[compiler] Fix bug with reassigning function param in destructuring #33624

josephsavona · 2025-06-23T23:14:39Z

Closes #33577, a bug with ExtractScopeDeclarationsFromDestructuring and codegen when a function param is reassigned.

Stack created with Sapling. Best reviewed with ReviewStack.

Substantially improves the last major known issue with the new inference model's implementation: inferring effects of function expressions. I knowingly used a really simple (dumb) approach in InferFunctionExpressionAliasingEffects but it worked surprisingly well on a ton of code. However, investigating during the sync I saw that we the algorithm was literally running out of memory, or crashing from arrays that exceeded the maximum capacity. We were accumluating data flow in a way that could lead to lists of data flow captures compounding on themselves and growing very large very quickly. Plus, we were incorrectly recording some data flow, leading to cases where we reported false positive "can't mutate frozen value" for example. So I went back to the drawing board. InferMutationAliasingRanges already builds up a data flow graph which it uses to figure out what values would be affected by mutations of other values, and update mutable ranges. Well, the key question that we really want to answer for inferring a function expression's aliasing effects is which values alias/capture where. Per the docs I wrote up, we only have to record such aliasing _if they are observable via mutations_. So, lightbulb: simulate mutations of the params, free variables, and return of the function expression and see which params/free-vars would be affected! That's what we do now, giving us precise information about which such values alias/capture where. When the "into" is a param/context-var we use Capture, iwhen the destination is the return we use Alias to be conservative. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33584). * #33626 * #33625 * #33624 * __->__ #33584

Substantially improves the last major known issue with the new inference model's implementation: inferring effects of function expressions. I knowingly used a really simple (dumb) approach in InferFunctionExpressionAliasingEffects but it worked surprisingly well on a ton of code. However, investigating during the sync I saw that we the algorithm was literally running out of memory, or crashing from arrays that exceeded the maximum capacity. We were accumluating data flow in a way that could lead to lists of data flow captures compounding on themselves and growing very large very quickly. Plus, we were incorrectly recording some data flow, leading to cases where we reported false positive "can't mutate frozen value" for example. So I went back to the drawing board. InferMutationAliasingRanges already builds up a data flow graph which it uses to figure out what values would be affected by mutations of other values, and update mutable ranges. Well, the key question that we really want to answer for inferring a function expression's aliasing effects is which values alias/capture where. Per the docs I wrote up, we only have to record such aliasing _if they are observable via mutations_. So, lightbulb: simulate mutations of the params, free variables, and return of the function expression and see which params/free-vars would be affected! That's what we do now, giving us precise information about which such values alias/capture where. When the "into" is a param/context-var we use Capture, iwhen the destination is the return we use Alias to be conservative. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33584). * #33626 * #33625 * #33624 * __->__ #33584 DiffTrain build for [94cf60b](94cf60b)

mofeiZ · 2025-06-25T16:29:31Z

...s/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/repro-reassign-props.expect.md

+  } else {
+    t1 = $[4];
+  }
+  [props, ref] = useIdentity(t1);


nice! I see, in the current output we have without a declaration of t2

[t2, ref] = useIdentity(t1);

Closes #33577, a bug with ExtractScopeDeclarationsFromDestructuring and codegen when a function param is reassigned.

mofeiZ

ahh makes sense, we were first missing that parameters are already declared in ExtractScopeDeclarationsFromDestructuring so we incorrectly added an scope declaration. Then after fixing that, we needed to make sure codegen doesn't emit declarations for function parameters

Small cosmetic win, found this when i was looking at some code internally with lots of cases that all share the same logic. Previously, all the but last one would have an empty block. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33625). * #33643 * #33642 * #33640 * __->__ #33625 * #33624

We now have `HIRFunction.returns: Place` as well as `returnType: Type`. I want to add additional return information, so as a first step i'm consolidating everything under an object at `HIRFunction.returns: {place: Place}`. We use the type of this place as the return type. Next step is to add more properties to this object to represent things like the return kind. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33640). * #33643 * #33642 * __->__ #33640 * #33625 * #33624

Small cosmetic win, found this when i was looking at some code internally with lots of cases that all share the same logic. Previously, all the but last one would have an empty block. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33625). * #33643 * #33642 * #33640 * __->__ #33625 * #33624 DiffTrain build for [e130c08](e130c08)

We now have `HIRFunction.returns: Place` as well as `returnType: Type`. I want to add additional return information, so as a first step i'm consolidating everything under an object at `HIRFunction.returns: {place: Place}`. We use the type of this place as the return type. Next step is to add more properties to this object to represent things like the return kind. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33640). * #33643 * #33642 * __->__ #33640 * #33625 * #33624 DiffTrain build for [123ff13](123ff13)

…33624) Closes #33577, a bug with ExtractScopeDeclarationsFromDestructuring and codegen when a function param is reassigned. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33624). * #33643 * #33642 * #33640 * #33625 * __->__ #33624 DiffTrain build for [9894c48](9894c48)

josephsavona mentioned this pull request Jun 23, 2025

[compiler] New inference repros/fixes #33584

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facebook-github-bot added the CLA Signed label Jun 23, 2025

github-actions bot added the React Core Team Opened by a member of the React Core Team label Jun 23, 2025

This was referenced Jun 23, 2025

[compiler] Avoid empty switch cases #33625

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[compiler] Alternate take on ref validation #33626

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josephsavona mentioned this pull request Jun 25, 2025

[compiler] Consolidate HIRFunction return information #33640

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mofeiZ reviewed Jun 25, 2025

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[compiler] Fix bug with reassigning function param in destructuring

440d8c2

Closes #33577, a bug with ExtractScopeDeclarationsFromDestructuring and codegen when a function param is reassigned.

josephsavona force-pushed the pr33624 branch from 81c8f65 to 440d8c2 Compare June 25, 2025 16:49

This was referenced Jun 25, 2025

[compiler] Propagate CreateFunction effects for functions that return functions #33642

Draft

[compiler] Repro for missed case of global mutation #33643

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mofeiZ approved these changes Jun 25, 2025

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josephsavona merged commit 9894c48 into main Jun 25, 2025
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This was referenced Jun 25, 2025

[compiler] Infer render helpers for additional validation #33647

Open

[compiler] InferEffects uses effects as value keys #33650

Open

vercel-release-bot mentioned this pull request Jun 26, 2025

Upgrade React from cee7939b-20250625 to 4db4b21c-20250626 vercel/next.js#80959

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[compiler] Fix bug with reassigning function param in destructuring #33624

[compiler] Fix bug with reassigning function param in destructuring #33624

josephsavona commented Jun 23, 2025 •

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[compiler] Fix bug with reassigning function param in destructuring #33624

[compiler] Fix bug with reassigning function param in destructuring #33624

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josephsavona commented Jun 23, 2025 •

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