[compiler] @enablePreserveExistingMemoizationGuarantees on by default
#34689
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) The `@enablePreserveExistingMemoizationGuarantees` mode can still fail to preserve manual memoization due to mismtached dependencies. Specifically, where the user's dependencies are more precise than the compiler infers bc the compiler is being conservative about what might be nullable. In this mode though we're intentionally using information from the manual memoization and can also rely on the deps as a signal for what's non-nullable. The idea of the PR is that we treat manual memo deps just like other inferred-as-non-nullable objects during PropagateScopeDeps. We're careful to not treat the full path as non-nullable, only up to the last property index. So `x.y.z` as a manual dep treats `x` and `x.y` as non-nullable, allowing us to preserve a conditional dependency on `x.y.z`. Optionals within manual dependencies are a bit trickier and aren't handled yet, but hopefully that's less common and something we can improve in a follow-up. Not handling them just means that developers may hit false positives on validating existing memoization if they use optional chains in manual dependencies. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34503). * #34689 * __->__ #34503
This enables `@enablePreserveExistingMemoizationGuarantees` by default. As of the previous PR (#34503), this mode now enables the following behaviors: - Treating variables referenced within a `useMemo()` or `useCallback()` as "frozen" (immutable) as of the start of the call. Ie, the compiler will assume that the values you reference are not mutated by the body of the useMemo, not are they mutated later. Directly modifying them (eg `var.property = true`) will be an error. - Similarly, the results of the useMemo/useCallback are treated as frozen (immutable) after the call. These two rules match the behavior for other hooks: this means that developers will see similar behavior to swapping out `useMemo()` for a custom `useMyMemo()` wrapper/alias. Additionally, as of #34503 the compiler uses information from the manual dependencies to know which variables are non-nullable. Even if a useMemo block conditionally accesses a nested property — `if (cond) { log(x.y.z) }` — where the compiler would not usually know that `x` is non-nullable, if the user specifies `x.y.z` as a manual dependency then the compiler knows that `x` and `x.y` are non-nullable and can infer a more precise dependency. Finally, this mode also ensures that we always memoize function calls that return primitives. See #34343 for more details. For now, I've explicitly opted out of this feature in all test fixtures where the behavior changed.
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) The `@enablePreserveExistingMemoizationGuarantees` mode can still fail to preserve manual memoization due to mismtached dependencies. Specifically, where the user's dependencies are more precise than the compiler infers bc the compiler is being conservative about what might be nullable. In this mode though we're intentionally using information from the manual memoization and can also rely on the deps as a signal for what's non-nullable. The idea of the PR is that we treat manual memo deps just like other inferred-as-non-nullable objects during PropagateScopeDeps. We're careful to not treat the full path as non-nullable, only up to the last property index. So `x.y.z` as a manual dep treats `x` and `x.y` as non-nullable, allowing us to preserve a conditional dependency on `x.y.z`. Optionals within manual dependencies are a bit trickier and aren't handled yet, but hopefully that's less common and something we can improve in a follow-up. Not handling them just means that developers may hit false positives on validating existing memoization if they use optional chains in manual dependencies. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34503). * #34689 * __->__ #34503 DiffTrain build for [57d5a59](57d5a59)
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) The `@enablePreserveExistingMemoizationGuarantees` mode can still fail to preserve manual memoization due to mismtached dependencies. Specifically, where the user's dependencies are more precise than the compiler infers bc the compiler is being conservative about what might be nullable. In this mode though we're intentionally using information from the manual memoization and can also rely on the deps as a signal for what's non-nullable. The idea of the PR is that we treat manual memo deps just like other inferred-as-non-nullable objects during PropagateScopeDeps. We're careful to not treat the full path as non-nullable, only up to the last property index. So `x.y.z` as a manual dep treats `x` and `x.y` as non-nullable, allowing us to preserve a conditional dependency on `x.y.z`. Optionals within manual dependencies are a bit trickier and aren't handled yet, but hopefully that's less common and something we can improve in a follow-up. Not handling them just means that developers may hit false positives on validating existing memoization if they use optional chains in manual dependencies. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34503). * #34689 * __->__ #34503 DiffTrain build for [57d5a59](57d5a59)
