perf: O(n²)→O(n) Map lookups for legacy reactive statements

[ljodea]

Before submitting the PR, please make sure you do the following

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• Prefix your PR title with feat:, fix:, chore:, or docs:. (Using perf: as a compile-time complexity win; happy to retitle if preferred.)
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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

Summary

While ordering and emitting legacy $: reactive statements, the compiler treated an already-keyed Map<LabeledStatement, Statement> as an array:

const statement = [...state.legacy_reactive_statements].find(([n]) => n === node);
instance.body.push(statement[1]);

That is O(n²) in the number of $: statements per component (n), plus an O(n) temporary array per lookup. The map is already keyed by that node.

Fix (same public behavior):

1. Client + server transform: state.legacy_reactive_statements.get(node) then push(statement).
2. Related path in order_reactive_statements: replace [...values()].includes(declaration) with reactive_declarations.has(node) (1:1 node↔declaration; DFS "visited" intent).

Complexity: O(n²) → O(n) for lookup/membership over $: count.

Verification

• pnpm test runtime-legacy with reactive-focused filters — pass
• pnpm test runtime-legacy — 3304 passed
• pnpm test validator -t reactive — pass
• pnpm test server-side-rendering — pass
• pnpm test runtime-browser — 123 passed
• Full pnpm test — 7576+ passed

Follow-ups

Related complexity cleanups filed separately (not in this PR):

• perf: O(n²) membership in sort_const_tags topological sort #18599 — sort_const_tags O(n²) membership
• perf: O(k²) key dedup in spread_props ownKeys #18600 — spread_props ownKeys quadratic key dedup
• perf: use Set for rest-prop exclude lists (server, legacy, exclude_from_object) #18601 — Set for rest-prop exclude lists (server / legacy / exclude_from_object)

[pkg-svelte-dev]

Install the latest version of svelte from 2224715:

pnpm add https://pkg.svelte.dev/svelte/c/22247150c8ddf01cd4d2dd8baeb29363e4e84e28

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/svelte/pr/18602

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: 2224715

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

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

[ljodea]

Bench (Node v26.5.0). Lookup path. n = $: count per component.

n	[...map].find	map.get	ratio
1	280 ns	54 ns	0.19×
4	1066 ns	72 ns	0.07×
10	3873 ns	98 ns	0.03×
50	65 µs	522 ns	0.01×

Early-exit (values().includes vs map.has): same direction at every n.

[dummdidumm]

thank you!