Upgrade: Rewrite imports of relative files to use relative file paths

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - _Upgrade (experimental)_: Migrate `plugins` with options to CSS ([#14700](https://github.com/tailwindlabs/tailwindcss/pull/14700))
 - _Upgrade (experimental)_: Allow JS configuration files with `corePlugins` options to be migrated to CSS ([#14742](https://github.com/tailwindlabs/tailwindcss/pull/14742))
+- _Upgrade (experimental)_: Migrate `@import` statements for relative CSS files to use relative path syntax (e.g. `./file.css`) ([#14755](https://github.com/tailwindlabs/tailwindcss/pull/14755))
 
 ### Fixed
 
@@ -26,6 +27,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- No longer resolve filenames without a prefix as relative files (e.g. `@import 'styles.css'` instead of `@import './styles.css'`) ([#14755](https://github.com/tailwindlabs/tailwindcss/pull/14755))
 - _Upgrade (experimental)_: Don't create `@source` rules for `content` paths that are already covered by automatic source detection ([#14714](https://github.com/tailwindlabs/tailwindcss/pull/14714))
 
 ## [4.0.0-alpha.28] - 2024-10-17

packages/@tailwindcss-node/src/compile.ts

@@ -2,7 +2,7 @@ import EnhancedResolve from 'enhanced-resolve'
 import { createJiti, type Jiti } from 'jiti'
 import fs from 'node:fs'
 import fsPromises from 'node:fs/promises'
-import path, { dirname, extname } from 'node:path'
+import path, { dirname } from 'node:path'
 import { pathToFileURL } from 'node:url'
 import {
   __unstable__loadDesignSystem as ___unstable__loadDesignSystem,
@@ -120,21 +120,6 @@ async function resolveCssId(id: string, base: string): Promise<string | false |
     }
   }
 
-  // CSS imports that do not have a dir prefix are considered relative. Since
-  // the resolver does not account for this, we need to do a first pass with an
-  // assumed relative import by prefixing `./${path}`. We don't have to do this
-  // when the path starts with a `.` or when the path has no extension (at which
-  // case it's likely an npm package and not a relative stylesheet).
-  let skipRelativeCheck = extname(id) === '' || id.startsWith('.')
-
-  if (!skipRelativeCheck) {
-    try {
-      let dotResolved = await runResolver(cssResolver, `./${id}`, base)
-      if (!dotResolved) throw new Error()
-      return dotResolved
-    } catch {}
-  }
-
   return runResolver(cssResolver, id, base)
 }
 

