add cloneAstNode helper

Our AST nodes are simple plain objects with a `kind` as the discriminant
instead of of an actual class instance. Since we're dealing with simple
plain objects, there is no need for a heavy handed `structuredClone` and
we can use this function instead.

Also added some benchmarks which you can run to see how much the
difference is. On my M1 Pro Max while using battery, these are the
results:

```
  ✓ Cloning AST nodes (2) 2462ms
     name                         hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · cloneAstNode()     5,769,838.55  0.0000  0.0467  0.0002  0.0002  0.0004  0.0004  0.0005  ±0.09%  2884920   fastest
   · structuredClone()    222,598.38  0.0042  4.5958  0.0045  0.0045  0.0048  0.0049  0.0100  ±1.81%   111300

 BENCH  Summary

  cloneAstNode() - ast.bench.ts > Cloning AST nodes
    25.92x faster than structuredClone()
```

Author: RobinMalfait

packages/tailwindcss/src/apply.ts

@@ -1,5 +1,5 @@
 import { Features } from '.'
-import { rule, toCss, walk, WalkAction, type AstNode } from './ast'
+import { cloneAstNode, rule, toCss, walk, WalkAction, type AstNode } from './ast'
 import { compileCandidates } from './compile'
 import type { DesignSystem } from './design-system'
 import type { SourceLocation } from './source-maps/source'
@@ -249,7 +249,7 @@ export function substituteAtApply(ast: AstNode[], designSystem: DesignSystem) {
           let candidate = compiled.nodeSorting.get(node)?.candidate
           let candidateOffset = candidate ? candidateOffsets[candidate] : undefined
 
-          node = structuredClone(node)
+          node = cloneAstNode(node)
 
           if (!src || !candidate || candidateOffset === undefined) {
             // While the original nodes may have come from an `@utility` we still

packages/tailwindcss/src/ast.bench.ts

@@ -1,5 +1,5 @@
-import { bench } from 'vitest'
-import { toCss } from './ast'
+import { bench, describe } from 'vitest'
+import { cloneAstNode, toCss } from './ast'
 import * as CSS from './css-parser'
 
 const css = String.raw
@@ -19,10 +19,22 @@ const input = css`
 `
 const ast = CSS.parse(input)
 
-bench('toCss', () => {
-  toCss(ast)
+describe('AST to CSS', () => {
+  bench('toCss', () => {
+    toCss(ast)
+  })
+
+  bench('toCss with source maps', () => {
+    toCss(ast, true)
+  })
 })
 
-bench('toCss with source maps', () => {
-  toCss(ast, true)
+describe('Cloning AST nodes', () => {
+  bench('cloneAstNode()', () => {
+    ast.map(cloneAstNode)
+  })
+
+  bench('structuredClone()', () => {
+    structuredClone(ast)
+  })
 })

packages/tailwindcss/src/ast.ts

@@ -122,6 +122,26 @@ export function atRoot(nodes: AstNode[]): AtRoot {
   }
 }
 
+export function cloneAstNode<T extends AstNode>(node: T): T {
+  switch (node.kind) {
+    case 'rule':
+    case 'at-rule':
+    case 'at-root':
+      return { ...node, nodes: node.nodes.map(cloneAstNode) }
+
+    case 'context':
+      return { ...node, context: { ...node.context }, nodes: node.nodes.map(cloneAstNode) }
+
+    case 'declaration':
+    case 'comment':
+      return { ...node }
+
+    default:
+      node satisfies never
+      throw new Error(`Unknown node kind: ${(node as any).kind}`)
+  }
+}
+
 export const enum WalkAction {
   /** Continue walking, which is the default */
   Continue,

packages/tailwindcss/src/compat/container.ts

@@ -1,4 +1,4 @@
-import { atRule, decl, type AstNode, type AtRule } from '../ast'
+import { atRule, cloneAstNode, decl, type AstNode, type AtRule } from '../ast'
 import type { DesignSystem } from '../design-system'
 import { compareBreakpoints } from '../utils/compare-breakpoints'
 import type { ResolvedConfig } from './config/types'
@@ -16,7 +16,7 @@ export function registerContainerCompat(userConfig: ResolvedConfig, designSystem
     return
   }
 
-  designSystem.utilities.static('container', () => structuredClone(rules))
+  designSystem.utilities.static('container', () => rules.map(cloneAstNode))
 }
 
 export function buildCustomContainerUtilityRules(

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

@@ -1,6 +1,6 @@
 import type { Features } from '..'
 import { substituteAtApply } from '../apply'
-import { atRule, decl, rule, walk, type AstNode } from '../ast'
+import { atRule, cloneAstNode, decl, rule, walk, type AstNode } from '../ast'
 import type { Candidate, CandidateModifier, NamedUtilityValue } from '../candidate'
 import { substituteFunctions } from '../css-functions'
 import * as CSS from '../css-parser'
@@ -329,7 +329,7 @@ export function buildPluginApi({
         }
 
         designSystem.utilities.static(className, (candidate) => {
-          let clonedAst = structuredClone(ast)
+          let clonedAst = ast.map(cloneAstNode)
           replaceNestedClassNameReferences(clonedAst, className, candidate.raw)
           featuresRef.current |= substituteAtApply(clonedAst, designSystem)
           return clonedAst

packages/tailwindcss/src/utilities.ts

@@ -1,6 +1,7 @@
 import {
   atRoot,
   atRule,
+  cloneAstNode,
   decl,
   rule,
   styleRule,
@@ -6008,7 +6009,7 @@ export function createCssUtility(node: AtRule) {
       })
 
       designSystem.utilities.functional(name.slice(0, -2), (candidate) => {
-        let atRule = structuredClone(node)
+        let atRule = cloneAstNode(node)
 
         let value = candidate.value
         let modifier = candidate.modifier
@@ -6176,7 +6177,7 @@ export function createCssUtility(node: AtRule) {
 
   if (IS_VALID_STATIC_UTILITY_NAME.test(name)) {
     return (designSystem: DesignSystem) => {
-      designSystem.utilities.static(name, () => structuredClone(node.nodes))
+      designSystem.utilities.static(name, () => node.nodes.map(cloneAstNode))
     }
   }
 

packages/tailwindcss/src/variants.ts

@@ -3,6 +3,7 @@ import {
   WalkAction,
   atRoot,
   atRule,
+  cloneAstNode,
   decl,
   rule,
   styleRule,
@@ -100,7 +101,7 @@ export class Variants {
     this.static(
       name,
       (r) => {
-        let body = structuredClone(ast)
+        let body = ast.map(cloneAstNode)
         if (usesAtVariant) substituteAtVariant(body, designSystem)
         substituteAtSlot(body, r.nodes)
         r.nodes = body