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…lt (#34689) This enables `@enablePreserveExistingMemoizationGuarantees` by default. As of the previous PR (#34503), this mode now enables the following behaviors: - Treating variables referenced within a `useMemo()` or `useCallback()` as "frozen" (immutable) as of the start of the call. Ie, the compiler will assume that the values you reference are not mutated by the body of the useMemo, not are they mutated later. Directly modifying them (eg `var.property = true`) will be an error. - Similarly, the results of the useMemo/useCallback are treated as frozen (immutable) after the call. These two rules match the behavior for other hooks: this means that developers will see similar behavior to swapping out `useMemo()` for a custom `useMyMemo()` wrapper/alias. Additionally, as of #34503 the compiler uses information from the manual dependencies to know which variables are non-nullable. Even if a useMemo block conditionally accesses a nested property — `if (cond) { log(x.y.z) }` — where the compiler would not usually know that `x` is non-nullable, if the user specifies `x.y.z` as a manual dependency then the compiler knows that `x` and `x.y` are non-nullable and can infer a more precise dependency. Finally, this mode also ensures that we always memoize function calls that return primitives. See #34343 for more details. For now, I've explicitly opted out of this feature in all test fixtures where the behavior changed. DiffTrain build for [70b52be](70b52be)
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…lt (#34689) This enables `@enablePreserveExistingMemoizationGuarantees` by default. As of the previous PR (#34503), this mode now enables the following behaviors: - Treating variables referenced within a `useMemo()` or `useCallback()` as "frozen" (immutable) as of the start of the call. Ie, the compiler will assume that the values you reference are not mutated by the body of the useMemo, not are they mutated later. Directly modifying them (eg `var.property = true`) will be an error. - Similarly, the results of the useMemo/useCallback are treated as frozen (immutable) after the call. These two rules match the behavior for other hooks: this means that developers will see similar behavior to swapping out `useMemo()` for a custom `useMyMemo()` wrapper/alias. Additionally, as of #34503 the compiler uses information from the manual dependencies to know which variables are non-nullable. Even if a useMemo block conditionally accesses a nested property — `if (cond) { log(x.y.z) }` — where the compiler would not usually know that `x` is non-nullable, if the user specifies `x.y.z` as a manual dependency then the compiler knows that `x` and `x.y` are non-nullable and can infer a more precise dependency. Finally, this mode also ensures that we always memoize function calls that return primitives. See #34343 for more details. For now, I've explicitly opted out of this feature in all test fixtures where the behavior changed. DiffTrain build for [70b52be](70b52be)
This was referenced Oct 3, 2025
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This enables
@enablePreserveExistingMemoizationGuaranteesby default. As of the previous PR (#34503), this mode now enables the following behaviors:useMemo()oruseCallback()as "frozen" (immutable) as of the start of the call. Ie, the compiler will assume that the values you reference are not mutated by the body of the useMemo, not are they mutated later. Directly modifying them (egvar.property = true) will be an error.These two rules match the behavior for other hooks: this means that developers will see similar behavior to swapping out
useMemo()for a customuseMyMemo()wrapper/alias.Additionally, as of #34503 the compiler uses information from the manual dependencies to know which variables are non-nullable. Even if a useMemo block conditionally accesses a nested property —
if (cond) { log(x.y.z) }— where the compiler would not usually know thatxis non-nullable, if the user specifiesx.y.zas a manual dependency then the compiler knows thatxandx.yare non-nullable and can infer a more precise dependency.Finally, this mode also ensures that we always memoize function calls that return primitives. See #34343 for more details.
For now, I've explicitly opted out of this feature in all test fixtures where the behavior changed.