_image EGL_KHR_image_base EGL_KHR_no_config_context EGL_KHR_reusable_sync EGL_KHR_surfaceless_context EGL_KHR_swap_buffers_with_damage EGL_EXT_pixel_format_float EGL_KHR_wait_sync EGL_MESA_configless_context EGL_MESA_drm_image EGL_MESA_image_dma_buf_export EGL_WL_bind_wayland_display EGL_WL_create_wayland_buffer_from_image 
[wlroots-sys/wlroots/render/egl.c:151] EGL vendor: Mesa Project
[wlroots-sys/wlroots/render/egl.c:97] Supported dmabuf buffer formats: AR30 XR30 AB30 XB30 AR24 AB24 XR24 XB24 AR15 RG16 R8   R16  GR88 GR32 YUV9 YU11 YU12 YU16 YU24 YVU9 YV11 YV12 YV16 YV24 NV12 NV16 YUYV UYVY 
[wlroots-sys/wlroots/render/gles2/renderer.c:553] Using OpenGL ES 3.2 Mesa 18.3.1
[wlroots-sys/wlroots/render/gles2/renderer.c:554] GL vendor: Intel Open Source Technology Center
[wlroots-sys/wlroots/render/gles2/renderer.c:555] Supported GLES2 extensions: GL_EXT_blend_minmax GL_EXT_multi_draw_arrays GL_EXT_texture_filter_anisotropic GL_EXT_texture_compression_s3tc GL_EXT_occlusion_query_boolean GL_EXT_texture_compression_dxt1 GL_EXT_texture_format_BGRA8888 GL_OES_compressed_ETC1_RGB8_texture GL_OES_depth24 GL_OES_element_index_uint GL_OES_fbo_render_mipmap GL_OES_mapbuffer GL_OES_rgb8_rgba8 GL_OES_standard_derivatives GL_OES_stencil8 GL_OES_texture_3D GL_OES_texture_float GL_OES_texture_float_linear GL_OES_texture_half_float GL_OES_texture_half_float_linear GL_OES_texture_npot GL_OES_vertex_half_float GL_EXT_texture_sRGB_decode GL_OES_EGL_image GL_OES_depth_texture GL_OES_packed_depth_stencil GL_EXT_texture_type_2_10_10_10_REV GL_OES_get_program_binary GL_APPLE_texture_max_level GL_EXT_discard_framebuffer GL_EXT_read_format_bgra GL_EXT_frag_depth GL_NV_fbo_color_attachments GL_OES_EGL_image_external GL_OES_EGL_sync GL_OES_vertex_array_object GL_OES_viewport_array GL_ANGLE_texture_compression_dxt3 GL_ANGLE_texture_compression_dxt5 GL_EXT_robustness GL_EXT_texture_rg GL_EXT_unpack_subimage GL_NV_draw_buffers GL_NV_read_buffer GL_NV_read_depth GL_NV_read_depth_stencil GL_NV_read_stencil GL_EXT_draw_buffers GL_EXT_map_buffer_range GL_KHR_debug GL_KHR_robustness GL_KHR_texture_compression_astc_ldr GL_OES_depth_texture_cube_map GL_OES_required_internalformat GL_OES_surfaceless_context GL_EXT_color_buffer_float GL_EXT_separate_shader_objects GL_EXT_shader_framebuffer_fetch GL_EXT_shader_integer_mix GL_EXT_tessellation_point_size GL_EXT_tessellation_shader GL_INTEL_conservative_rasterization GL_INTEL_performance_query GL_ANDROID_extension_pack_es31a GL_EXT_base_instance GL_EXT_compressed_ETC1_RGB8_sub_texture GL_EXT_copy_image GL_EXT_draw_buffers_indexed GL_EXT_draw_elements_base_vertex GL_EXT_gpu_shader5 GL_EXT_polygon_offset_clamp GL_EXT_primitive_bounding_box GL_EXT_render_snorm GL_EXT_shader_io_blocks GL_EXT_texture_border_clamp GL_EXT_texture_buffer GL_EXT_texture_cube_map_array GL_EXT_texture_norm16 GL_KHR_blend_equation_advanced GL_KHR_blend_equation_advanced_coherent GL_KHR_context_flush_control GL_KHR_robust_buffer_access_behavior GL_NV_image_formats GL_OES_copy_image GL_OES_draw_buffers_indexed GL_OES_draw_elements_base_vertex GL_OES_gpu_shader5 GL_OES_primitive_bounding_box GL_OES_sample_shading GL_OES_sample_variables GL_OES_shader_io_blocks GL_OES_shader_multisample_interpolation GL_OES_tessellation_point_size GL_OES_tessellation_shader GL_OES_texture_border_clamp GL_OES_texture_buffer GL_OES_texture_cube_map_array GL_OES_texture_stencil8 GL_OES_texture_storage_multisample_2d_array GL_OES_texture_view GL_EXT_blend_func_extended GL_EXT_buffer_storage GL_EXT_geometry_point_size GL_EXT_geometry_shader GL_EXT_shader_samples_identical GL_KHR_no_error GL_KHR_texture_compression_astc_sliced_3d GL_NV_fragment_shader_interlock GL_OES_EGL_image_external_essl3 GL_OES_geometry_point_size GL_OES_geometry_shader GL_OES_shader_image_atomic GL_EXT_clip_cull_distance GL_EXT_disjoint_timer_query GL_MESA_shader_integer_functions GL_EXT_shader_framebuffer_fetch_non_coherent GL_MESA_framebuffer_flip_y 
[src/compositor.rs:434] Running compositor on wayland display wayland-2
[src/compositor.rs:497] Starting compositor
[wlroots-sys/wlroots/backend/wayland/backend.c:104] Initializating wayland backend
[GLES2] FS SIMD8 shader: 5 inst, 0 loops, 24 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 80 to 48 bytes.
[GLES2] FS SIMD16 shader: 5 inst, 0 loops, 34 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 80 to 48 bytes.
[GLES2] VS SIMD8 shader: 28 inst, 0 loops, 116 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 448 to 336 bytes.
[GLES2] FS SIMD16 shader: 2 inst, 0 loops, 0 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 32 to 32 bytes.
 

