Wasm vs host

The stack has two cells in the sense that matters to a page author, and one compiler in the sense that matters to setup. Confusing those two sentences is how people reintroduce a 1.42 host toolchain “just for vibe.0.”

The wasm cell is libwasm. It is what <script lang="d"> and src-d/ compile to. The target triple is wasm32-unknown-wasi. The flags that make D try/catch real are --wasm-enable-eh and --foptimize-nothrow=false. After the link, Binaryen ≥123 parses that try_table module and ships it through wasm-opt -Oz (release) or -g -O0 (debug). Flatten --asyncify is still UNREACHABLE on try_table (123 and 132), so this cell never asyncifies. You write mixin NodeDef, this.update, UnorderedList, and same-function try/catch. You do not write import vibe.*, std.file, or .await.

The host cell is vibe.0. It is what +page.server.d, +server.d, +layout.server.d, and hooks.server.d compile to, inside ws/webserver. The target is native. You write HTTPServerRequest, registerWebInterface, res.writeBody, res.redirect. You do not write import libwasm or mixin Spa. Class names are dest-unique (AdminPageServer, UsersPageServer) so every ingested route can hang on webserver/source/app.d without colliding.

One LDC 1.43+ compiles both. findHostLdc and findWasmLdc are the same function. Each dub build still clears DFLAGS and DC so a wasm object cannot be linked into the host and a host Phobos object cannot be linked into wasm. That is “two cells” as targets, not as versions.

Each cell also has two build types. Debug (svelte-d wasm --debug, svelte-d host, bun run dev) keeps symbols (wasm-opt -g -O0 still parses try_table). Release (svelte-d wasm, svelte-d host --release, svelte-d build, bun run build) is optimize plus lflags -strip-all, then wasm-opt -Oz. The kit-admin measurements — 12.64 MiB debug wasm down to 0.93 MiB / 224 KB gzipped, 14.08 MiB debug host down to 10.85 MiB — are in Wasm and host sizes.

<script lang="ts"> is neither cell. It is spliced into src-ts/modules/generated/ and folded into libwasm.init(modules) as jsExports, and registered on window.__svelteD.ts. jQuery, SCSS, and vanilla helpers stay on Vite. Same-file D reaches those exports by the simple name (callTs / Lodash invoke). A window host object that is not a lang=ts export is still Eval("window.$"), the PgLite pattern. A printed import jquery in src-d/ would be a third FFI. The calling convention is cross-calling.

Layouts are worth a special mention because they look like they might be a third cell. They are not. A +layout.svelte prints as an @child wrapper that stays mounted. KitRoutes remounts the page inside @entering. There is one wasm module and one mixin Spa!App. A layout that became its own wasm module would be a second DOM.

The two cells have their own API sections: libwasm for the wasm runtime a <script lang="d"> compiles against, and vibe.0 for the host runtime a +page.server.d compiles against.