fix: disregard presence/absence of trailing slash in prerendered redirect (#12966)

* fix: disregard presence/absence of trailing slash in prerendered redirect

* handle edge case

* tweak comment placement

Author: Rich-Harris

.changeset/seven-kangaroos-talk.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+fix: disregard presence/absence of trailing slash in prerendered redirect

packages/adapter-vercel/index.js

@@ -444,7 +444,17 @@ function static_vercel_config(builder, config, dir) {
 	/** @type {import('./index.js').ImagesConfig | undefined} */
 	const images = config.images;
 
-	for (const [src, redirect] of builder.prerendered.redirects) {
+	for (let [src, redirect] of builder.prerendered.redirects) {
+		if (src.replace(/\/$/, '') === redirect.location.replace(/\/$/, '')) {
+			// ignore the extreme edge case of a `/foo` -> `/foo/` redirect,
+			// which would only arise if the response was generated by a
+			// `handle` hook or outside the app altogether (since you
+			// can't declaratively create both routes)
+		} else {
+			// redirect both `/foo` and `/foo/` to `redirect.location`
+			src = src.replace(/\/?$/, '/?');
+		}
+
 		prerendered_redirects.push({
 			src,
 			headers: {