[Experiment] Reuse memo cache after interruption (#28878)

Adds an experimental feature flag to the implementation of useMemoCache,
the internal cache used by the React Compiler (Forget).

When enabled, instead of treating the cache as copy-on-write, like we do
with fibers, we share the same cache instance across all render
attempts, even if the component is interrupted before it commits.

If an update is interrupted, either because it suspended or because of
another update, we can reuse the memoized computations from the previous
attempt. We can do this because the React Compiler performs atomic
writes to the memo cache, i.e. it will not record the inputs to a
memoization without also recording its output.

This gives us a form of "resuming" within components and hooks.

This only works when updating a component that already mounted. It has
no impact during initial render, because the memo cache is stored on the
fiber, and since we have not implemented resuming for fibers, it's
always a fresh memo cache, anyway.

However, this alone is pretty useful — it happens whenever you update
the UI with fresh data after a mutation/action, which is extremely
common in a Suspense-driven (e.g. RSC or Relay) app.

So the impact of this feature is faster data mutations/actions (when the
React Compiler is used).

DiffTrain build for [ea26e38](ea26e38)

compiled/facebook-www/JSXDEVRuntime-dev.classic.js

@@ -100,7 +100,8 @@ var enableDebugTracing = dynamicFeatureFlags.enableDebugTracing,
     enableTransitionTracing = dynamicFeatureFlags.enableTransitionTracing,
     enableRenderableContext = dynamicFeatureFlags.enableRenderableContext,
     enableRefAsProp = dynamicFeatureFlags.enableRefAsProp,
-    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses; // On WWW, false is used for a new modern build.
+    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses;
+ // On WWW, false is used for a new modern build.
 // because JSX is an extremely hot path.
 
 var disableStringRefs = false;

compiled/facebook-www/JSXDEVRuntime-dev.modern.js

@@ -100,7 +100,8 @@ var enableDebugTracing = dynamicFeatureFlags.enableDebugTracing,
     enableTransitionTracing = dynamicFeatureFlags.enableTransitionTracing,
     enableRenderableContext = dynamicFeatureFlags.enableRenderableContext,
     enableRefAsProp = dynamicFeatureFlags.enableRefAsProp,
-    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses; // On WWW, true is used for a new modern build.
+    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses;
+ // On WWW, true is used for a new modern build.
 // because JSX is an extremely hot path.
 
 var disableStringRefs = false;

compiled/facebook-www/REVISION

@@ -1 +1 @@
-446aa9a632670eb9a373d897309452e24c10c55e
+ea26e38e33bffeba1ecc42688d7e8cd7e0da1c02

compiled/facebook-www/React-dev.classic.js

@@ -25,7 +25,7 @@ if (
 ) {
   __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStart(new Error());
 }
-          var ReactVersion = '19.0.0-www-classic-a296fa2c';
+          var ReactVersion = '19.0.0-www-classic-8ac39151';
 
 // ATTENTION
 // When adding new symbols to this file,
@@ -429,7 +429,8 @@ var enableDebugTracing = dynamicFeatureFlags.enableDebugTracing,
     enableTransitionTracing = dynamicFeatureFlags.enableTransitionTracing,
     enableRenderableContext = dynamicFeatureFlags.enableRenderableContext,
     enableRefAsProp = dynamicFeatureFlags.enableRefAsProp,
-    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses; // On WWW, false is used for a new modern build.
+    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses;
+ // On WWW, false is used for a new modern build.
 // because JSX is an extremely hot path.
 
 var disableStringRefs = false;

compiled/facebook-www/React-dev.modern.js

@@ -25,7 +25,7 @@ if (
 ) {
   __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStart(new Error());
 }
-          var ReactVersion = '19.0.0-www-modern-fe830ed2';
+          var ReactVersion = '19.0.0-www-modern-f6026193';
 
 // ATTENTION
 // When adding new symbols to this file,
@@ -429,7 +429,8 @@ var enableDebugTracing = dynamicFeatureFlags.enableDebugTracing,
     enableTransitionTracing = dynamicFeatureFlags.enableTransitionTracing,
     enableRenderableContext = dynamicFeatureFlags.enableRenderableContext,
     enableRefAsProp = dynamicFeatureFlags.enableRefAsProp,
-    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses; // On WWW, true is used for a new modern build.
+    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses;
+ // On WWW, true is used for a new modern build.
 // because JSX is an extremely hot path.
 
