The implementation ended up not merging the boundary resource concept…

… with hoistable state due to the very different nature in which these things need to hoist (one during task completion and the other during flush but only when flushing late boundaries). Given that I've gone back to resource specific naming rather than calling it BoundaryState.

packages/react-dom-bindings/src/server/ReactFizzConfigDOM.js

@@ -485,7 +485,6 @@ export function createRenderState(
 
     nonce,
     // like a module global for currently rendering boundary
-    boundaryState: null,
     stylesToHoist: false,
   };
 
@@ -2252,7 +2251,7 @@ function pushLink(
   props: Object,
   resumableState: ResumableState,
   renderState: RenderState,
-  boundaryState: null | BoundaryState,
+  boundaryResources: null | BoundaryResources,
   hoistableState: HoistableState,
   textEmbedded: boolean,
   insertionMode: InsertionMode,
@@ -2374,8 +2373,8 @@ function pushLink(
           // We add the newly created resource to our StyleQueue and if necessary
           // track the resource with the currently rendering boundary
           styleQueue.sheets.set(key, resource);
-          if (boundaryState) {
-            boundaryState.stylesheets.add(resource);
+          if (boundaryResources) {
+            boundaryResources.stylesheets.add(resource);
           }
         } else {
           // We need to track whether this boundary should wait on this resource or not.
@@ -2386,8 +2385,8 @@ function pushLink(
           if (styleQueue) {
             const resource = styleQueue.sheets.get(key);
             if (resource) {
-              if (boundaryState) {
-                boundaryState.stylesheets.add(resource);
+              if (boundaryResources) {
+                boundaryResources.stylesheets.add(resource);
               }
             }
           }
@@ -2454,7 +2453,7 @@ function pushStyle(
   props: Object,
   resumableState: ResumableState,
   renderState: RenderState,
-  boundaryState: null | BoundaryState,
+  boundaryResources: null | BoundaryResources,
   textEmbedded: boolean,
   insertionMode: InsertionMode,
   noscriptTagInScope: boolean,
@@ -2554,8 +2553,8 @@ function pushStyle(
       // it. However, it's possible when you resume that the style has already been emitted
       // and then it wouldn't be recreated in the RenderState and there's no need to track
       // it again since we should've hoisted it to the shell already.
-      if (boundaryState) {
-        boundaryState.styles.add(styleQueue);
+      if (boundaryResources) {
+        boundaryResources.styles.add(styleQueue);
       }
     }
 
@@ -3455,7 +3454,7 @@ export function pushStartInstance(
   props: Object,
   resumableState: ResumableState,
   renderState: RenderState,
-  boundaryState: null | BoundaryState,
+  boundaryResources: null | BoundaryResources,
   hoistableState: HoistableState,
   formatContext: FormatContext,
   textEmbedded: boolean,
@@ -3535,7 +3534,7 @@ export function pushStartInstance(
         props,
         resumableState,
         renderState,
-        boundaryState,
+        boundaryResources,
         hoistableState,
         textEmbedded,
         formatContext.insertionMode,
@@ -3559,7 +3558,7 @@ export function pushStartInstance(
         props,
         resumableState,
         renderState,
-        boundaryState,
+        boundaryResources,
         textEmbedded,
         formatContext.insertionMode,
         !!(formatContext.tagScope & NOSCRIPT_SCOPE),
@@ -4108,7 +4107,7 @@ export function writeCompletedBoundaryInstruction(
   resumableState: ResumableState,
   renderState: RenderState,
   id: number,
-  boundaryResources: BoundaryState,
+  boundaryResources: BoundaryResources,
 ): boolean {
   let requiresStyleInsertion;
   if (enableFloat) {
@@ -4439,7 +4438,7 @@ function hasStylesToHoist(stylesheet: StylesheetResource): boolean {
 
 export function writeResourcesForBoundary(
   destination: Destination,
-  boundaryResources: BoundaryState,
+  boundaryResources: BoundaryResources,
   renderState: RenderState,
 ): boolean {
   // Reset these on each invocation, they are only safe to read in this function
@@ -4739,7 +4738,7 @@ const arrayCloseBracket = stringToPrecomputedChunk(']');
 //  [["JS_escaped_string1", "JS_escaped_string2"]]
 function writeStyleResourceDependenciesInJS(
   destination: Destination,
-  boundaryResources: BoundaryState,
+  boundaryResources: BoundaryResources,
 ): void {
   writeChunk(destination, arrayFirstOpenBracket);
 