Your output will probably not match exactly, but it should roughly have this output.

When build_auto is called on Compositor it will dynamically detect which backend makes the most sense to spin up. If the compositor is ran in X11 or a Wayland compositor then it will run as a client with all the contents rendered to a window. If ran on a TTY then it uses the kernel's DRM module. It also possible to specify a backend directly.

[wlroots-sys/wlroots/backend/wayland/backend.c:186] Creating wayland backend

This first line shows which backend was selected. The Wayland backend was selected here, because it was ran in another Wayland instance.¹

[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_data_device_manager v3
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_shm v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_linux_dmabuf_v1 v3
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_drm v2
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zxdg_output_manager_v1 v2
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_compositor v4
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_subcompositor v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zxdg_shell_v6 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: xdg_wm_base v2
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_shell v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwlr_layer_shell_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_tablet_manager_v2 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: gamma_control_manager v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwlr_gamma_control_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: orbital_screenshooter v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwlr_export_dmabuf_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: org_kde_kwin_server_decoration_manager v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: gtk_primary_selection_device_manager v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: org_kde_kwin_idle v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_idle_inhibit_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwlr_input_inhibit_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_input_method_manager_v2 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_text_input_manager_v3 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_virtual_keyboard_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwlr_screencopy_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zxdg_decoration_manager_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: zwp_pointer_constraints_v1 v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wp_presentation v1
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_output v3
[wlroots-sys/wlroots/backend/wayland/backend.c:65] Remote wayland global: wl_seat v6

This is a list of all the globals that the parent compositor was advertising. This is output is specific to the Wayland backend. Protocols that are unstable have their names prepended with z by convention.

Our compositor also exposes some globals even in this minimal state. Globals are the way clients can start up communication with the Wayland compositor. There are some default ones that come bundled with Wayland, such as wl_compositor, and then there are custom ones defined on a per compositor basis. wlroots comes with some popular custom protocols already implemented, but you have to explicitly opt in to using them explicitly in the builder. xdg shell, for example, is an optional protocol that wasn't used in this example.

In order to see what globals the toy compositor is advertising you need to use a useful Wayland utility called weston-info.² It lists the Wayland globals advertised by the current compositor. The current compositor is determined by looking at the $WAYLAND_DISPLAY environment variable similar to how in X the current xserver is determined with $DISPLAY.

In the log output it prints out the $WAYLAND_DISPLAY:

[src/compositor.rs:434] Running compositor on wayland display wayland-2

Lets see what globals are being advertised by our compositor:

interface: 'wl_compositor', version: 4, name: 1
interface: 'wl_subcompositor', version: 1, name: 2

weston-info prints out the name of each exposed interface on a new line along with the highest advertised interface version and the relative, atomically increasing index (confusingly prepended with "name").

wl_compositor and wl_subcompositor are both standard Wayland interfaces. wl_subcompositor is automatically started by wlroots when a wl_compositor is used. A wl_compositor is the base global that all clients depend on. From this global a client can create a wl_surface and a wl_region.