 var disableStringRefs = false;

compiled/facebook-www/ReactART-dev.classic.js

@@ -63,7 +63,7 @@ function _assertThisInitialized(self) {
   return self;
 }
 
-var ReactVersion = '19.0.0-www-classic-94931f64';
+var ReactVersion = '19.0.0-www-classic-d14dfff2';
 
 var LegacyRoot = 0;
 var ConcurrentRoot = 1;
@@ -161,7 +161,8 @@ var enableDebugTracing = dynamicFeatureFlags.enableDebugTracing,
     enableInfiniteRenderLoopDetection = dynamicFeatureFlags.enableInfiniteRenderLoopDetection,
     enableRenderableContext = dynamicFeatureFlags.enableRenderableContext,
     enableRefAsProp = dynamicFeatureFlags.enableRefAsProp,
-    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses; // On WWW, false is used for a new modern build.
+    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses,
+    enableNoCloningMemoCache = dynamicFeatureFlags.enableNoCloningMemoCache; // On WWW, false is used for a new modern build.
 var enableProfilerTimer = true;
 var enableProfilerCommitHooks = true;
 var enableProfilerNestedUpdatePhase = true;
@@ -8403,7 +8404,30 @@ function useMemoCache(size) {
 
         if (currentMemoCache != null) {
           memoCache = {
-            data: currentMemoCache.data.map(function (array) {
+            // When enableNoCloningMemoCache is enabled, instead of treating the
+            // cache as copy-on-write, like we do with fibers, we share the same
+            // cache instance across all render attempts, even if the component
+            // is interrupted before it commits.
+            //
+            // If an update is interrupted, either because it suspended or
+            // because of another update, we can reuse the memoized computations
+            // from the previous attempt. We can do this because the React
+            // Compiler performs atomic writes to the memo cache, i.e. it will
+            // not record the inputs to a memoization without also recording its
+            // output.
+            //
+            // This gives us a form of "resuming" within components and hooks.
+            //
+            // This only works when updating a component that already mounted.
+            // It has no impact during initial render, because the memo cache is
+            // stored on the fiber, and since we have not implemented resuming
+            // for fibers, it's always a fresh memo cache, anyway.
+            //
+            // However, this alone is pretty useful — it happens whenever you
+            // update the UI with fresh data after a mutation/action, which is
+            // extremely common in a Suspense-driven (e.g. RSC or Relay) app.
+            data: enableNoCloningMemoCache ? currentMemoCache.data : // Clone the memo cache before each render (copy-on-write)
+            currentMemoCache.data.map(function (array) {
               return array.slice();
             }),
             index: 0

compiled/facebook-www/ReactART-dev.modern.js

@@ -63,7 +63,7 @@ function _assertThisInitialized(self) {
   return self;
 }
 
-var ReactVersion = '19.0.0-www-modern-c57fe631';
+var ReactVersion = '19.0.0-www-modern-41846fdd';
 
 var LegacyRoot = 0;
 var ConcurrentRoot = 1;
@@ -161,7 +161,8 @@ var enableDebugTracing = dynamicFeatureFlags.enableDebugTracing,
     enableInfiniteRenderLoopDetection = dynamicFeatureFlags.enableInfiniteRenderLoopDetection,
     enableRenderableContext = dynamicFeatureFlags.enableRenderableContext,
     enableRefAsProp = dynamicFeatureFlags.enableRefAsProp,
-    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses; // On WWW, true is used for a new modern build.
+    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses,
+    enableNoCloningMemoCache = dynamicFeatureFlags.enableNoCloningMemoCache; // On WWW, true is used for a new modern build.
 var enableProfilerTimer = true;
 var enableProfilerCommitHooks = true;
 var enableProfilerNestedUpdatePhase = true;
@@ -8192,7 +8193,30 @@ function useMemoCache(size) {
 
