Re-enable legacyBehavior and passHref on Link (#84500)

In #83003, `legacyBehavior` and
`passHref` were removed from `<Link>`. However, this turned out to be a
bit premature—despite most usage being covered by the newer `<Link>`
APIs, there's some use cases we don't yet have good alternatives for.

We expect to solve this with an API that uses Fragment Refs once they're
available in React. For now, we're re-enabling these flags.

---------

Co-authored-by: JJ Kasper <[email protected]>

Author: samselikoff

crates/next-core/src/next_import_map.rs

@@ -983,6 +983,7 @@ async fn apply_vendored_react_aliases_server(
         // This is used in the server runtime to import React Server Components.
         alias.extend(fxindexmap! {
             rcstr!("next/navigation") => rcstr!("next/dist/api/navigation.react-server"),
+            rcstr!("next/link") => rcstr!("next/dist/client/app-dir/link.react-server"),
         });
     }
 
@@ -1012,6 +1013,7 @@ async fn rsc_aliases(
         // This is used in the server runtime to import React Server Components.
         alias.extend(fxindexmap! {
             rcstr!("next/navigation") => rcstr!("next/dist/api/navigation.react-server"),
+            rcstr!("next/link") => rcstr!("next/dist/client/app-dir/link.react-server"),
         });
     }
 

packages/next/errors.json

@@ -860,5 +860,6 @@
   "859": "An IO error occurred while attempting to create and acquire the lockfile",
   "860": "Client Max Body Size must be a valid number (bytes) or filesize format string (e.g., \"5mb\")",
   "861": "Client Max Body Size must be larger than 0 bytes",
-  "862": "Request body exceeded %s"
+  "862": "Request body exceeded %s",
+  "863": "\\`<Link legacyBehavior>\\` received a direct child that is either a Server Component, or JSX that was loaded with React.lazy(). This is not supported. Either remove legacyBehavior, or make the direct child a Client Component that renders the Link's \\`<a>\\` tag."
 }

packages/next/src/build/create-compiler-aliases.ts

@@ -238,6 +238,7 @@ export function createAppRouterApiAliases(isServerOnlyLayer: boolean) {
 
   if (isServerOnlyLayer) {
     mapping['navigation'] = 'next/dist/api/navigation.react-server'
+    mapping['link'] = 'next/dist/client/app-dir/link.react-server'
   }
 
   const aliasMap: Record<string, string> = {}

packages/next/src/client/app-dir/link.react-server.tsx

@@ -0,0 +1,31 @@
+import type { ComponentProps } from 'react'
+import ClientLinkComponent, { type LinkProps, useLinkStatus } from './link'
+
+export default function LinkComponent(
+  props: ComponentProps<typeof ClientLinkComponent>
+) {
+  const isLegacyBehavior = props.legacyBehavior
+  const childIsHostComponent =
+    typeof props.children === 'string' ||
+    typeof props.children === 'number' ||
+    typeof (props.children as any)?.type === 'string'
+  const childIsClientComponent =
+    (props.children as any)?.type?.$$typeof ===
+    Symbol.for('react.client.reference')
+
+  if (isLegacyBehavior && !childIsHostComponent && !childIsClientComponent) {
+    if ((props.children as any)?.type?.$$typeof === Symbol.for('react.lazy')) {
+      console.error(
+        `Using a Lazy Component as a direct child of \`<Link legacyBehavior>\` from a Server Component is not supported. If you need legacyBehavior, wrap your Lazy Component in a Client Component that renders the Link's \`<a>\` tag.`
+      )
+    } else {
+      console.error(
+        `Using a Server Component as a direct child of \`<Link legacyBehavior>\` is not supported. If you need legacyBehavior, wrap your Server Component in a Client Component that renders the Link's \`<a>\` tag.`
+      )
+    }
+  }
+
+  return <ClientLinkComponent {...props} />
+}
+
+export { type LinkProps, useLinkStatus }