Fix link to file url behavior with trailingSlash (#14681)

Avoid trailing slashes on urls that look like files. The redirect for `trailingSlash: true` will now look like:

```
Redirects

┌ source: /:path*/:file.:ext/
├ destination: /:path*/:file.:ext
└ permanent: true

┌ source: /:path*/:notfile([^/.]+)
├ destination: /:path*/:notfile/
└ permanent: true
```

The default still looks like:

```
Redirects

┌ source: /:path+/
├ destination: /:path+
└ permanent: true
```
After this gets merged, I have a few optimizations planned on the normalization code that should reduce the client bundle a little and that consolidates the `trailingSlash` and `exportTrailingSlash` options

packages/next/lib/load-custom-routes.ts

@@ -346,17 +346,26 @@ export default async function loadCustomRoutes(
   ])
 
   redirects.unshift(
-    config.experimental.trailingSlash
-      ? {
-          source: '/:path+',
-          destination: '/:path+/',
-          permanent: true,
-        }
-      : {
-          source: '/:path+/',
-          destination: '/:path+',
-          permanent: true,
-        }
+    ...(config.experimental.trailingSlash
+      ? [
+          {
+            source: '/:path*/:file.:ext/',
+            destination: '/:path*/:file.:ext',
+            permanent: true,
+          },
+          {
+            source: '/:path*/:notfile([^/.]+)',
+            destination: '/:path*/:notfile/',
+            permanent: true,
+          },
+        ]
+      : [
+          {
+            source: '/:path+/',
+            destination: '/:path+',
+            permanent: true,
+          },
+        ])
   )
 
   return {

packages/next/next-server/lib/router/normalize-trailing-slash.ts

@@ -2,11 +2,19 @@ export function normalizeTrailingSlash(
   path: string,
   requireSlash?: boolean
 ): string {
-  if (path === '/') {
-    return path
-  } else if (path.endsWith('/')) {
-    return requireSlash ? path : path.slice(0, -1)
+  if (requireSlash) {
+    if (!path.endsWith('/') && !/\.[^/]+$/.test(path)) {
+      return path + '/'
+    } else if (/\.[^/]+\/$/.test(path)) {
+      return path.slice(0, -1)
+    } else {
+      return path
+    }
   } else {
-    return requireSlash ? path + '/' : path
+    if (path.endsWith('/') && path !== '/') {
+      return path.slice(0, -1)
+    } else {
+      return path
+    }
   }
 }

test/integration/trailing-slashes/test/index.test.js

@@ -118,6 +118,7 @@ function testWithoutTrailingSlash() {
   testShouldRedirect([
     ['/about/', '/about'],
     ['/catch-all/hello/world/', '/catch-all/hello/world'],
+    ['/catch-all/hello.world/', '/catch-all/hello.world'],
   ])
 
   testShouldResolve([
@@ -138,6 +139,8 @@ function testWithoutTrailingSlash() {
     ['/about/', '/about'],
     ['/about?hello=world', '/about?hello=world'],
     ['/about/?hello=world', '/about?hello=world'],
+    ['/catch-all/hello/', '/catch-all/hello'],
+    ['/catch-all/hello.world/', '/catch-all/hello.world'],
   ])
 }
 
@@ -145,6 +148,7 @@ function testWithTrailingSlash() {
   testShouldRedirect([
     ['/about', '/about/'],
     ['/catch-all/hello/world', '/catch-all/hello/world/'],
+    ['/catch-all/hello.world/', '/catch-all/hello.world'],
   ])
 
   testShouldResolve([
@@ -165,6 +169,8 @@ function testWithTrailingSlash() {
     ['/about/', '/about/'],
     ['/about?hello=world', '/about/?hello=world'],
     ['/about/?hello=world', '/about/?hello=world'],
+    ['/catch-all/hello/', '/catch-all/hello/'],
+    ['/catch-all/hello.world/', '/catch-all/hello.world'],
   ])
 }
 
@@ -273,8 +279,13 @@ describe('Trailing slashes', () => {
         expect.objectContaining({
           redirects: expect.arrayContaining([
             expect.objectContaining({
-              source: '/:path+',
-              destination: '/:path+/',
+              source: '/:path*/:file.:ext/',
+              destination: '/:path*/:file.:ext',
+              statusCode: 308,
+            }),
+            expect.objectContaining({
+              source: '/:path*/:notfile([^/.]+)',
+              destination: '/:path*/:notfile/',
               statusCode: 308,
             }),
           ]),