         if (currentMemoCache != null) {
           memoCache = {
-            data: currentMemoCache.data.map(function (array) {
+            // When enableNoCloningMemoCache is enabled, instead of treating the
+            // cache as copy-on-write, like we do with fibers, we share the same
+            // cache instance across all render attempts, even if the component
+            // is interrupted before it commits.
+            //
+            // If an update is interrupted, either because it suspended or
+            // because of another update, we can reuse the memoized computations
+            // from the previous attempt. We can do this because the React
+            // Compiler performs atomic writes to the memo cache, i.e. it will
+            // not record the inputs to a memoization without also recording its
+            // output.
+            //
+            // This gives us a form of "resuming" within components and hooks.
+            //
+            // This only works when updating a component that already mounted.
+            // It has no impact during initial render, because the memo cache is
+            // stored on the fiber, and since we have not implemented resuming
+            // for fibers, it's always a fresh memo cache, anyway.
+            //
+            // However, this alone is pretty useful — it happens whenever you
+            // update the UI with fresh data after a mutation/action, which is
+            // extremely common in a Suspense-driven (e.g. RSC or Relay) app.
+            data: enableNoCloningMemoCache ? currentMemoCache.data : // Clone the memo cache before each render (copy-on-write)
+            currentMemoCache.data.map(function (array) {
               return array.slice();
             }),
             index: 0

compiled/facebook-www/ReactART-prod.classic.js

@@ -86,6 +86,7 @@ var ReactSharedInternals =
   enableRefAsProp = dynamicFeatureFlags.enableRefAsProp,
   disableDefaultPropsExceptForClasses =
     dynamicFeatureFlags.disableDefaultPropsExceptForClasses,
+  enableNoCloningMemoCache = dynamicFeatureFlags.enableNoCloningMemoCache,
   REACT_ELEMENT_TYPE = Symbol.for("react.element"),
   REACT_PORTAL_TYPE = Symbol.for("react.portal"),
   REACT_FRAGMENT_TYPE = Symbol.for("react.fragment"),
@@ -2739,9 +2740,11 @@ function useMemoCache(size) {
         ((current = current.memoCache),
         null != current &&
           (memoCache = {
-            data: current.data.map(function (array) {
-              return array.slice();
-            }),
+            data: enableNoCloningMemoCache
+              ? current.data
+              : current.data.map(function (array) {
+                  return array.slice();
+                }),
             index: 0
           })));
   }
@@ -10619,7 +10622,7 @@ var slice = Array.prototype.slice,
       return null;
     },
     bundleType: 0,
-    version: "19.0.0-www-classic-e5fcf28b",
+    version: "19.0.0-www-classic-4cca10ad",
     rendererPackageName: "react-art"
   };
 var internals$jscomp$inline_1322 = {
@@ -10650,7 +10653,7 @@ var internals$jscomp$inline_1322 = {
   scheduleRoot: null,
   setRefreshHandler: null,
   getCurrentFiber: null,
-  reconcilerVersion: "19.0.0-www-classic-e5fcf28b"
+  reconcilerVersion: "19.0.0-www-classic-4cca10ad"
 };
 if ("undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__) {
   var hook$jscomp$inline_1323 = __REACT_DEVTOOLS_GLOBAL_HOOK__;

compiled/facebook-www/ReactART-prod.modern.js

@@ -86,6 +86,7 @@ var ReactSharedInternals =
   enableRefAsProp = dynamicFeatureFlags.enableRefAsProp,
   disableDefaultPropsExceptForClasses =
     dynamicFeatureFlags.disableDefaultPropsExceptForClasses,
+  enableNoCloningMemoCache = dynamicFeatureFlags.enableNoCloningMemoCache,
   REACT_ELEMENT_TYPE = Symbol.for("react.element"),
   REACT_PORTAL_TYPE = Symbol.for("react.portal"),
   REACT_FRAGMENT_TYPE = Symbol.for("react.fragment"),
@@ -2537,9 +2538,11 @@ function useMemoCache(size) {
         ((current = current.memoCache),
         null != current &&
           (memoCache = {
-            data: current.data.map(function (array) {
-              return array.slice();
-            }),
+            data: enableNoCloningMemoCache
+              ? current.data
+              : current.data.map(function (array) {
+                  return array.slice();
+                }),
             index: 0
           })));
   }
@@ -10098,7 +10101,7 @@ var slice = Array.prototype.slice,
       return null;
     },
     bundleType: 0,
-    version: "19.0.0-www-modern-20a4f736",
+    version: "19.0.0-www-modern-c36cdef3",
     rendererPackageName: "react-art"
   };
 var internals$jscomp$inline_1307 = {
@@ -10129,7 +10132,7 @@ var internals$jscomp$inline_1307 = {
   scheduleRoot: null,
   setRefreshHandler: null,
   getCurrentFiber: null,
-  reconcilerVersion: "19.0.0-www-modern-20a4f736"
+  reconcilerVersion: "19.0.0-www-modern-c36cdef3"
 };
 if ("undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__) {
   var hook$jscomp$inline_1308 = __REACT_DEVTOOLS_GLOBAL_HOOK__;

