Fix @apply and CSS functions inside imported files (#14576)

Part-of #14558

After handling `@import` of stylesheets in core, we can no longer gate
out features based on finding a specific sequence in the input CSS, as
this string will not contain contents from other stylesheets.

So, consider the following input CSS:

```css
@import "tailwindcsss";
@import "./styles.css";
```

We can't opt-out of the traversal to search for `@apply` based on it
because it, of course, does not contain the contents of `./styles.css`
yet.

---------

Co-authored-by: Adam Wathan <adam.wathan@gmail.com>

CHANGELOG.md

@@ -24,7 +24,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Use the right import base path when using the CLI to reading files from stdin ([#14522](https://github.com/tailwindlabs/tailwindcss/pull/14522))
 - Ensure that `@utility` is top-level and cannot be nested ([#14525](https://github.com/tailwindlabs/tailwindcss/pull/14525))
-- Editing imported CSS files should trigger a rebuild ([#14561](https://github.com/tailwindlabs/tailwindcss/pull/14561))
+- Ensure editing imported CSS files triggers a rebuild ([#14561](https://github.com/tailwindlabs/tailwindcss/pull/14561))
+- Ensure `@apply` and CSS functions work inside imported stylesheets ([#14576](https://github.com/tailwindlabs/tailwindcss/pull/14576))
 - _Experimental_: Improve codemod output, keep CSS after last Tailwind directive unlayered ([#14512](https://github.com/tailwindlabs/tailwindcss/pull/14512))
 - _Experimental_: Fix incorrect empty `layer()` at the end of `@import` at-rules when running codemods ([#14513](https://github.com/tailwindlabs/tailwindcss/pull/14513))
 - _Experimental_: Do not wrap comment nodes in `@layer` when running codemods ([#14517](https://github.com/tailwindlabs/tailwindcss/pull/14517))

packages/tailwindcss/src/css-functions.test.ts

@@ -901,3 +901,37 @@ describe('in JS config files', () => {
     `)
   })
 })
+
+test('replaces CSS theme() function with values inside imported stylesheets', async () => {
+  expect(
+    await compileCss(
+      css`
+        @theme {
+          --color-red-500: #f00;
+        }
+        @import './bar.css';
+      `,
+      [],
+      {
+        async loadStylesheet() {
+          return {
+            base: '/bar.css',
+            content: css`
+              .red {
+                color: theme(colors.red.500);
+              }
+            `,
+          }
+        },
+      },
+    ),
+  ).toMatchInlineSnapshot(`
+    ":root {
+      --color-red-500: red;
+    }
+
+    .red {
+      color: red;
+    }"
+  `)
+})

packages/tailwindcss/src/index.test.ts

@@ -254,6 +254,34 @@ describe('@apply', () => {
     `)
   })
 
+  it('should replace @apply with the correct result inside imported stylesheets', async () => {
+    expect(
+      await compileCss(
+        css`
+          @import './bar.css';
+          @tailwind utilities;
+        `,
+        [],
+        {
+          async loadStylesheet() {
+            return {
+              base: '/bar.css',
+              content: css`
+                .foo {
+                  @apply underline;
+                }
+              `,
+            }
+          },
+        },
+      ),
+    ).toMatchInlineSnapshot(`
+      ".foo {
+        text-decoration-line: underline;
+      }"
+    `)
+  })
+
   it('should @apply in order the utilities would be sorted in if they were used in HTML', async () => {
     expect(
       await compileCss(css`

packages/tailwindcss/src/index.ts

@@ -16,7 +16,7 @@ import { applyCompatibilityHooks } from './compat/apply-compat-hooks'
 import type { UserConfig } from './compat/config/types'
 import { type Plugin } from './compat/plugin-api'
 import { compileCandidates } from './compile'
-import { substituteFunctions, THEME_FUNCTION_INVOCATION } from './css-functions'
+import { substituteFunctions } from './css-functions'
 import * as CSS from './css-parser'
 import { buildDesignSystem, type DesignSystem } from './design-system'
 import { Theme, ThemeOptions } from './theme'
@@ -341,13 +341,9 @@ async function parseCss(
   }
 
   // Replace `@apply` rules with the actual utility classes.
-  if (css.includes('@apply')) {
-    substituteAtApply(ast, designSystem)
-  }
+  substituteAtApply(ast, designSystem)
 
-  if (css.includes(THEME_FUNCTION_INVOCATION)) {
-    substituteFunctions(ast, designSystem.resolveThemeValue)
-  }
+  substituteFunctions(ast, designSystem.resolveThemeValue)
 
   // Remove `@utility`, we couldn't replace it before yet because we had to
   // handle the nested `@apply` at-rules first.

packages/tailwindcss/src/test-utils/run.ts

@@ -1,8 +1,8 @@
 import { Features, transform } from 'lightningcss'
 import { compile } from '..'
 
-export async function compileCss(css: string, candidates: string[] = []) {
-  let { build } = await compile(css)
+export async function compileCss(css: string, candidates: string[] = [], options = {}) {
+  let { build } = await compile(css, options)
   return optimizeCss(build(candidates)).trim()
 }