Evaluate theme functions in plugins (#14326)

This PR fixes a bug where CSS `theme()` functions were not evaluated
when present in rules added by plugins, using either `@plugin` or
registering a plugin in a JS config file.

For example, prior to this PR the `theme()` functions in this plugin
would make it into the final CSS without being evaluated:

```js
// ./my-plugin.js
export default plugin(({ addBase }) => {
  addBase({
    '.my-rule': {
      background: 'theme(colors.primary)',
      color: 'theme(colors.secondary)',
    },
  })
})
```

---------

Co-authored-by: Adam Wathan <4323180+adamwathan@users.noreply.github.com>

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure content globs defined in `@config` files are relative to that file ([#14314](https://github.com/tailwindlabs/tailwindcss/pull/14314))
 - Ensure CSS `theme()` functions are evaluated in media query ranges with collapsed whitespace ((#14321)[https://github.com/tailwindlabs/tailwindcss/pull/14321])
 - Fix support for Nuxt projects in the Vite plugin (requires Nuxt 3.13.1+) ([#14319](https://github.com/tailwindlabs/tailwindcss/pull/14319))
+- Evaluate theme functions in plugins and JS config files ([#14326](https://github.com/tailwindlabs/tailwindcss/pull/14326))
 
 ## [4.0.0-alpha.21] - 2024-09-02
 

packages/tailwindcss/src/functions.test.ts

@@ -1,7 +1,9 @@
 import fs from 'node:fs/promises'
 import path from 'node:path'
 import { describe, expect, test } from 'vitest'
-import { compileCss } from './test-utils/run'
+import { compile } from '.'
+import plugin from './plugin'
+import { compileCss, optimizeCss } from './test-utils/run'
 
 const css = String.raw
 
@@ -618,3 +620,121 @@ describe('theme function', () => {
     })
   })
 })
+
+describe('in plugins', () => {
+  test('CSS theme functions in plugins are properly evaluated', async () => {
+    let compiled = await compile(
+      css`
+        @layer base, utilities;
+        @plugin "my-plugin";
+        @theme reference {
+          --color-red: red;
+          --color-orange: orange;
+          --color-blue: blue;
+          --color-pink: pink;
+        }
+        @layer utilities {
+          @tailwind utilities;
+        }
+      `,
+      {
+        async loadPlugin() {
+          return plugin(({ addBase, addUtilities }) => {
+            addBase({
+              '.my-base-rule': {
+                color: 'theme(colors.red)',
+                'outline-color': 'theme(colors.orange / 15%)',
+                'background-color': 'theme(--color-blue)',
+                'border-color': 'theme(--color-pink / 10%)',
+              },
+            })
+
+            addUtilities({
+              '.my-utility': {
+                color: 'theme(colors.red)',
+              },
+            })
+          })
+        },
+      },
+    )
+
+    expect(optimizeCss(compiled.build(['my-utility'])).trim()).toMatchInlineSnapshot(`
+      "@layer base {
+        .my-base-rule {
+          color: red;
+          background-color: #00f;
+          border-color: #ffc0cb1a;
+          outline-color: #ffa50026;
+        }
+      }
+
+      @layer utilities {
+        .my-utility {
+          color: red;
+        }
+      }"
+    `)
+  })
+})
+
+describe('in JS config files', () => {
+  test('CSS theme functions in config files are properly evaluated', async () => {
+    let compiled = await compile(
+      css`
+        @layer base, utilities;
+        @config "./my-config.js";
+        @theme reference {
+          --color-red: red;
+          --color-orange: orange;
+        }
+        @layer utilities {
+          @tailwind utilities;
+        }
+      `,
+      {
+        loadConfig: async () => ({
+          theme: {
+            extend: {
+              colors: {
+                primary: 'theme(colors.red)',
+                secondary: 'theme(--color-orange)',
+              },
+            },
+          },
+          plugins: [
+            plugin(({ addBase, addUtilities }) => {
+              addBase({
+                '.my-base-rule': {
+                  background: 'theme(colors.primary)',
+                  color: 'theme(colors.secondary)',
+                },
+              })
+
+              addUtilities({
+                '.my-utility': {
+                  color: 'theme(colors.red)',
+                },
+              })
+            }),
+          ],
+        }),
+      },
+    )
+
+    expect(optimizeCss(compiled.build(['my-utility'])).trim()).toMatchInlineSnapshot(`
+      "@layer base {
+        .my-base-rule {
+          color: orange;
+          background: red;
+        }
+      }
+
+      @layer utilities {
+        .my-utility {
+          color: red;
+        }
+      }"
+    `)
+  })
+})

packages/tailwindcss/src/index.ts

@@ -372,8 +372,11 @@ async function parseCss(
     substituteAtApply(ast, designSystem)
   }
 
-  // Replace `theme()` function calls with the actual theme variables.
-  if (css.includes(THEME_FUNCTION_INVOCATION)) {
+  // Replace `theme()` function calls with the actual theme variables. Plugins
+  // could register new rules that include functions, and JS config files could
+  // also contain functions or plugins that use functions so we need to evaluate
+  // functions if either of those are present.
+  if (plugins.length > 0 || configs.length > 0 || css.includes(THEME_FUNCTION_INVOCATION)) {
     substituteFunctions(ast, pluginApi)
   }
 
@@ -482,6 +485,10 @@ export async function compile(
           return compiledCss
         }
 
+        // Arbitrary values (`text-[theme(--color-red-500)]`) and arbitrary
+        // properties (`[--my-var:theme(--color-red-500)]`) can contain function
+        // calls so we need evaluate any functions we find there that weren't in
+        // the source CSS.
         substituteFunctions(newNodes, pluginApi)
 
         previousAstNodeCount = newNodes.length