compiled/facebook-www/ReactDOM-dev.classic.js

@@ -126,7 +126,8 @@ var enableTrustedTypesIntegration = dynamicFeatureFlags.enableTrustedTypesIntegr
     enableRenderableContext = dynamicFeatureFlags.enableRenderableContext,
     enableRefAsProp = dynamicFeatureFlags.enableRefAsProp,
     favorSafetyOverHydrationPerf = dynamicFeatureFlags.favorSafetyOverHydrationPerf,
-    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses; // On WWW, false is used for a new modern build.
+    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses,
+    enableNoCloningMemoCache = dynamicFeatureFlags.enableNoCloningMemoCache; // On WWW, false is used for a new modern build.
 var enableProfilerTimer = true;
 var enableProfilerCommitHooks = true;
 var enableProfilerNestedUpdatePhase = true;
@@ -12043,7 +12044,30 @@ function useMemoCache(size) {
 
         if (currentMemoCache != null) {
           memoCache = {
-            data: currentMemoCache.data.map(function (array) {
+            // When enableNoCloningMemoCache is enabled, instead of treating the
+            // cache as copy-on-write, like we do with fibers, we share the same
+            // cache instance across all render attempts, even if the component
+            // is interrupted before it commits.
+            //
+            // If an update is interrupted, either because it suspended or
+            // because of another update, we can reuse the memoized computations
+            // from the previous attempt. We can do this because the React
+            // Compiler performs atomic writes to the memo cache, i.e. it will
+            // not record the inputs to a memoization without also recording its
+            // output.
+            //
+            // This gives us a form of "resuming" within components and hooks.
+            //
+            // This only works when updating a component that already mounted.
+            // It has no impact during initial render, because the memo cache is
+            // stored on the fiber, and since we have not implemented resuming
+            // for fibers, it's always a fresh memo cache, anyway.
+            //
+            // However, this alone is pretty useful — it happens whenever you
+            // update the UI with fresh data after a mutation/action, which is
+            // extremely common in a Suspense-driven (e.g. RSC or Relay) app.
+            data: enableNoCloningMemoCache ? currentMemoCache.data : // Clone the memo cache before each render (copy-on-write)
+            currentMemoCache.data.map(function (array) {
               return array.slice();
             }),
             index: 0
@@ -30808,7 +30832,7 @@ identifierPrefix, onUncaughtError, onCaughtError, onRecoverableError, transition
   return root;
 }
 
-var ReactVersion = '19.0.0-www-classic-bf2e83d8';
+var ReactVersion = '19.0.0-www-classic-3bfa22a0';
 
 function createPortal$1(children, containerInfo, // TODO: figure out the API for cross-renderer implementation.
 implementation) {

compiled/facebook-www/ReactDOM-dev.modern.js

@@ -103,7 +103,8 @@ var enableTrustedTypesIntegration = dynamicFeatureFlags.enableTrustedTypesIntegr
     enableRenderableContext = dynamicFeatureFlags.enableRenderableContext,
     enableRefAsProp = dynamicFeatureFlags.enableRefAsProp,
     favorSafetyOverHydrationPerf = dynamicFeatureFlags.favorSafetyOverHydrationPerf,
-    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses; // On WWW, true is used for a new modern build.
+    disableDefaultPropsExceptForClasses = dynamicFeatureFlags.disableDefaultPropsExceptForClasses,
+    enableNoCloningMemoCache = dynamicFeatureFlags.enableNoCloningMemoCache; // On WWW, true is used for a new modern build.
 var enableProfilerTimer = true;
 var enableProfilerCommitHooks = true;
 var enableProfilerNestedUpdatePhase = true;
@@ -18179,7 +18180,30 @@ function useMemoCache(size) {
 