A wl_surface is a basic building block for drawing and rendering contents to the screen in Wayland. A client needs more than a wl_surface in order to render to the screen, but that is the basic object a compositor needs in order to render.³

A wl_region is the object that allows clients to tell the compositor where, in surface level coordinates⁴, it wants to handle input and where it is rendering content. Where it wants input is very important, but the default is that it accepts input everywhere in the surface. Specifying an area where the client is rendering content is important because it allows the compositor to know that any content behind that doesn't need to be redrawn. As a very simple example of this if there is a moving background on the screen and there is a fullscreen window then there is no need to draw the background saving precious cycles.

The ability to specify only parts of the screen to update is a major feature of Wayland which will be totally ignored until a much later chapter. When starting out it's simple enough to simply redraw the entire screen each time a new frame is available. For non-toy compositors though it is vital that proper damage tracking (as the feature is called) is implemented. It reduces power consumption and makes the compositor faster.

[wlroots-sys/wlroots/backend/wayland/wl_seat.c:398] seat 0x7fb3a2235e10 offered pointer
[wlroots-sys/wlroots/backend/wayland/wl_seat.c:411] seat 0x7fb3a2235e10 offered keyboard

Rootson automatically offers the keyboard and mouse to all new windows that appear. This allows input to passthrough directly to the toy compositor, but it also hints at this concept of Wayland "seats".

A Wayland seat is a collection of inputs devices usually handled under the hood by libinput. Seats are created by the compositor, advertised to any new clients including when new input methods are added, and are used to facilitate user input to clients including drag-in-drop.

Seats are necessary to communicate properly with clients and will be explored in a later chapter.

