Svelte component AST, retentive syntax and compiler-oracle boundary

Document status: verified. Canonical source.

Summary

The official Svelte compiler is the semantic oracle for .svelte syntax. It exposes component and CSS parsers with source-offset AST nodes and separately models markup, instance/module scripts and component CSS. A NUIF production adapter still needs a concrete syntax tree because the official print API is explicitly allowed to change whitespace and quoting. The selected split is the official compiler for pinned foreign conformance and tree-sitter-svelte-next 0.1.1 for bounded UTF-8 byte spans in Rust.

The first executable profile is deliberately smaller than Svelte’s static surface: regular div/span elements, double-quoted literal identity and name attributes, one literal inline-style declaration list and one literal text run. Scripts, component CSS, expressions and all directives or blocks are rejected inside the mapped component. Top-level comments and whitespace may be retained, but the profile does not infer their runtime relationship to the marked root; other top-level nodes are rejected.

Evidence

• svelte/compiler exposes compile, parse, parseCss, preprocess and print. Modern AST nodes contain start and end offsets. The root separates markup, CSS, instance script and module script. https://svelte.dev/docs/svelte/svelte-compiler#parse (retrieved 2026-08-29).
• print emits valid Svelte plus a source map but may change whitespace and quoting. It is therefore not an edit-locality mechanism for retained source. https://svelte.dev/docs/svelte/svelte-compiler#print (retrieved 2026-08-29).
• The AST distinguishes static text, expression tags, HTML tags, regular elements, components, blocks and directives. This supplies a structural boundary between literals and executable expressions. https://svelte.dev/docs/svelte/svelte-compiler#AST (retrieved 2026-08-29).
• Component CSS is scoped by default through a hash-derived class added to affected elements and selectors. Scoped keyframe names are also rewritten. https://svelte.dev/docs/svelte/scoped-styles (retrieved 2026-08-29).
• tree-sitter-svelte-next 0.1.1 is a dual MIT/Apache-2.0 grammar compatible with Tree-sitter 0.25 and later. Its crate is generated from commit bdea454a8ae7272498b8fe9d4b6b24fbd3dfe7b6; its Rust API exposes the language and node types without a Svelte runtime. The grammar includes elements, literal and expression attributes, blocks, scripts and styles, so the adapter can reject executable nodes structurally. Locators: crate manifest, src/lib.rs, src/node-types.json, .cargo_vcs_info.json, retrieved 2026-08-30: https://github.com/PRRPCHT/tree-sitter-svelte-next and https://docs.rs/tree-sitter-svelte-next/0.1.1/.
• The npm registry reported official svelte 5.57.0 as latest on 2026-08-30. A foreign fixture must pin that exact package and invoke both parse(source, { modern: true }) and compile(source, ...); floating latest is not evidence because the compiler and modern-AST defaults evolve.
• The independent Rust svelte-compiler 0.1.4 declares Rust 1.94 and depends on a broad compiler stack. Its own AUDIT.md reports about 11,300 lines in the API module, duplicated modern/legacy paths and manual recovery for grammar gaps. This is useful comparative work but is neither the official compiler nor a smaller retentive boundary. Locators: crate manifest and project audit, retrieved 2026-08-30: https://docs.rs/svelte-compiler/0.1.4/svelte_compiler/ and https://github.com/themixednuts/svelte/blob/main/AUDIT.md.

Alternatives and decision

Inline literal style is selected over a component <style> block for profile zero. It binds every mapped scalar to the element that owns it, avoids cascade, specificity and Svelte scope-hash semantics, and provides one exact replaceable span per property. A later class/CSS profile needs separate selector, cascade, scope and unused-selector conformance; it is not an implicit expansion of this profile.

Mechanism

The Rust parser first enforces encoded-size, syntax-node and mapped-depth limits. It locates exactly one marked regular-element root, verifies the closed attribute and inline-style vocabularies, decodes only canonical entity escapes, and records every mapped UTF-8 span. Export self-imports. Synchronization renders the before and after profile forms to obtain canonical replacement values, checks that each retained span is still current, applies replacements from the end of the source, and self-imports the result. The foreign gate separately parses and compiles generated and synchronized sources with the exact official compiler package.

NUIF relevance

Borrow compiler AST offsets and literal-node categories for correspondence.

Adapt a static subset of regular elements, literal attributes, literal text and profile-owned inline CSS declarations. Edits replace original spans rather than printing the AST. Grammar revision, compiler version and modern-AST mode are part of provenance.

Reject automatic semantic lifting of runes, scripts, expressions, snippets, blocks, directives, actions, transitions, dynamic components or preprocessors. They are programs whose behavior depends on inputs and runtime state.

Falsification and update triggers

The adapter decision fails if the Tree-sitter grammar accepts a generated fixture that official Svelte rejects, assigns unusable byte spans, or cannot structurally distinguish one of the excluded executable constructs. The gate therefore compiles every exported and synchronized fixture with the pinned official package and maintains negative fixtures for scripts, expressions, directives, blocks, components and component CSS. A Svelte or grammar update is one isolated dependency commit that regenerates the foreign lockfile, runs the complete adapter corpus, and records any AST or diagnostic change before the pin moves.

Open questions

• Whether a future component-CSS profile should own one style block per mapped root or preserve user CSS through a selector-aware correspondence layer.
• Whether official compiler warnings should become hard failures or a separately versioned diagnostic baseline once the profile accepts more than generated source.
• Whether the community grammar will publish a stable compatibility and release policy; until then, its exact version and source commit remain provenance.