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ddidderr 42f05c926d feat(wasm): port Ghostty's wasm32 build

Ports main_wasm.zig as crates/ghostty-wasm (cdylib exporting the Zig
names: atlas_*, face_*, deferred_face_*, shaper_*, config_*), the
web_canvas font face and shaper (backend selected on wasm32), os/wasm
(logging, target, JS interop) and config/Wasm.zig.

C-library dependencies (freetype, harfbuzz, fontconfig, onig, gettext,
D-Bus, rustix) are target-gated. Like Zig's wasm build, link regex
search (StringMap::search_iterator) is unavailable without Oniguruma;
Collection/SharedGrid/harfbuzz are compiled out on wasm32. ghostty-os
switches from forbid to deny unsafe_code for the JS import blocks.

The JS import ABI is currently a documented handle-based interface with
a reference host (js/ghostty_host.js); aligning it with zig-js's exact
import names follows separately.

Test Plan:
- cargo build --target wasm32-unknown-unknown -p ghostty-font \
  -p ghostty-config -p ghostty-terminal -p ghostty-vt -p ghostty-wasm
- cargo test -p ghostty-wasm (node runs the module against a fake canvas)

2026-10-12 05:21:14 +02:00

5.2 KiB

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WASM port (W1)

Port of Ghostty's wasm-specific Zig. Everything is behind cfg(target_arch = "wasm32") (or target-gated Cargo deps); native behaviour is unchanged.

File map

Zig	Rust
`main_wasm.zig`	`crates/ghostty-wasm` (new, `cdylib`; empty on native)
`config/Wasm.zig`	`ghostty-wasm/src/config.rs`
`os/wasm.zig`, `os/wasm/target.zig`, `os/wasm/log.zig`	`ghostty-os/src/wasm/{mod,target,log}.rs`
zig-js (`js.*` used by Atlas/web_canvas)	`ghostty-os/src/wasm/js.rs` + host `ghostty-wasm/js/ghostty_host.js`
`font/face/web_canvas.zig`	`ghostty-font/src/face/web_canvas.rs` (+ exports in `ghostty-wasm/src/face.rs`)
`font/shaper/web_canvas.zig`	`ghostty-font/src/shaper/web_canvas.rs` (+ `ghostty-wasm/src/shaper.rs`)
`font/DeferredFace.zig` (`wc` variant, `Wasm`)	`ghostty-font/src/deferred_face_web_canvas.rs`
`font/Atlas.zig` `Wasm`	`ghostty-wasm/src/atlas.rs`
`font/backend.zig`	`ghostty-font/src/backend.rs` (`OPTIONS`/`default_for_target` = `WebCanvas` on wasm32)
`renderer/WebGL.zig` (empty stub), `App.zig` `Wasm` (commented out upstream)	nothing to export; `ghostty-wasm/src/app.rs` documents it
`os/passwd.zig` (`@compileError` on wasm)	`ghostty-os/src/unsupported.rs` shims

Build gating

ghostty-os: nix/rustix/zbus/gettext target-gated; POSIX modules (env, file, flatpak, ...) are not compiled on wasm32. unsupported.rs gives API-compatible "unavailable" stand-ins (xdg, passwd, expand_home, resources_dir, is_flatpak) so ghostty-config compiles unchanged (Zig never analyzes them on wasm). unsafe is deny (not forbid) for the wasm import blocks only.
ghostty-config: only std::os::unix bits changed (file-type names, OsStrExt).
ghostty-terminal: onig, ghostty-onig, rustix target-gated. Like Zig with oniguruma = false, StringMap::search_iterator/SearchIterator do not exist on wasm (match types stay). Kitty shared-memory transmission returns InvalidData on wasm; OsStrExt shim for the file medium.
ghostty-font: freetype/harfbuzz/fontconfig -sys target-gated; on wasm the collection, codepoint_resolver, shared_grid(_set), library, discovery, fontconfig, embedded, face::freetype, harfbuzz shaper and shaper::{run,cache,noop} modules are compiled out.
Builds: cargo build --target wasm32-unknown-unknown -p ghostty-font -p ghostty-config -p ghostty-terminal -p ghostty-vt -p ghostty-wasm passes (debug + release); clippy -D warnings clean on both targets.

JS protocol (deviation)

zig-js (mitchellh/zig-js) is not vendored in the Ghostty tree, so its import names could not be mirrored. js.rs documents a small handle-based ABI in import module ghostty_js (global/get/set/call/new/string/number/ boolean/to_f64/string_len/string_copy/is_nullish/bytes/copy_bytes/release) plus env.log(ptr,len) (same as Zig's log). The Rust-side API mirrors js.Object. ghostty-wasm/js/ghostty_host.js is a reference host. Allocation helpers wasm_alloc/wasm_free (host-owned buffers) replace the stale toHostOwned (does not exist in current os/wasm.zig).

Exports

Same names as Zig: atlas_{new,free,reserve,set,grow,clear,debug_canvas}, face_{new,free,render_glyph,debug_canvas}, deferred_face_{new,free,load}, shaper_{new,free,test}, config_{new,free,load_string,finalize}. Differences/extras: region_free/glyph_free (Zig: "global free"), atlas_data/atlas_size, face_metrics, face_glyph_index, shaper_{prepare,add_codepoint,shape}, config_format, config_diagnostics_count, ghostty_wasm_init, wasm_alloc/free. shaper_test takes a Face (Zig takes a stale GroupCache) and prints into a ghostty-terminal Terminal.

Deviations from the (stale) Zig web_canvas code

Zig refers to Atlas.Format.rgba and a 7-field Metrics that no longer exist; here Bgra atlases get the canvas RGBA bitmap with R/B swapped, grayscale gets alpha, and Metrics::init() + face size = cell size.
The RunIterator hook API is tied to the FreeType Collection (not built on wasm), so Shaper exposes prepare/add_codepoint/finalize/shape directly and takes the grapheme-index callback (Face::grapheme_glyph_index).
Collection/SharedGrid are not generic over the face type in this port, so they are unavailable on wasm (Zig's wasm exports never use them).
render_glyph drops the unused RenderOptions parameter.

Tests

Native: pure parts of web_canvas (grapheme table, metrics math, presentation detection, glyph sizing, bitmap/atlas conversion; 9 tests) and the shaper clustering (6 tests) run natively; backend::web_canvas_capabilities.
crates/ghostty-wasm/tests/node.rs builds the module (separate target dir target/wasm-test) and runs tests/node_test.js under node with a fake canvas: config parse/finalize/format, atlas, face metrics/glyph_index/ render, emoji presentation, shaper clustering, deferred face, debug canvas. Skipped if node or the wasm32 target is missing.
Not ported: Zig tests in Atlas.Wasm (toHostOwned, stale).

Known gaps

ghostty-os/tests/common/flatpak_mock.rs (another agent's, untracked) fails clippy --all-targets -D warnings (needless_pass_by_value); unrelated.

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