[wlroots-sys/wlroots/render/egl.c:149] Using EGL 1.4
[wlroots-sys/wlroots/render/egl.c:150] Supported EGL extensions: EGL_ANDROID_blob_cache EGL_ANDROID_native_fence_sync EGL_EXT_buffer_age EGL_EXT_create_context_robustness EGL_EXT_image_dma_buf_import EGL_EXT_image_dma_buf_import_modifiers EGL_EXT_swap_buffers_with_damage EGL_IMG_context_priority EGL_KHR_config_attribs EGL_KHR_create_context EGL_KHR_create_context_no_error EGL_KHR_fence_sync EGL_KHR_get_all_proc_addresses EGL_KHR_gl_colorspace EGL_KHR_gl_renderbuffer_image EGL_KHR_gl_texture_2D_image EGL_KHR_gl_texture_3D_image EGL_KHR_gl_texture_cubemap_image EGL_KHR_image_base EGL_KHR_no_config_context EGL_KHR_reusable_sync EGL_KHR_surfaceless_context EGL_KHR_swap_buffers_with_damage EGL_EXT_pixel_format_float EGL_KHR_wait_sync EGL_MESA_configless_context EGL_MESA_drm_image EGL_MESA_image_dma_buf_export EGL_WL_bind_wayland_display EGL_WL_create_wayland_buffer_from_image 
[wlroots-sys/wlroots/render/egl.c:151] EGL vendor: Mesa Project
[wlroots-sys/wlroots/render/egl.c:97] Supported dmabuf buffer formats: AR30 XR30 AB30 XB30 AR24 AB24 XR24 XB24 AR15 RG16 R8   R16  GR88 GR32 YUV9 YU11 YU12 YU16 YU24 YVU9 YV11 YV12 YV16 YV24 NV12 NV16 YUYV UYVY 
[wlroots-sys/wlroots/render/gles2/renderer.c:553] Using OpenGL ES 3.2 Mesa 18.3.1
[wlroots-sys/wlroots/render/gles2/renderer.c:554] GL vendor: Intel Open Source Technology Center
[wlroots-sys/wlroots/render/gles2/renderer.c:555] Supported GLES2 extensions: GL_EXT_blend_minmax GL_EXT_multi_draw_arrays GL_EXT_texture_filter_anisotropic GL_EXT_texture_compression_s3tc GL_EXT_occlusion_query_boolean GL_EXT_texture_compression_dxt1 GL_EXT_texture_format_BGRA8888 GL_OES_compressed_ETC1_RGB8_texture GL_OES_depth24 GL_OES_element_index_uint GL_OES_fbo_render_mipmap GL_OES_mapbuffer GL_OES_rgb8_rgba8 GL_OES_standard_derivatives GL_OES_stencil8 GL_OES_texture_3D GL_OES_texture_float GL_OES_texture_float_linear GL_OES_texture_half_float GL_OES_texture_half_float_linear GL_OES_texture_npot GL_OES_vertex_half_float GL_EXT_texture_sRGB_decode GL_OES_EGL_image GL_OES_depth_texture GL_OES_packed_depth_stencil GL_EXT_texture_type_2_10_10_10_REV GL_OES_get_program_binary GL_APPLE_texture_max_level GL_EXT_discard_framebuffer GL_EXT_read_format_bgra GL_EXT_frag_depth GL_NV_fbo_color_attachments GL_OES_EGL_image_external GL_OES_EGL_sync GL_OES_vertex_array_object GL_OES_viewport_array GL_ANGLE_texture_compression_dxt3 GL_ANGLE_texture_compression_dxt5 GL_EXT_robustness GL_EXT_texture_rg GL_EXT_unpack_subimage GL_NV_draw_buffers GL_NV_read_buffer GL_NV_read_depth GL_NV_read_depth_stencil GL_NV_read_stencil GL_EXT_draw_buffers GL_EXT_map_buffer_range GL_KHR_debug GL_KHR_robustness GL_KHR_texture_compression_astc_ldr GL_OES_depth_texture_cube_map GL_OES_required_internalformat GL_OES_surfaceless_context GL_EXT_color_buffer_float GL_EXT_separate_shader_objects GL_EXT_shader_framebuffer_fetch GL_EXT_shader_integer_mix GL_EXT_tessellation_point_size GL_EXT_tessellation_shader GL_INTEL_conservative_rasterization GL_INTEL_performance_query GL_ANDROID_extension_pack_es31a GL_EXT_base_instance GL_EXT_compressed_ETC1_RGB8_sub_texture GL_EXT_copy_image GL_EXT_draw_buffers_indexed GL_EXT_draw_elements_base_vertex GL_EXT_gpu_shader5 GL_EXT_polygon_offset_clamp GL_EXT_primitive_bounding_box GL_EXT_render_snorm GL_EXT_shader_io_blocks GL_EXT_texture_border_clamp GL_EXT_texture_buffer GL_EXT_texture_cube_map_array GL_EXT_texture_norm16 GL_KHR_blend_equation_advanced GL_KHR_blend_equation_advanced_coherent GL_KHR_context_flush_control GL_KHR_robust_buffer_access_behavior GL_NV_image_formats GL_OES_copy_image GL_OES_draw_buffers_indexed GL_OES_draw_elements_base_vertex GL_OES_gpu_shader5 GL_OES_primitive_bounding_box GL_OES_sample_shading GL_OES_sample_variables GL_OES_shader_io_blocks GL_OES_shader_multisample_interpolation GL_OES_tessellation_point_size GL_OES_tessellation_shader GL_OES_texture_border_clamp GL_OES_texture_buffer GL_OES_texture_cube_map_array GL_OES_texture_stencil8 GL_OES_texture_storage_multisample_2d_array GL_OES_texture_view GL_EXT_blend_func_extended GL_EXT_buffer_storage GL_EXT_geometry_point_size GL_EXT_geometry_shader GL_EXT_shader_samples_identical GL_KHR_no_error GL_KHR_texture_compression_astc_sliced_3d GL_NV_fragment_shader_interlock GL_OES_EGL_image_external_essl3 GL_OES_geometry_point_size GL_OES_geometry_shader GL_OES_shader_image_atomic GL_EXT_clip_cull_distance GL_EXT_disjoint_timer_query GL_MESA_shader_integer_functions GL_EXT_shader_framebuffer_fetch_non_coherent GL_MESA_framebuffer_flip_y 
...
[GLES2] FS SIMD8 shader: 5 inst, 0 loops, 24 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 80 to 48 bytes.
[GLES2] FS SIMD16 shader: 5 inst, 0 loops, 34 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 80 to 48 bytes.
[GLES2] VS SIMD8 shader: 28 inst, 0 loops, 116 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 448 to 336 bytes.
[GLES2] FS SIMD16 shader: 2 inst, 0 loops, 0 cycles, 0:0 spills:fills, Promoted 0 constants, compacted 32 to 32 bytes.

Currently all backends need a renderer in wlroots which is automatically setup when you create one. This output is from the Wayland backend setting up the EGL drawing for rendering. In the future this may change, as the rendering API is in flux.

[src/compositor.rs:434] Running compositor on wayland display wayland-2
[src/compositor.rs:497] Starting compositor
[wlroots-sys/wlroots/backend/wayland/backend.c:104] Initializating wayland backend

Everything after these lines, including these lines, is printed to the log after run is called. Since there are no clients that connected there is no logging from them and since there are no event handlers nothing else happens.

¹ On my machine it was ran in rootson, the wlroots reference compositor, which is why Wayland was selected.

² In most Linux distributions this utility is packaged along with weston, the reference Wayland compositor.

³ Usually a surface is wrapped in a shell. What a shell adds to a wl_surface is context. Without the proper context a compositor doesn't know if the surface it was just handed by the client is a standalone window, a popup, a background, a status bar, or a cursor to be rendered. All of them need to be handled differently and they are all handled using a dedicated wayland "shell" or a specialized non-shell protocol.

⁴ It has to be surface level because clients doesn't know about anything but the content it renders.