Plugin API: Allow custom utilities to start with @ (#14793)

Closes #14791

Add support to the JS Plugin interop layer for utilities that _start
with_ `@`. This ensures no breaking when trying to load plugins that
contribute utilities like `@container` from
`@tailwindcss/container-queries` (even though the `@container` utility
is now part of core).

## Test Plan

Added the `@tailwindcss/container-queries` plugin to to the Vite
example:
![Screenshot 2024-10-25 at
11.18.19.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/0Y77ilPI2WoJfMLFiAEw/3761c0a8-8c54-42eb-a1fd-213c4215c024.png)

However, in order for the Vite example to load the extension, I also had
to apply the following patch:

![Screenshot 2024-10-25 at
11.18.54.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/0Y77ilPI2WoJfMLFiAEw/bd151684-ff7b-4805-b305-71ac0378c610.png)

I think this is related to our dev system though, the compiled plugin
file is going to be a flat file with no requires in our public release.

---------

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

CHANGELOG.md

@@ -39,6 +39,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure spacing utilities with no value (e.g. `px` or `translate-y`) don't generate CSS ([#14911](https://github.com/tailwindlabs/tailwindcss/pull/14911))
 - Don't override user-agent background color for input elements in Preflight ([#14913](https://github.com/tailwindlabs/tailwindcss/pull/14913))
 - Don't attempt to convert CSS variables (which should already be percentages) to percentages when used as opacity modifiers ([#14916](https://github.com/tailwindlabs/tailwindcss/pull/14916))
+- Ensure custom utilities registered with the plugin API can start with `@` ([#14793](https://github.com/tailwindlabs/tailwindcss/pull/14793))
 - _Upgrade (experimental)_: Install `@tailwindcss/postcss` next to `tailwindcss` ([#14830](https://github.com/tailwindlabs/tailwindcss/pull/14830))
 - _Upgrade (experimental)_: Remove whitespace around `,` separator when print arbitrary values ([#14838](https://github.com/tailwindlabs/tailwindcss/pull/14838))
 - _Upgrade (experimental)_: Fix crash during upgrade when content globs escape root of project ([#14896](https://github.com/tailwindlabs/tailwindcss/pull/14896))

packages/tailwindcss/src/compat/plugin-api.test.ts

@@ -2943,6 +2943,50 @@ describe('matchUtilities()', () => {
     ).toEqual('')
   })
 
+  test('custom functional utilities can start with @', async () => {
+    async function run(candidates: string[]) {
+      let compiled = await compile(
+        css`
+          @plugin "my-plugin";
+          @tailwind utilities;
+        `,
+
+        {
+          async loadModule(id, base) {
+            return {
+              base,
+              module: ({ matchUtilities }: PluginAPI) => {
+                matchUtilities(
+                  { '@w': (value) => ({ width: value }) },
+                  {
+                    values: {
+                      1: '1px',
+                    },
+                  },
+                )
+              },
+            }
+          },
+        },
+      )
+
+      return compiled.build(candidates)
+    }
+
+    expect(optimizeCss(await run(['@w-1','hover:@w-1'])).trim())
+      .toMatchInlineSnapshot(`
+        ".\\@w-1 {
+          width: 1px;
+        }
+
+        @media (hover: hover) {
+          .hover\\:\\@w-1:hover {
+            width: 1px;
+          }
+        }"
+      `)
+  })
+
   test('custom functional utilities can return an array of rules', async () => {
     let compiled = await compile(
       css`

packages/tailwindcss/src/compat/plugin-api.ts

@@ -75,7 +75,7 @@ export type PluginAPI = {
   prefix(className: string): string
 }
 
-const IS_VALID_UTILITY_NAME = /^[a-z][a-zA-Z0-9/%._-]*$/
+const IS_VALID_UTILITY_NAME = /^[a-z@][a-zA-Z0-9/%._-]*$/
 
 export function buildPluginApi(
   designSystem: DesignSystem,