Emit unowned-this closures for synchronous @JS bindings#20
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Synchronous `@JS` functions and property accessors never read the JS `this` (the receiver is the module's real `self`), yet the synthesized `setProperty` closure bound `this` as an owning `JavaScriptValue`, allocating it and forming/destroying its `weak` runtime reference on every call. They now bind through the unowned-`this` `setProperty` overload, typing the closure's first parameter `borrowing JavaScriptUnownedValue` to select it. The parameter list is parenthesized and fully typed because Swift rejects a type annotation on a shorthand closure parameter. Async functions keep the owning-`this` overload: there is no unowned-`this` async variant and the buffer escapes into the task.
The macro now emits sync `@JS` bindings with a fully-typed closure parameter list (`(this: borrowing JavaScriptUnownedValue, arguments: consuming JavaScriptValuesBuffer) in`) to select the unowned-`this` `setProperty` overload. The fixture expectations still carried the old `this, arguments in` shorthand, so every sync-binding expansion test was failing. Update them to match. Async fixtures keep the shorthand and the owning-`this` overload, so they are left untouched.
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Kudo
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Jun 17, 2026
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Why
Synchronous
@JSfunctions and property accessors never read the JSthis— the receiver is the module's realself, and the synthesized body callsself.<name>(...)directly. Yet the generatedsetPropertyclosure boundthisas an owningJavaScriptValue, which allocates the value and forms/destroys itsweakruntime reference on every call. Profiling the no-op@JScall floor showed this per-call churn was a meaningful slice of the Swift-side cost.How
Sync
@JSfunctions and propertyget/setaccessors now bind through the unowned-thissetPropertyoverload added in expo-modules-jsi, which handsthisto the closure as a borrowedJavaScriptUnownedValueinstead of an owningJavaScriptValue. The closure's first parameter is typedborrowing JavaScriptUnownedValueso overload resolution selects that (otherwise@_disfavoredOverload) overload. Because Swift rejects a type annotation on a shorthand{ [capture] name, name in }parameter, the synthesized parameter list is now parenthesized and fully typed.Async functions are unchanged: they keep the untyped shorthand and the owning-
thisoverload, since there is no unowned-thisasync variant and the arguments buffer escapes into the task anyway.Requires the unowned-
thiscreateFunction/setPropertyoverloads in expo-modules-jsi: expo/expo#46949.Test Plan
Existing macro expansion tests pass. End-to-end, built locally and verified against the Modules Core benchmarks: the synthesized
@JShost-function path drops the per-call owning-thisallocation andweak-runtime traffic, improving the no-op call floor ~28% and the argument cases ~15–18% on the iPhone simulator, and making@JSthe fastest host-function tier (overtaking@OptimizedFunction).