@@ -4932,7 +4931,7 @@ function writeStyleResourceAttributeInJS(
 //  [["JSON_escaped_string1", "JSON_escaped_string2"]]
 function writeStyleResourceDependenciesInAttr(
   destination: Destination,
-  boundaryResources: BoundaryState,
+  boundaryResources: BoundaryResources,
 ): void {
   writeChunk(destination, arrayFirstOpenBracket);
 
@@ -5198,7 +5197,7 @@ export function createHoistableState(): HoistableState {
   };
 }
 
-export type BoundaryState = {
+export type BoundaryResources = {
   // style dependencies
   styles: Set<StyleQueue>,
   stylesheets: Set<StylesheetResource>,
@@ -5211,7 +5210,7 @@ export type StyleQueue = {
   sheets: Map<string, StylesheetResource>,
 };
 
-export function createBoundaryState(): BoundaryState {
+export function createBoundaryResources(): BoundaryResources {
   return {
     styles: new Set(),
     stylesheets: new Set(),
@@ -6064,35 +6063,35 @@ function escapeStringForLinkHeaderQuotedParamValueContextReplacer(
   }
 }
 
-export function hoistHoistableState(
-  parentHoistableState: HoistableState,
-  hoistableState: HoistableState,
+export function hoistHoistables(
+  target: HoistableState,
+  source: HoistableState,
 ) {
-  parentHoistableState.charset.push(...hoistableState.charset);
-  parentHoistableState.viewport.push(...hoistableState.viewport);
-  parentHoistableState.chunks.push(...hoistableState.chunks);
+  target.charset.push(...source.charset);
+  target.viewport.push(...source.viewport);
+  target.chunks.push(...source.chunks);
 }
 
 function hoistStyleQueueDependency(
-  this: BoundaryState,
+  this: BoundaryResources,
   styleQueue: StyleQueue,
 ) {
   this.styles.add(styleQueue);
 }
 
 function hoistStylesheetDependency(
-  this: BoundaryState,
+  this: BoundaryResources,
   stylesheet: StylesheetResource,
 ) {
   this.stylesheets.add(stylesheet);
 }
 
-export function hoistBoundaryState(
-  targetState: BoundaryState,
-  sourceState: BoundaryState,
+export function hoistBoundaryResources(
+  target: BoundaryResources,
+  source: BoundaryResources,
 ): void {
-  sourceState.styles.forEach(hoistStyleQueueDependency, targetState);
-  sourceState.stylesheets.forEach(hoistStylesheetDependency, targetState);
+  source.styles.forEach(hoistStyleQueueDependency, target);
+  source.stylesheets.forEach(hoistStylesheetDependency, target);
 }
 
 // This function is called at various times depending on whether we are rendering

packages/react-dom-bindings/src/server/ReactFizzConfigDOMLegacy.js

@@ -127,7 +127,8 @@ export const doctypeChunk: PrecomputedChunk = stringToPrecomputedChunk('');
 
 export type {
   ResumableState,
-  BoundaryState,
+  HoistableState,
+  BoundaryResources,
   FormatContext,
 } from './ReactFizzConfigDOM';
 
@@ -152,13 +153,13 @@ export {
   writeCompletedRoot,
   createRootFormatContext,
   createResumableState,
-  createBoundaryState,
+  createBoundaryResources,
   createHoistableState,
   writePreamble,
   writeHoistables,
   writePostamble,
-  hoistBoundaryState,
-  hoistHoistableState,
+  hoistBoundaryResources,
+  hoistHoistables,
   prepareHostDispatcher,
   resetResumableState,
   completeResumableState,

packages/react-noop-renderer/src/ReactNoopServer.js

@@ -52,8 +52,8 @@ type Destination = {
 };
 
 type RenderState = null;
-type BoundaryState = null;
 type HoistableState = null;
+type BoundaryResources = null;
 
 const POP = Buffer.from('/', 'utf8');
 
@@ -266,15 +266,15 @@ const ReactNoopServer = ReactFizzServer({
   writeHoistables() {},
   writePostamble() {},
 
-  createBoundaryState(): BoundaryState {
+  createBoundaryResources(): BoundaryResources {
     return null;
   },
 
   createHoistableState(): HoistableState {
     return null;
   },
 
-  hoistHoistableState(
+  hoistHoistables(
     parentHoistableState: HoistableState,
     hoistableState: HoistableState,
   ) {},