packages/@tailwindcss-upgrade/src/codemods/fixtures/test.css

@@ -0,0 +1,3 @@
+.foo {
+  color: red;
+}

packages/@tailwindcss-upgrade/src/codemods/migrate-import.test.ts

@@ -0,0 +1,102 @@
+import { __unstable__loadDesignSystem } from '@tailwindcss/node'
+import dedent from 'dedent'
+import postcss from 'postcss'
+import { expect, it } from 'vitest'
+import type { UserConfig } from '../../../tailwindcss/src/compat/config/types'
+import { migrateImport } from './migrate-import'
+
+const css = dedent
+
+async function migrate(input: string, userConfig: UserConfig = {}) {
+  return postcss()
+    .use(
+      migrateImport({
+        designSystem: await __unstable__loadDesignSystem(`@import 'tailwindcss';`, {
+          base: __dirname,
+        }),
+        userConfig,
+      }),
+    )
+    .process(input, { from: expect.getState().testPath })
+    .then((result) => result.css)
+}
+
+it('prints relative file imports as relative paths', async () => {
+  expect(
+    await migrate(css`
+      @import 'fixtures/test';
+      @import 'fixtures/test.css';
+      @import './fixtures/test.css';
+      @import './fixtures/test';
+
+      @import 'fixtures/test' screen;
+      @import 'fixtures/test.css' screen;
+      @import './fixtures/test.css' screen;
+      @import './fixtures/test' screen;
+
+      @import 'fixtures/test' supports(display: grid);
+      @import 'fixtures/test.css' supports(display: grid);
+      @import './fixtures/test.css' supports(display: grid);
+      @import './fixtures/test' supports(display: grid);
+
+      @import 'fixtures/test' layer(utilities);
+      @import 'fixtures/test.css' layer(utilities);
+      @import './fixtures/test.css' layer(utilities);
+      @import './fixtures/test' layer(utilities);
+
+      @import 'fixtures/test' theme(inline);
+      @import 'fixtures/test.css' theme(inline);
+      @import './fixtures/test.css' theme(inline);
+      @import './fixtures/test' theme(inline);
+
+      @import 'fixtures/test' layer(utilities) supports(display: grid) screen and (min-width: 600px);
+      @import 'fixtures/test.css' layer(utilities) supports(display: grid) screen and
+        (min-width: 600px);
+      @import './fixtures/test.css' layer(utilities) supports(display: grid) screen and
+        (min-width: 600px);
+      @import './fixtures/test' layer(utilities) supports(display: grid) screen and
+        (min-width: 600px);
+
+      @import 'tailwindcss';
+      @import 'tailwindcss/theme.css';
+      @import 'tailwindcss/theme';
+    `),
+  ).toMatchInlineSnapshot(`
+    "@import './fixtures/test.css';
+    @import './fixtures/test.css';
+    @import './fixtures/test.css';
+    @import './fixtures/test.css';
+
+    @import './fixtures/test.css' screen;
+    @import './fixtures/test.css' screen;
+    @import './fixtures/test.css' screen;
+    @import './fixtures/test.css' screen;
+
+    @import './fixtures/test.css' supports(display: grid);
+    @import './fixtures/test.css' supports(display: grid);
+    @import './fixtures/test.css' supports(display: grid);
+    @import './fixtures/test.css' supports(display: grid);
+
+    @import './fixtures/test.css' layer(utilities);
+    @import './fixtures/test.css' layer(utilities);
+    @import './fixtures/test.css' layer(utilities);
+    @import './fixtures/test.css' layer(utilities);
+
+    @import './fixtures/test.css' theme(inline);
+    @import './fixtures/test.css' theme(inline);
+    @import './fixtures/test.css' theme(inline);
+    @import './fixtures/test.css' theme(inline);
+
+    @import './fixtures/test.css' layer(utilities) supports(display: grid) screen and (min-width: 600px);
+    @import './fixtures/test.css' layer(utilities) supports(display: grid) screen and
+      (min-width: 600px);
+    @import './fixtures/test.css' layer(utilities) supports(display: grid) screen and
+      (min-width: 600px);
+    @import './fixtures/test.css' layer(utilities) supports(display: grid) screen and
+      (min-width: 600px);
+
+    @import 'tailwindcss';
+    @import 'tailwindcss/theme.css';
+    @import 'tailwindcss/theme';"
+  `)
+})

packages/@tailwindcss-upgrade/src/codemods/migrate-import.ts

@@ -0,0 +1,45 @@
+import fs from 'node:fs/promises'
+import { dirname, resolve } from 'node:path'
+import { type Plugin, type Root } from 'postcss'
+import { parseImportParams } from '../../../tailwindcss/src/at-import'
+import { segment } from '../../../tailwindcss/src/utils/segment'
+import * as ValueParser from '../../../tailwindcss/src/value-parser'
+
+export function migrateImport(): Plugin {
+  async function migrate(root: Root) {
+    let file = root.source?.input.file
+    if (!file) return
+
+    let promises: Promise<void>[] = []
+    root.walkAtRules('import', (rule) => {
+      let [firstParam, ...rest] = segment(rule.params, ' ')
+
+      let params = parseImportParams(ValueParser.parse(firstParam))
+
+      let isRelative = params.uri[0] === '.'
+      let hasCssExtension = params.uri.endsWith('.css')
+
+      if (isRelative && hasCssExtension) {
+        return
+      }
+
+      let fullPath = resolve(dirname(file), params.uri)
+      if (!hasCssExtension) fullPath += '.css'
+
+      promises.push(
+        fs.stat(fullPath).then(() => {
+          let ext = hasCssExtension ? '' : '.css'
+          let path = isRelative ? params.uri : `./${params.uri}`
+          rule.params = [`'${path}${ext}'`, ...rest].join(' ')
+        }),
+      )
+    })
+
+    await Promise.allSettled(promises)
+  }
+
+  return {
+    postcssPlugin: '@tailwindcss/upgrade/migrate-import',
+    OnceExit: migrate,
+  }
+}

packages/tailwindcss/src/at-import.ts

@@ -67,7 +67,7 @@ export async function substituteAtImports(
 // `postcss-import` <https://github.com/postcss/postcss-import>
 // Copyright (c) 2014 Maxime Thirouin, Jason Campbell & Kevin Mårtensson
 // Released under the MIT License.
-function parseImportParams(params: ValueParser.ValueAstNode[]) {
+export function parseImportParams(params: ValueParser.ValueAstNode[]) {
   let uri
   let layer: string | null = null
   let media: string | null = null