Separate the rendering of viewport from metadata

packages/next/src/lib/metadata/metadata-context.tsx

@@ -0,0 +1,46 @@
+import type { AppRenderContext } from '../../server/app-render/app-render'
+import type { MetadataContext } from './types/resolvers'
+import type { StaticGenerationStore } from '../../client/components/static-generation-async-storage.external'
+import { trackFallbackParamAccessed } from '../../server/app-render/dynamic-rendering'
+
+export function createMetadataContext(
+  pathname: string,
+  renderOpts: AppRenderContext['renderOpts']
+): MetadataContext {
+  return {
+    pathname,
+    trailingSlash: renderOpts.trailingSlash,
+    isStandaloneMode: renderOpts.nextConfigOutput === 'standalone',
+  }
+}
+
+export function createTrackedMetadataContext(
+  pathname: string,
+  renderOpts: AppRenderContext['renderOpts'],
+  staticGenerationStore: StaticGenerationStore | null
+): MetadataContext {
+  return {
+    // Use the regular metadata context, but we trap the pathname access.
+    ...createMetadataContext(pathname, renderOpts),
+
+    // Setup the trap around the pathname access so we can track when the
+    // pathname is accessed while resolving metadata which would indicate it's
+    // being used to resolve a relative URL. If that's the case, we don't want
+    // to provide it, and instead we should error.
+    get pathname() {
+      if (
+        staticGenerationStore &&
+        staticGenerationStore.isStaticGeneration &&
+        staticGenerationStore.fallbackRouteParams &&
+        staticGenerationStore.fallbackRouteParams.size > 0
+      ) {
+        trackFallbackParamAccessed(
+          staticGenerationStore,
+          'metadata relative url resolving'
+        )
+      }
+
+      return pathname
+    },
+  }
+}