Optimize unchanged FiberRefs updates

[coyaSONG]

Type

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

Description

FiberRefs.updateAs cloned the complete locals map before unsafeUpdateAs could discover that the current fiber already held an equal value. Repeated unchanged FiberRef updates therefore remained O(R) in the number of unrelated refs.

This change computes the replacement stack first and returns the original FiberRefs when the same fiber's value is unchanged. Updates from a different fiber still append a history entry, and changed values still copy the map to preserve immutable snapshots.

A local 7-sample median microbenchmark with 500 populated refs and 20,000 unchanged updates measured the map-cloning baseline at 476.9 ms and the optimized path at 0.69 ms (about 691x faster).

Implementation and validation were assisted by OpenAI Codex. No independent human review is claimed.

Validation:

• corepack pnpm lint-fix
• corepack pnpm vitest packages/effect/test/FiberRefs.test.ts packages/effect/test/FiberRef.test.ts --run (40 tests)
• corepack pnpm check
• corepack pnpm build
• corepack pnpm docgen
• git diff --check

Related

• Related Issue Every FiberRef.set copies the fiber's entire FiberRefs Map — O(R) per mutation, even no-op mutations #6306

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Latest commit: bd05883

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

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