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[compiler] Validate against locals being reassigned after render
Adds a pass which validates that local variables are not reassigned by functions which may be called after render. This is a straightforward forward data-flow analysis, where we: 1. Build up a mapping of context variables in the outer component/hook 2. Find ObjectMethod/FunctionExpressions which may reassign those context variables 3. Propagate aliases of those functions via StoreLocal/LoadLocal 4. Disallow passing those functions with a Freeze effect. This includes JSX arguments, hook arguments, hook return types, etc. Conceptually, a function that reassigns a local is inherently mutable. Frozen functions must be side-effect free, so these two categories are incompatible and we can use the freeze effect to find all instances of where such functions are disallowed rather than special-casing eg hook calls and JSX. ghstack-source-id: c2b22e3d62a1ab490a6a2150e28b934b9dc8676b Pull Request resolved: #30107
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...ages/babel-plugin-react-compiler/src/Validation/ValidateLocalsNotReassignedAfterRender.ts
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,146 @@ | ||
| /** | ||
| * Copyright (c) Meta Platforms, Inc. and affiliates. | ||
| * | ||
| * This source code is licensed under the MIT license found in the | ||
| * LICENSE file in the root directory of this source tree. | ||
| */ | ||
|
|
||
| import { CompilerError, Effect } from ".."; | ||
| import { HIRFunction, IdentifierId, Place } from "../HIR"; | ||
| import { | ||
| eachInstructionValueOperand, | ||
| eachTerminalOperand, | ||
| } from "../HIR/visitors"; | ||
|
|
||
| /** | ||
| * Validates that local variables cannot be reassigned after render. | ||
| * This prevents a category of bugs in which a closure captures a | ||
| * binding from one render but does not update | ||
| */ | ||
| export function validateLocalsNotReassignedAfterRender(fn: HIRFunction): void { | ||
| const contextVariables = new Set<IdentifierId>(); | ||
| const reassignment = getContextReassignment(fn, contextVariables, false); | ||
| if (reassignment !== null) { | ||
| CompilerError.throwInvalidReact({ | ||
| reason: | ||
| "Reassigning a variable after render has completed can cause inconsistent behavior on subsequent renders. Consider using state instead", | ||
| description: | ||
| reassignment.identifier.name !== null && | ||
| reassignment.identifier.name.kind === "named" | ||
| ? `Variable \`${reassignment.identifier.name.value}\` cannot be reassigned after render` | ||
| : "", | ||
| loc: reassignment.loc, | ||
| }); | ||
| } | ||
| } | ||
|
|
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| function getContextReassignment( | ||
| fn: HIRFunction, | ||
| contextVariables: Set<IdentifierId>, | ||
| isFunctionExpression: boolean | ||
| ): Place | null { | ||
| const reassigningFunctions = new Map<IdentifierId, Place>(); | ||
| for (const [, block] of fn.body.blocks) { | ||
| for (const instr of block.instructions) { | ||
| const { lvalue, value } = instr; | ||
| switch (value.kind) { | ||
| case "FunctionExpression": | ||
| case "ObjectMethod": { | ||
| let reassignment = getContextReassignment( | ||
| value.loweredFunc.func, | ||
| contextVariables, | ||
| true | ||
| ); | ||
| if (reassignment === null) { | ||
| // If the function itself doesn't reassign, does one of its dependencies? | ||
| for (const operand of eachInstructionValueOperand(value)) { | ||
| const reassignmentFromOperand = reassigningFunctions.get( | ||
| operand.identifier.id | ||
| ); | ||
| if (reassignmentFromOperand !== undefined) { | ||
| reassignment = reassignmentFromOperand; | ||
| break; | ||
| } | ||
| } | ||
| } | ||
| // if the function or its depends reassign, propagate that fact on the lvalue | ||
