The native core

6. The native core (Rust)

6.1 Crates

Concern	Crate	Notes
Windows, input, IME, event loop	winit	run_app on the main thread; EventLoopProxy for wake-ups from other threads
GPU	wgpu	Metal / D3D12 / Vulkan, with GL as a fallback; WGSL shaders
Layout	taffy	flexbox, grid, block. Do our own rounding to device pixels
Text layout and shaping	parley + fontique	Bidi, line breaking, font fallback. cosmic-text is the alternative
Glyph rasterization	swash	Coverage + color (emoji) atlases
Accessibility	accesskit + accesskit_winit	VoiceOver, Narrator/UIA, Orca/AT-SPI
Menus, tray	muda, tray-icon
Dialogs, clipboard	rfd, arboard
Images	image (PNG/JPEG/WebP), resvg (SVG)	Decoded off the main thread

6.2 Layout of the core’s code

core/
  crates/
    mantou-ffi        C ABI surface (cdylib); the only crate with `extern "C"`
    mantou-tree       shadow tree, op decoder, styles, Taffy integration, hit-testing, focus
    mantou-text       parley wrapper, measure cache, glyph atlas management
    mantou-scene      immutable scene / display list built from the tree after layout
    mantou-render     wgpu renderer: instanced SDF quads, glyphs, images, clips, layers
    mantou-platform   winit app, windows, IME, AccessKit, menus, dialogs, clipboard

mantou-tree, mantou-text and mantou-scene have no platform dependencies, so they can be tested headless (§12).

Status: mantou-ffi, mantou-tree and mantou-render exist. For now the scene types live in mantou-tree, and the winit code in mantou-ffi; mantou-scene, mantou-text and mantou-platform split out when text and platform services arrive. The C ABI is at version 3: every per-UI function takes an instance handle, and headless instances exist for tests (§12.2).

6.3 C ABI

bun:ffi can’t pass structs by value. Every call therefore takes only scalars, pointers and lengths.

uint32_t mt_abi_version(void);

// Bootstrap (main thread)
void     mt_run(const uint8_t* config, uint32_t len);   // blocks until quit; returns exit code via mt_exit_code()
int32_t  mt_exit_code(void);

// Worker thread
void     mt_set_wake(void (*wake)(void));                // threadsafe JSCallback pointer
void     mt_commit(const uint8_t* ops, uint32_t len);   // apply ops, layout, build scene, request redraw
uint32_t mt_take_events(const uint8_t** out);            // swaps the event buffer; *out valid until the next call
uint32_t mt_measure_text(const uint8_t* req, uint32_t len, uint8_t* out, uint32_t cap); // optional, for app-side measuring
void     mt_quit(int32_t code);

Both threads dlopen the same library. A commit made before mt_run has started is queued.

Revision planned (see §12.2): the core becomes multi-instance. Every function will take an instance handle: mt_run creates the app instance, and mt_headless_new creates headless instances for tests. The signatures above leave the handle out for brevity.