Improve error message when given invalid hosting configuration.

CHANGELOG.md

@@ -1 +1,2 @@
 - Allow configuration of the Cloud Function generated for full-stack web frameworks (#5504)
+- Improve error message during deploy when given invalid hosting rewrite rule (#5533)

src/deploy/hosting/convertConfig.ts

@@ -247,7 +247,14 @@ export async function convertConfig(
 
     // This line makes sure this function breaks if there is ever added a new
     // kind of rewrite and we haven't yet handled it.
-    assertExhaustive(rewrite);
+    try {
+      assertExhaustive(rewrite);
+    } catch (e: any) {
+      throw new FirebaseError(
+        "Invalid hosting rewrite config in firebase.json. " +
+          "A rewrite config must specify 'destination', 'function', 'dynamicLinks', or 'run'"
+      );
+    }
   });
 
   if (config.rewrites) {

src/functional.ts

@@ -88,7 +88,7 @@ export const zipIn =
 /** Used with type guards to guarantee that all cases have been covered. */
 export function assertExhaustive(val: never): never {
   // eslint-disable-next-line @typescript-eslint/restrict-template-expressions
-  throw new Error(`Never has a value (${val}). This should be impossible`);
+  throw new Error(`Never has a value (${val}).`);
 }
 
 /**