[compiler] Revert error description #35525
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note: This implements the idea discussed in reactwg/react#389 (comment) Currently we create `Impure` effects for impure functions like `Date.now()` or `Math.random()`, and then throw if the effect is reachable during render. However, impurity is a property of the resulting value: if the value isn't accessed during render then it's okay: maybe you're console-logging the time while debugging (fine), or storing the impure value into a ref and only accessing it in an effect or event handler (totally ok). This PR updates to validate that impure values are not transitively consumed during render, building on the `Render` effect. The validation currently uses an algorithm very similar to that of InferReactivePlaces - building a set of known impure values, starting with `Impure` effects as the sources and then flowing outward via data flow and mutations. If a transitively impure value reaches a `Render` effect, it's an error. We record both the source of the impure value and where it gets rendered to make it easier to understand and fix errors: ``` Error: Cannot access impure value during render Calling an impure function can produce unstable results that update unpredictably when the component happens to re-render. (https://react.dev/reference/rules/components-and-hooks-must-be-pure#components-and-hooks-must-be-idempotent). error.invalid-impure-functions-in-render-via-render-helper.ts:10:25 8 | const array = makeArray(now); 9 | const hasDate = identity(array); > 10 | return <Bar hasDate={hasDate} />; | ^^^^^^^ Cannot access impure value during render 11 | }; 12 | return <Foo renderItem={renderItem} />; 13 | } error.invalid-impure-functions-in-render-via-render-helper.ts:6:14 4 | 5 | function Component() { > 6 | const now = Date.now(); | ^^^^^^^^^^ `Date.now` is an impure function. 7 | const renderItem = () => { 8 | const array = makeArray(now); 9 | const hasDate = identity(array); ``` Impure values are allowed to flow into refs, meaning that we now allow `useRef(Date.now())` or `useRef(localFunctionThatReturnsMathDotRandom())` which would have errored previously. The next PR reuses this improved impurity tracking to validate ref access in render as well.
…idation Updates to guard against *reading* refs during render via the improved validateNoImpureValuesInRender() pass. InferMutationAliasingEffects generates `Impure` effects for ref reads, and then this pass validates that those accesses don't flow into `Render` effects. We now call the impure value validation first so that it takes priority over validateNoRefAccessInRender - the latter still has all the existing logic for now, but we can dramatically simplify it in a follow-up PR so that it only has to validate against ref writes.
Rewrites ValidateNoRefAccessInRender to use a simpler single-pass algorithm that only validates ref mutations (reads are now handled separately by ValidateNoImpureValuesInRender). The new approach: - Tracks refs and ref values through the function - Identifies functions that mutate refs (directly or transitively) - Only errors when ref-mutating functions are called at the top level - Supports null-guard exception: mutations inside `if (ref.current == null)` are allowed for the initialization pattern This reduces ~700 lines of complex fixpoint iteration to ~400 lines of straightforward forward data-flow analysis.
This was referenced Jan 16, 2026
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