hide implementation details in substituteAtApply itself

packages/tailwindcss/src/apply.ts

@@ -1,12 +1,18 @@
 import { Features } from '.'
-import { toCss, walk, WalkAction, type AstNode } from './ast'
+import { rule, toCss, walk, WalkAction, type AstNode } from './ast'
 import { compileCandidates } from './compile'
 import type { DesignSystem } from './design-system'
 import { DefaultMap } from './utils/default-map'
 
 export function substituteAtApply(ast: AstNode[], designSystem: DesignSystem) {
   let features = Features.None
 
+  // Wrap the whole AST in a root rule to make sure there is always a parent
+  // available for `@apply` at-rules. In some cases, the incoming `ast` just
+  // contains `@apply` at-rules which means that there is no proper parent to
+  // rely on.
+  let root = rule('&', ast)
+
   // Track all nodes containing `@apply`
   let parents = new Set<AstNode>()
 
@@ -17,7 +23,7 @@ export function substituteAtApply(ast: AstNode[], designSystem: DesignSystem) {
   let definitions = new DefaultMap(() => new Set<AstNode>())
 
   // Collect all new `@utility` definitions and all `@apply` rules first
-  walk(ast, (node, { parent }) => {
+  walk([root], (node, { parent }) => {
     if (node.kind !== 'at-rule') return
 
     // Do not allow `@apply` rules inside `@keyframes` rules.

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

@@ -268,9 +268,8 @@ export function buildPluginApi(
 
         designSystem.utilities.static(className, () => {
           let clonedAst = structuredClone(ast)
-          let parent = rule('&', clonedAst)
-          featuresRef.current |= substituteAtApply([parent], designSystem)
-          return parent.nodes
+          featuresRef.current |= substituteAtApply(clonedAst, designSystem)
+          return clonedAst
         })
       }
     },
@@ -391,9 +390,8 @@ export function buildPluginApi(
             }
 
             let ast = objectToAst(fn(value, { modifier }))
-            let parent = rule('&', ast)
-            featuresRef.current |= substituteAtApply([parent], designSystem)
-            return parent.nodes
+            featuresRef.current |= substituteAtApply(ast, designSystem)
+            return ast
           }
         }