         if (currentMemoCache != null) {
           memoCache = {
-            data: currentMemoCache.data.map(function (array) {
+            // When enableNoCloningMemoCache is enabled, instead of treating the
+            // cache as copy-on-write, like we do with fibers, we share the same
+            // cache instance across all render attempts, even if the component
+            // is interrupted before it commits.
+            //
+            // If an update is interrupted, either because it suspended or
+            // because of another update, we can reuse the memoized computations
+            // from the previous attempt. We can do this because the React
+            // Compiler performs atomic writes to the memo cache, i.e. it will
+            // not record the inputs to a memoization without also recording its
+            // output.
+            //
+            // This gives us a form of "resuming" within components and hooks.
+            //
+            // This only works when updating a component that already mounted.
+            // It has no impact during initial render, because the memo cache is
+            // stored on the fiber, and since we have not implemented resuming
+            // for fibers, it's always a fresh memo cache, anyway.
+            //
+            // However, this alone is pretty useful — it happens whenever you
+            // update the UI with fresh data after a mutation/action, which is
+            // extremely common in a Suspense-driven (e.g. RSC or Relay) app.
+            data: enableNoCloningMemoCache ? currentMemoCache.data : // Clone the memo cache before each render (copy-on-write)
+            currentMemoCache.data.map(function (array) {
               return array.slice();
             }),
             index: 0
@@ -38715,7 +38739,7 @@ identifierPrefix, onUncaughtError, onCaughtError, onRecoverableError, transition
   return root;
 }
 
-var ReactVersion = '19.0.0-www-modern-806cc996';
+var ReactVersion = '19.0.0-www-modern-b1784c6b';
 
 function createPortal$1(children, containerInfo, // TODO: figure out the API for cross-renderer implementation.
 implementation) {

compiled/facebook-www/ReactDOM-prod.classic.js

@@ -63,6 +63,7 @@ var ReactSharedInternals =
     dynamicFeatureFlags.favorSafetyOverHydrationPerf,
   disableDefaultPropsExceptForClasses =
     dynamicFeatureFlags.disableDefaultPropsExceptForClasses,
+  enableNoCloningMemoCache = dynamicFeatureFlags.enableNoCloningMemoCache,
   REACT_ELEMENT_TYPE = Symbol.for("react.element"),
   REACT_PORTAL_TYPE = Symbol.for("react.portal"),
   REACT_FRAGMENT_TYPE = Symbol.for("react.fragment"),
@@ -3448,9 +3449,11 @@ function useMemoCache(size) {
         ((current = current.memoCache),
         null != current &&
           (memoCache = {
-            data: current.data.map(function (array) {
-              return array.slice();
-            }),
+            data: enableNoCloningMemoCache
+              ? current.data
+              : current.data.map(function (array) {
+                  return array.slice();
+                }),
             index: 0
           })));
   }
@@ -17033,7 +17036,7 @@ Internals.Events = [
 var devToolsConfig$jscomp$inline_1729 = {
   findFiberByHostInstance: getClosestInstanceFromNode,
   bundleType: 0,
-  version: "19.0.0-www-classic-38c43cf6",
+  version: "19.0.0-www-classic-7b6a2c60",
   rendererPackageName: "react-dom"
 };
 var internals$jscomp$inline_2160 = {
@@ -17063,7 +17066,7 @@ var internals$jscomp$inline_2160 = {
   scheduleRoot: null,
   setRefreshHandler: null,
   getCurrentFiber: null,
-  reconcilerVersion: "19.0.0-www-classic-38c43cf6"
+  reconcilerVersion: "19.0.0-www-classic-7b6a2c60"
 };
 if ("undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__) {
   var hook$jscomp$inline_2161 = __REACT_DEVTOOLS_GLOBAL_HOOK__;
@@ -17529,4 +17532,4 @@ exports.useFormState = function (action, initialState, permalink) {
 exports.useFormStatus = function () {
   return ReactSharedInternals.H.useHostTransitionStatus();
 };
-exports.version = "19.0.0-www-classic-38c43cf6";
+exports.version = "19.0.0-www-classic-7b6a2c60";