Rename exportTrailingSlash to existing trailingSlash property (#15768)

In terms of url rewriting, `trailingSlash` supports everything `exportTrailingSlash` does. We can just share all other code paths and deprecate `exportTrailingSlash`.

This PR shows a deprecation warning when `exportTrailingSlash` is used.

Also fixes https://github.com/vercel/next.js/issues/15774

We can update the tests now or later. (I kept them the same to prove it's non-breaking)

To do:
- [x] Do we want to keep this? => nope https://github.com/vercel/next.js/blob/841d4efc517a379d8947f491e3c6a1c4784bbccf/packages/next/next-server/lib/router/router.ts#L329
- [x] I kept `exportTrailingSlash` here. Do we want to rename that as well? => nope https://github.com/vercel/next.js/blob/2d9d649d492b77adeddea0242ab6b7fe73d18079/packages/next/build/index.ts#L959

packages/next/build/index.ts

@@ -740,7 +740,7 @@ export default async function build(
 
         return defaultMap
       },
-      exportTrailingSlash: false,
+      trailingSlash: false,
     }
 
     await exportApp(dir, exportOptions, exportConfig)
@@ -956,7 +956,7 @@ export default async function build(
     JSON.stringify({
       version: 1,
       hasExportPathMap: typeof config.exportPathMap === 'function',
-      exportTrailingSlash: config.exportTrailingSlash === true,
+      exportTrailingSlash: config.trailingSlash === true,
     }),
     'utf8'
   )

packages/next/build/webpack-config.ts

@@ -861,9 +861,6 @@ export default async function getBaseWebpackConfig(
         'process.env.__NEXT_TRAILING_SLASH': JSON.stringify(
           config.trailingSlash
         ),
-        'process.env.__NEXT_EXPORT_TRAILING_SLASH': JSON.stringify(
-          config.exportTrailingSlash
-        ),
         'process.env.__NEXT_MODERN_BUILD': JSON.stringify(
           config.experimental.modern && !dev
         ),

packages/next/client/link.tsx

@@ -237,20 +237,6 @@ function Link(props: React.PropsWithChildren<LinkProps>) {
     childProps.href = addBasePath(as)
   }
 
-  // Add the ending slash to the paths. So, we can serve the
-  // "<page>/index.html" directly.
-  if (process.env.__NEXT_EXPORT_TRAILING_SLASH) {
-    const rewriteUrlForNextExport = require('../next-server/lib/router/rewrite-url-for-export')
-      .rewriteUrlForNextExport
-    if (
-      childProps.href &&
-      typeof __NEXT_DATA__ !== 'undefined' &&
-      __NEXT_DATA__.nextExport
-    ) {
-      childProps.href = rewriteUrlForNextExport(childProps.href)
-    }
-  }
-
   return React.cloneElement(child, childProps)
 }
 

packages/next/export/index.ts

@@ -133,7 +133,7 @@ export default async function exportApp(
     )
   }
 
-  const subFolders = nextConfig.exportTrailingSlash
+  const subFolders = nextConfig.trailingSlash
   const isLikeServerless = nextConfig.target !== 'server'
 
   log(`> using build directory: ${distDir}`)

packages/next/next-server/lib/router/rewrite-url-for-export.ts

-export function rewriteUrlForNextExport(url: string): string {
-  const [pathname, hash] = url.split('#')
-  // tslint:disable-next-line
-  let [path, qs] = pathname.split('?')
-  if (path) {
-    path = path.replace(/\/$/, '')
-    // Append a trailing slash if this path does not have an extension
-    if (!/\.[^/]+\/?$/.test(path)) path += `/`
-  }
-  if (typeof qs === 'string') path += '?' + qs
-  if (typeof hash === 'string') path += '#' + hash
-  return path
-}

packages/next/next-server/lib/router/router.ts

@@ -325,17 +325,6 @@ export default class Router implements BaseRouter {
     }
   }
 
-  // @deprecated backwards compatibility even though it's a private method.
-  static _rewriteUrlForNextExport(url: string): string {
-    if (process.env.__NEXT_EXPORT_TRAILING_SLASH) {
-      const rewriteUrlForNextExport = require('./rewrite-url-for-export')
-        .rewriteUrlForNextExport
-      return rewriteUrlForNextExport(url)
-    } else {
-      return url
-    }
-  }
-
   onPopState = (e: PopStateEvent): void => {
     const state = e.state as HistoryState
 
@@ -453,17 +442,6 @@ export default class Router implements BaseRouter {
       performance.mark('routeChange')
     }
 
-    // Add the ending slash to the paths. So, we can serve the
-    // "<page>/index.html" directly for the SSR page.
-    if (process.env.__NEXT_EXPORT_TRAILING_SLASH) {
-      const rewriteUrlForNextExport = require('./rewrite-url-for-export')
-        .rewriteUrlForNextExport
-      // @ts-ignore this is temporarily global (attached to window)
-      if (__NEXT_DATA__.nextExport) {
-        as = rewriteUrlForNextExport(as)
-      }
-    }
-
     if (this._inFlightRoute) {
       this.abortComponentLoad(this._inFlightRoute)
     }

packages/next/next-server/server/config.ts

@@ -35,7 +35,6 @@ const defaultConfig: { [key: string]: any } = {
     canonicalBase: '',
   },
   basePath: '',
-  exportTrailingSlash: false,
   sassOptions: {},
   trailingSlash: false,
   experimental: {
@@ -73,6 +72,17 @@ const experimentalWarning = execOnce(() => {
 })
 
 function assignDefaults(userConfig: { [key: string]: any }) {
+  if (typeof userConfig.exportTrailingSlash !== 'undefined') {
+    console.warn(
+      chalk.yellow.bold('Warning: ') +
+        'The "exportTrailingSlash" option has been renamed to "trailingSlash". Please update your next.config.js.'
+    )
+    if (typeof userConfig.trailingSlash === 'undefined') {
+      userConfig.trailingSlash = userConfig.exportTrailingSlash
+    }
+    delete userConfig.exportTrailingSlash
+  }
+
   const config = Object.keys(userConfig).reduce<{ [key: string]: any }>(
     (currentConfig, key) => {
       const value = userConfig[key]