| if (reassignment !== null) { | ||
| reassigningFunctions.set(lvalue.identifier.id, reassignment); | ||
| } | ||
| break; | ||
| } | ||
| case "StoreLocal": { | ||
| const reassignment = reassigningFunctions.get( | ||
| value.value.identifier.id | ||
| ); | ||
| if (reassignment !== undefined) { | ||
| reassigningFunctions.set( | ||
| value.lvalue.place.identifier.id, | ||
| reassignment | ||
| ); | ||
| reassigningFunctions.set(lvalue.identifier.id, reassignment); | ||
| } | ||
| break; | ||
| } | ||
| case "LoadLocal": { | ||
| const reassignment = reassigningFunctions.get( | ||
| value.place.identifier.id | ||
| ); | ||
| if (reassignment !== undefined) { | ||
| reassigningFunctions.set(lvalue.identifier.id, reassignment); | ||
| } | ||
| break; | ||
| } | ||
| case "DeclareContext": { | ||
| if (!isFunctionExpression) { | ||
| contextVariables.add(value.lvalue.place.identifier.id); | ||
| } | ||
| break; | ||
| } | ||
| case "StoreContext": { | ||
| if (isFunctionExpression) { | ||
| if (contextVariables.has(value.lvalue.place.identifier.id)) { | ||
| return value.lvalue.place; | ||
| } | ||
| } else { | ||
| /* | ||
| * We only track reassignments of variables defined in the outer | ||
| * component or hook. | ||
| */ | ||
| contextVariables.add(value.lvalue.place.identifier.id); | ||
| } | ||
| break; | ||
| } | ||
| default: { | ||
| for (const operand of eachInstructionValueOperand(value)) { | ||
| CompilerError.invariant(operand.effect !== Effect.Unknown, { | ||
| reason: `Expected effects to be inferred prior to ValidateLocalsNotReassignedAfterRender`, | ||
| loc: operand.loc, | ||
| }); | ||
| const reassignment = reassigningFunctions.get( | ||
| operand.identifier.id | ||
| ); | ||
| if ( | ||
| reassignment !== undefined && | ||
| operand.effect === Effect.Freeze | ||
| ) { | ||
| /* | ||
| * Functions that reassign local variables are inherently mutable and are unsafe to pass | ||
| * to a place that expects a frozen value. Propagate the reassignment upward. | ||
| */ | ||
| return reassignment; | ||
| } | ||
| } | ||
| break; | ||
| } | ||
| } | ||
| } | ||
| for (const operand of eachTerminalOperand(block.terminal)) { | ||
| const reassignment = reassigningFunctions.get(operand.identifier.id); | ||
| if (reassignment !== undefined) { | ||
| return reassignment; | ||
| } | ||
| } | ||
| } | ||
| return null; | ||
| } |
27 changes: 27 additions & 0 deletions
27
..._/fixtures/compiler/error.invalid-reassign-local-in-hook-return-value.expect.md
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| @@ -0,0 +1,27 @@ | ||
|
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| ## Input | ||
|
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| ```javascript | ||
| function useFoo() { | ||
| let x = 0; | ||
| return (value) => { | ||
| x = value; | ||
| }; | ||
| } | ||
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| ``` | ||
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| ## Error | ||
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| ``` | ||
| 2 | let x = 0; | ||
| 3 | return (value) => { | ||
| > 4 | x = value; | ||
| | ^ InvalidReact: Reassigning a variable after render has completed can cause inconsistent behavior on subsequent renders. Consider using state instead. Variable `x` cannot be reassigned after render (4:4) | ||
| 5 | }; | ||
| 6 | } | ||
| 7 | | ||
| ``` | ||
6 changes: 6 additions & 0 deletions
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...iler/src/__tests__/fixtures/compiler/error.invalid-reassign-local-in-hook-return-value.js
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,6 @@ | ||
| function useFoo() { | ||
| let x = 0; | ||
| return (value) => { | ||
| x = value; | ||
| }; | ||
| } |
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