Resolve theme keys when migrating JS config to CSS (#14675)

With the changes in #14672, it now becomes trivial to actually resolve
the config (while still retaining the reset behavior). This means that
we can now convert JS configs that use _functions_, e.g.:

```ts
import { type Config } from 'tailwindcss'

export default {
  theme: {
    extend: {
      colors: ({ colors }) => ({
        gray: colors.neutral,
      }),
    },
  },
} satisfies Config
```

This becomes:

```css
@import 'tailwindcss';

@theme {
  --color-gray-50: #fafafa;
  --color-gray-100: #f5f5f5;
  --color-gray-200: #e5e5e5;
  --color-gray-300: #d4d4d4;
  --color-gray-400: #a3a3a3;
  --color-gray-500: #737373;
  --color-gray-600: #525252;
  --color-gray-700: #404040;
  --color-gray-800: #262626;
  --color-gray-900: #171717;
  --color-gray-950: #0a0a0a;
}
```

---------

Co-authored-by: Adam Wathan <adam.wathan@gmail.com>
Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Support linear gradient angles as bare values ([#14707](https://github.com/tailwindlabs/tailwindcss/pull/14707))
 - Interpolate gradients in OKLCH by default ([#14708](https://github.com/tailwindlabs/tailwindcss/pull/14708))
 - _Upgrade (experimental)_: Migrate `theme(…)` calls to `var(…)` or to the modern `theme(…)` syntax ([#14664](https://github.com/tailwindlabs/tailwindcss/pull/14664), [#14695](https://github.com/tailwindlabs/tailwindcss/pull/14695))
+- _Upgrade (experimental)_: Support migrating JS configurations to CSS that contain functions inside the `theme` object ([#14675](https://github.com/tailwindlabs/tailwindcss/pull/14675))
 
 ### Fixed
 

integrations/upgrade/js-config.test.ts

@@ -193,7 +193,7 @@ test(
 )
 
 test(
-  'does not upgrade JS config files with functions in the theme config',
+  'upgrades JS config files with functions in the theme config',
   {
     fs: {
       'package.json': json`
@@ -230,24 +230,26 @@ test(
       "
       --- src/input.css ---
       @import 'tailwindcss';
-      @config '../tailwind.config.ts';
+
+      @theme {
+        --color-gray-50: oklch(0.985 0 none);
+        --color-gray-100: oklch(0.97 0 none);
+        --color-gray-200: oklch(0.922 0 none);
+        --color-gray-300: oklch(0.87 0 none);
+        --color-gray-400: oklch(0.708 0 none);
+        --color-gray-500: oklch(0.556 0 none);
+        --color-gray-600: oklch(0.439 0 none);
+        --color-gray-700: oklch(0.371 0 none);
+        --color-gray-800: oklch(0.269 0 none);
+        --color-gray-900: oklch(0.205 0 none);
+        --color-gray-950: oklch(0.145 0 none);
+      }
       "
     `)
 
     expect(await fs.dumpFiles('tailwind.config.ts')).toMatchInlineSnapshot(`
       "
-      --- tailwind.config.ts ---
-      import { type Config } from 'tailwindcss'
 
-      export default {
-        theme: {
-          extend: {
-            colors: ({ colors }) => ({
-              gray: colors.neutral,
-            }),
-          },
-        },
-      } satisfies Config
       "
     `)
   },

packages/@tailwindcss-upgrade/src/index.ts

@@ -87,7 +87,7 @@ async function run() {
   // Migrate JS config
 
   info('Migrating JavaScript configuration files using the provided configuration file.')
-  let jsConfigMigration = await migrateJsConfig(config.configFilePath, base)
+  let jsConfigMigration = await migrateJsConfig(config.designSystem, config.configFilePath, base)
 
   {
     // Stylesheet migrations

packages/@tailwindcss-upgrade/src/migrate-js-config.ts

@@ -10,10 +10,10 @@ import {
   themeableValues,
 } from '../../tailwindcss/src/compat/apply-config-to-theme'
 import { keyframesToRules } from '../../tailwindcss/src/compat/apply-keyframes-to-theme'
-import { deepMerge } from '../../tailwindcss/src/compat/config/deep-merge'
-import { mergeThemeExtension } from '../../tailwindcss/src/compat/config/resolve-config'
+import { resolveConfig, type ConfigFile } from '../../tailwindcss/src/compat/config/resolve-config'
 import type { ThemeConfig } from '../../tailwindcss/src/compat/config/types'
 import { darkModePlugin } from '../../tailwindcss/src/compat/dark-mode'
+import type { DesignSystem } from '../../tailwindcss/src/design-system'
 import { findStaticPlugins } from './utils/extract-static-plugins'
 import { info } from './utils/renderer'
 
@@ -29,6 +29,7 @@ export type JSConfigMigration =
   }
 
 export async function migrateJsConfig(
+  designSystem: DesignSystem,
   fullConfigPath: string,
   base: string,
 ): Promise<JSConfigMigration> {
@@ -57,7 +58,7 @@ export async function migrateJsConfig(
   }
 
   if ('theme' in unresolvedConfig) {
-    let themeConfig = await migrateTheme(unresolvedConfig as any)
+    let themeConfig = await migrateTheme(designSystem, unresolvedConfig, base)
     if (themeConfig) cssConfigs.push(themeConfig)
   }
 
