WIP: fix: avoid repeated Acorn location scans

[jschaf]

NOTE: not ready for review yet. I'm using this to explore whether we should adjust Acorn's API in acornjs/acorn#1448.

Svelte parses JavaScript embedded in component markup by repeatedly starting Acorn at different offsets in the same source string. Acorn initializes each parser by scanning the source prefix to recover the starting line, so components with many expression or declaration tags accumulate approximately quadratic parsing work.

This is the behavior described upstream in acornjs/acorn#1448.

• Reuse Svelte's existing source locator to initialize Acorn's line and token position when parsing directly from the component source.
• Keep Acorn's existing constructor path for adjusted source strings used by destructured contexts and TypeScript annotations.
• Fall back to Acorn's existing path for bare CR, U+2028, and U+2029 line separators, whose location semantics differ from Svelte's locator.
• Add regression coverage for multiline expressions, declaration tags, LF, CRLF, bare CR, and Unicode line separators.
• Add a patch changeset for svelte.

This preserves absolute offsets, AST locations, comments, diagnostics, and source-map behavior while avoiding the repeated prefix scan for normal LF/CRLF component sources.

Performance

The patch shows 5-9% faster full compilation with no output differences for Immich, Open WebUI, and Skeleton.

Project	Corpus	Mode	Before	After	Median improvement
Immich	411 files / 1.40 MB	Parse	263.5 ms	243.5 ms	7.7% faster
Immich	411 files / 1.40 MB	Client compile	758.9 ms	727.3 ms	4.7% faster
Immich	411 files / 1.40 MB	Server compile	691.4 ms	644.2 ms	7.3% faster
Open WebUI	593 files / 3.36 MB	Parse	627.3 ms	492.3 ms	19.0% faster
Open WebUI	592 files / 3.36 MB	Client compile	1,751.3 ms	1,621.9 ms	8.4% faster
Open WebUI	592 files / 3.36 MB	Server compile	1,580.8 ms	1,450.3 ms	9.4% faster
Skeleton	686 files / 0.83 MB	Parse	141.1 ms	135.5 ms	5.0% faster
Skeleton	686 files / 0.83 MB	Client compile	429.8 ms	424.3 ms	2.3% faster
Skeleton	686 files / 0.83 MB	Server compile	378.3 ms	366.9 ms	3.0% faster

Large individual components benefited substantially:

• Open WebUI MessageInput.svelte: 44% faster parsing
• Open WebUI Documents.svelte: 52% faster
• Immich album page: 25% faster
• Immich MemoryViewer.svelte: 27% faster
• Skeleton typography controls: 34% faster

Measured on macOS arm64 with Node 26.3.1 and pnpm 10.4.0. Each result is the median of 9 measurements after 3 warmups.

Synthetic benchmark to show quadratic behavior

Expression tags	Parse before	Parse after	Doubling ratio before	Doubling ratio after
128	0.758 ms	0.646 ms	—	—
256	1.633 ms	1.102 ms	2.16×	1.71×
512	4.132 ms	2.136 ms	2.53×	1.94×
1,024	12.534 ms	3.885 ms	3.03×	1.82×
2,048	41.135 ms	6.695 ms	3.28×	1.72×
4,096	148.524 ms	11.479 ms	3.61×	1.71×

The fitted parse-time exponent drops from 1.53 to 0.84. At 4,096 expression tags, the parse phase is 12.9× faster.

[pkg-svelte-dev]

Install the latest version of svelte from d7acfb0:

pnpm add https://pkg.svelte.dev/svelte/c/d7acfb026d62655b59fffca01ef0ac8854fbe0dc

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/svelte/pr/18586

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: d7acfb0

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

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR