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Mofei Zhang
54e602d891
Array and argument spreads may mutate stateful iterables. Spread sites should have `ConditionallyMutate` effects (e.g. mutate if the ValueKind is mutable, otherwise read). See - [ecma spec (13.2.4.1 Runtime Semantics: ArrayAccumulation. SpreadElement : ... AssignmentExpression)](https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-runtime-semantics-arrayaccumulation). - [ecma spec 13.3.8.1 Runtime Semantics: ArgumentListEvaluation](https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-runtime-semantics-argumentlistevaluation) Note that - Object and JSX Attribute spreads do not evaluate iterables (srcs [mozilla](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax#description), [ecma](https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-runtime-semantics-propertydefinitionevaluation)) - An ideal mutability inference system could model known collections (i.e. Arrays or Sets) as a "mutated collection of non-mutable objects" (see `todo-granular-iterator-semantics`), but this is not what we do today. As such, an array / argument spread will always extend the range of built-in arrays, sets, etc - Due to HIR limitations, call expressions with argument spreads may cause unnecessary bailouts and/or scope merging when we know the call itself has `freeze`, `capture`, or `read` semantics (e.g. `useHook(...mutableValue)`) We can deal with this by rewriting these call instructions to (1) create an intermediate array to consume the iterator and (2) capture and spread the array at the callsite