@@ -75,33 +76,27 @@ export async function migrateJsConfig(
   }
 }
 
-async function migrateTheme(unresolvedConfig: Config & { theme: any }): Promise<string | null> {
-  let { extend: extendTheme, ...overwriteTheme } = unresolvedConfig.theme
-
-  let resetNamespaces = new Map<string, boolean>()
-  // Before we merge theme overrides with theme extensions, we capture all
-  // namespaces that need to be reset.
-  for (let [key, value] of themeableValues(overwriteTheme)) {
-    if (typeof value !== 'string' && typeof value !== 'number') {
-      continue
-    }
-
-    if (!resetNamespaces.has(key[0])) {
-      resetNamespaces.set(key[0], false)
-    }
+async function migrateTheme(
+  designSystem: DesignSystem,
+  unresolvedConfig: Config,
+  base: string,
+): Promise<string | null> {
+  // Resolve the config file without applying plugins and presets, as these are
+  // migrated to CSS separately.
+  let configToResolve: ConfigFile = {
+    base,
+    config: { ...unresolvedConfig, plugins: [], presets: undefined },
   }
+  let { resolvedConfig, replacedThemeKeys } = resolveConfig(designSystem, [configToResolve])
 
-  let themeValues: Record<string, Record<string, unknown>> = deepMerge(
-    {},
-    [overwriteTheme, extendTheme],
-    mergeThemeExtension,
+  let resetNamespaces = new Map<string, boolean>(
+    Array.from(replacedThemeKeys.entries()).map(([key]) => [key, false]),
   )
 
   let prevSectionKey = ''
-
   let css = `@theme {`
   let containsThemeKeys = false
-  for (let [key, value] of themeableValues(themeValues)) {
+  for (let [key, value] of themeableValues(resolvedConfig.theme)) {
     if (typeof value !== 'string' && typeof value !== 'number') {
       continue
     }
@@ -125,9 +120,9 @@ async function migrateTheme(unresolvedConfig: Config & { theme: any }): Promise<
     css += `  --${keyPathToCssProperty(key)}: ${value};\n`
   }
 
-  if ('keyframes' in themeValues) {
+  if ('keyframes' in resolvedConfig.theme) {
     containsThemeKeys = true
-    css += '\n' + keyframesToCss(themeValues.keyframes)
+    css += '\n' + keyframesToCss(resolvedConfig.theme.keyframes)
   }
 
   if (!containsThemeKeys) {
@@ -179,11 +174,6 @@ function migrateContent(
 
 // Applies heuristics to determine if we can attempt to migrate the config
 function canMigrateConfig(unresolvedConfig: Config, source: string): boolean {
-  // The file may not contain any functions
-  if (source.includes('function') || source.includes(' => ')) {
-    return false
-  }
-
   // The file may not contain non-serializable values
   function isSimpleValue(value: unknown): boolean {
     if (typeof value === 'function') return false
@@ -194,8 +184,8 @@ function canMigrateConfig(unresolvedConfig: Config, source: string): boolean {
     return ['string', 'number', 'boolean', 'undefined'].includes(typeof value)
   }
 
-  // Plugins are more complex, so we have a special heuristics for them.
-  let { plugins, ...remainder } = unresolvedConfig
+  // `theme` and `plugins` are handled separately and allowed to be more complex
+  let { plugins, theme, ...remainder } = unresolvedConfig
   if (!isSimpleValue(remainder)) {
     return false
   }
@@ -224,7 +214,6 @@ function canMigrateConfig(unresolvedConfig: Config, source: string): boolean {
 
   // Only migrate the config file if all top-level theme keys are allowed to be
   // migrated
-  let theme = unresolvedConfig.theme
   if (theme && typeof theme === 'object') {
     if (theme.extend && !onlyAllowedThemeValues(theme.extend)) return false
     let { extend: _extend, ...themeCopy } = theme
@@ -234,14 +223,18 @@ function canMigrateConfig(unresolvedConfig: Config, source: string): boolean {
   return true
 }
 
-const DEFAULT_THEME_KEYS = [
+const ALLOWED_THEME_KEYS = [
   ...Object.keys(defaultTheme),
   // Used by @tailwindcss/container-queries
   'containers',
 ]
+const BLOCKED_THEME_KEYS = ['supports', 'data', 'aria']
 function onlyAllowedThemeValues(theme: ThemeConfig): boolean {
   for (let key of Object.keys(theme)) {
-    if (!DEFAULT_THEME_KEYS.includes(key)) {
+    if (!ALLOWED_THEME_KEYS.includes(key)) {
+      return false
+    }
+    if (BLOCKED_THEME_KEYS.includes(key)) {
       return false
     }
   }

packages/tailwindcss/src/compat/apply-config-to-theme.ts

@@ -24,8 +24,8 @@ export function applyConfigToTheme(
   { theme }: ResolvedConfig,
   replacedThemeKeys: Set<string>,
 ) {
-  for (let resetThemeKey of replacedThemeKeys) {
-    let name = keyPathToCssProperty([resetThemeKey])
+  for (let replacedThemeKey of replacedThemeKeys) {
+    let name = keyPathToCssProperty([replacedThemeKey])
     if (!name) continue
 
     designSystem.theme.clearNamespace(`--${name}`, ThemeOptions.DEFAULT)