fix: port adapter-vercel's prerendered redirect fixes to adapter-static

.changeset/large-pugs-clap.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-static': patch
+---
+
+fix: match adapter-vercel's prerendered redirect handling when deploying to Vercel

packages/adapter-static/platforms.js

@@ -19,7 +19,17 @@ function static_vercel_config(builder) {
 	/** @type {Record<string, { path: string }>} */
 	const overrides = {};
 
-	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: {
@@ -30,16 +40,24 @@ function static_vercel_config(builder) {
 	}
 
 	for (const [path, page] of builder.prerendered.pages) {
-		if (path.endsWith('/') && path !== '/') {
+		let overrides_path = path.slice(1);
+
+		if (path !== '/') {
+			/** @type {string | undefined} */
+			let counterpart_route = path + '/';
+
+			if (path.endsWith('/')) {
+				counterpart_route = path.slice(0, -1);
+				overrides_path = path.slice(1, -1);
+			}
+
 			prerendered_redirects.push(
-				{ src: path, dest: path.slice(0, -1) },
-				{ src: path.slice(0, -1), status: 308, headers: { Location: path } }
+				{ src: path, dest: counterpart_route },
+				{ src: counterpart_route, status: 308, headers: { Location: path } }
 			);
-
-			overrides[page.file] = { path: path.slice(1, -1) };
-		} else {
-			overrides[page.file] = { path: path.slice(1) };
 		}
+
+		overrides[page.file] = { path: overrides_path };
 	}
 
 	return {