How svelte-d works

svelte-d is a D program that happens to be importable from bun. The native CLI is built with vibe.0 as a library (VibeCustomMain); the TypeScript module in packages/svelte-d/ts is a thin façade over that executable and, when present, a dynamic library. Neither side parses Svelte with the official compiler. Pegged (with a markup scan as a named fallback) produces a tree whose node kinds are already libwasm’s. libdparse checks the lang=d bodies and the host *.d files. The printer pretty-prints. That is the entire front end.

The back end is not a code generator in the usual sense. There is no private intermediate language that then becomes D. The tree is NodeDef, @child, @prop, Slot, @visible, UnorderedList. Pretty-printing that tree into svelte-engine-ws/src-d/ is the compile. JSON written under ws/.svelte-d/ir/ is a content-addressed cache of those nodes so the next compile can skip a dest whose bytes would not change. It is not a language you are supposed to edit.

Around that print sit four mechanical steps that exist so the print has a place to land. First, drop copies the packaged svelte-engine bootstrap to svelte-engine-ws. The template is a golden; the workspace is disposable. Second, ingest overlays the bun project’s src/ — routes, lib, imported node_modules/*.svelte, local TypeScript and SCSS helpers, public/ — onto that workspace. Third, print walks the ingested .svelte files and the host +page.server.d files. Fourth, optional wasm and host cells invoke the same LDC 1.43 against two different dub.sdl files, with DFLAGS and DC cleared so objects cannot leak from one cell into the other.

Kit features are accommodated in svelte-engine, libwasm, and vibe.0, not in svelte-d. If {#each}{#if row.ok} needs a new list primitive, that primitive is added to libwasm, demonstrated in the engine, and only then printed. svelte-d does not grow a third runtime to paper over a missing idiom. Updating the engine and dropping again is how a new construct enters the product.

Two principles organize everything that follows.

Fall-through means a kit path keeps its relative shape. Only the cell prefix changes. src/routes/admin/users/+page.svelte is still routes/admin/users under src-svelte/ and src-d/; the file name +page.svelte becomes page.d because D cannot start an identifier with +. Mapping a path is not a claim that every construct on that file has been printed. Status lives in the language section and in architecture/sveltekit-feature-map.md.

AST ≡ D IR means the kinds in the walk are the kinds compile!() already understands. A {#if} that became a bespoke IfNode in a private JSON IR would force a second lowering, and that second lowering is how a toolchain quietly becomes a second DOM. There is one graph. The IR contract is the long form of that sentence.