feat: add customNamespaces option for custom namespaced attributes

[khromov]

Problem

Frameworks built on top of Svelte attach custom namespaced attributes to elements and components to carry framework-level metadata, e.g. an SSR/islands framework:

<Counter mochi:hydrate />

At runtime Svelte passes these through, but at type-check time svelte2tsx emits the attribute as a real prop/attribute, so TypeScript reports a hard error:

Type 'true' is not assignable to type 'never'.

There was no supported way to exempt these attributes. Downstream projects monkey-patch the bundled svelte-check output (a hard-coded carve-out in handleAttribute), which breaks on every version bump, hard-codes a single prefix, and only covers the component branch.

Solution

An opt-in, declarative customNamespaces option, configured once in svelte.config.js and honored by both the CLI (svelte-check) and the editor (language server):

// svelte.config.js
export default {
  customNamespaces: ['mochi']
};

An entry 'mochi' matches an attribute named exactly mochi or any attribute starting with mochi: (matching rule: name === ns || name.startsWith(ns + ':'); a trailing : is normalized away). Matching attributes reuse the existing __sveltets_2_empty({ ... }) wrapper that data-* already uses, in both the element (addAttribute) and component (addProp) branches — so the attribute name is exempt from prop/attribute validation while any value expression stays type-checked (mochi:defer={someVar} still errors if someVar is undefined).

Why this shape

• Opt-in, not automatic — treating all unknown colon-namespaced attributes as opaque would silently hide typos and conflict with Svelte's directive semantics. Type safety stays the default.
• Namespace list, not a predicate — declarative string[] mirrors the existing data-/-- handling, is trivial to serialize/validate/document, and works everywhere config is consumed. A predicate could be added later as a superset.
• customNamespaces over a generic prefix match — real-world cases are colon-delimited namespaces; "namespace" matches Svelte's own vocabulary (bind:, on:, use:).
• Config in svelte.config.js — one source of truth keeps CLI and editor in lockstep; a CLI-only fix would still leave red squiggles in the IDE.

Changes

• svelte2tsx: new customNamespaces?: string[] option (normalized), threaded svelte2tsx → convertHtmlxToJsx → handleAttribute; matching branch added to both attribute paths.
• svelte-language-server: reads customNamespaces from svelte.config.js (SvelteConfig) and passes it through the document snapshot path.
• svelte-check: reads it from the loaded config in the incremental emit path. (The non-incremental path already flows through the language server.)

Tests

• svelte2tsx fixtures: custom-namespaces (with option; covers component + element, value-less, expression value, and bare-namespace match) and custom-namespaces-disabled (control — confirms unwrapped emit, i.e. no global behavior change).
• language-server diagnostics fixtures: enabled → 0 errors; value-checked → only Cannot find name 'typoVar' (proves expressions still checked); disabled → exactly Type 'true' is not assignable to type 'never'.

Docs / changeset

• svelte-check README: new "Custom attribute namespaces" section, cross-linking --compiler-warnings "attribute_illegal_colon:ignore" (frameworks typically need both: the warning suppressed and the namespace registered).
• minor changeset for svelte2tsx, svelte-check, svelte-language-server.

Notes / open questions for maintainers

• CLI flag: a --custom-namespaces mochi,foo convenience flag was intentionally left out (config-first gives editor parity for free; threading a CLI value into the per-file config loader is invasive). Happy to add it as a follow-up if desired.
• Prefix vs namespace: scoped to the namespace shape; a generic prefix match is broader than needed. Open to changing if maintainers prefer.

Out of scope

• Suppressing the compiler attribute_illegal_colon warning (already configurable via --compiler-warnings).
• Changing Svelte core to register known namespaces.

[changeset-bot]

🦋 Changeset detected

Latest commit: d77082b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
svelte2tsx	Minor
svelte-check	Minor
svelte-language-server	Minor
typescript-svelte-plugin	Patch

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[dummdidumm]

Since we want to get rid of svelte.config.js at some point this would go the wrong way for this goal.
As discussed on discord ideally we have a way to globally enhance the component typings.

[khromov]

@dummdidumm Makes sense, how about this approach? #3060