Properly skip over nodes when using replaceNode (#16300)

Fixes #16298 

This PR fixes an issue where using an AST walk in combination with
`replaceNode` and various `SkipAction` would either cause children to be
visited multiple times or not visited at all even though it should. This
PR fixes the issue which also means we can get rid of a custom walk for
`@variant` inside the `@media` that was used to apply `@variant` because
we never recursively visited children inside the `@media` rule.

Because we now can use the regular walk for `@variant`, we now properly
convert `@variant` to `@custom-variant` inside `@reference`-ed
stylesheet which also fixes #16298

## Test plan

Lots of tests added to ensure the combinations of `WalkAction` and
`replaceWith()` works as expected.

Author: philipp-spiess

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix support for older instruction sets on Linux x64 builds of the standalone CLI ([#16244](https://github.com/tailwindlabs/tailwindcss/pull/16244))
 - Ensure `NODE_PATH` is respected when resolving JavaScript and CSS files ([#16274](https://github.com/tailwindlabs/tailwindcss/pull/16274))
 - Ensure Node addons are packaged correctly with FreeBSD builds ([#16277](https://github.com/tailwindlabs/tailwindcss/pull/16277))
+- Fix an issue where `@variant` inside a referenced stylesheet could cause a stack overflow ([#16300](https://github.com/tailwindlabs/tailwindcss/pull/16300))
 
 ## [4.0.3] - 2025-02-01
 

packages/tailwindcss/src/ast.test.ts

@@ -1,5 +1,15 @@
 import { expect, it } from 'vitest'
-import { context, decl, optimizeAst, styleRule, toCss, walk, WalkAction } from './ast'
+import {
+  atRule,
+  context,
+  decl,
+  optimizeAst,
+  styleRule,
+  toCss,
+  walk,
+  WalkAction,
+  type AstNode,
+} from './ast'
 import * as CSS from './css-parser'
 
 const css = String.raw
@@ -188,3 +198,310 @@ it('should not emit empty rules once optimized', () => {
     "
   `)
 })
+
+it('should only visit children once when calling `replaceWith` with single element array', () => {
+  let visited = new Set()
+
+  let ast = [
+    atRule('@media', '', [styleRule('.foo', [decl('color', 'blue')])]),
+    styleRule('.bar', [decl('color', 'blue')]),
+  ]
+
+  walk(ast, (node, { replaceWith }) => {
+    if (visited.has(node)) {
+      throw new Error('Visited node twice')
+    }
+    visited.add(node)
+
+    if (node.kind === 'at-rule') replaceWith(node.nodes)
+  })
+})
+
+it('should only visit children once when calling `replaceWith` with multi-element array', () => {
+  let visited = new Set()
+
+  let ast = [
+    atRule('@media', '', [
+      context({}, [
+        styleRule('.foo', [decl('color', 'red')]),
+        styleRule('.baz', [decl('color', 'blue')]),
+      ]),
+    ]),
+    styleRule('.bar', [decl('color', 'green')]),
+  ]
+
+  walk(ast, (node, { replaceWith }) => {
+    let key = id(node)
+    if (visited.has(key)) {
+      throw new Error('Visited node twice')
+    }
+    visited.add(key)
+
+    if (node.kind === 'at-rule') replaceWith(node.nodes)
+  })
+
+  expect(visited).toMatchInlineSnapshot(`
+    Set {
+      "@media ",
+      ".foo",
+      "color: red",
+      ".baz",
+      "color: blue",
+      ".bar",
+      "color: green",
+    }
+  `)
+})
+
+it('should never visit children when calling `replaceWith` with `WalkAction.Skip`', () => {
+  let visited = new Set()
+
+  let inner = styleRule('.foo', [decl('color', 'blue')])
+
+  let ast = [atRule('@media', '', [inner]), styleRule('.bar', [decl('color', 'blue')])]
+
+  walk(ast, (node, { replaceWith }) => {
+    visited.add(node)
+
+    if (node.kind === 'at-rule') {
+      replaceWith(node.nodes)
+      return WalkAction.Skip
+    }
+  })
+
+  expect(visited).not.toContain(inner)
+  expect(visited).toMatchInlineSnapshot(`
+    Set {
+      {
+        "kind": "at-rule",
+        "name": "@media",
+        "nodes": [
+          {
+            "kind": "rule",
+            "nodes": [
+              {
+                "important": false,
+                "kind": "declaration",
+                "property": "color",
+                "value": "blue",
+              },
+            ],
+            "selector": ".foo",
+          },
+        ],
+        "params": "",
+      },
+      {
+        "kind": "rule",
+        "nodes": [
+          {
+            "important": false,
+            "kind": "declaration",
+            "property": "color",
+            "value": "blue",
+          },
+        ],
+        "selector": ".bar",
+      },
+      {
+        "important": false,
+        "kind": "declaration",
+        "property": "color",
+        "value": "blue",
+      },
+    }
+  `)
+})
+
+it('should skip the correct number of children based on the the replaced children nodes', () => {
+  {
+    let ast = [
+      decl('--index', '0'),
+      decl('--index', '1'),
+      decl('--index', '2'),
+      decl('--index', '3'),
+      decl('--index', '4'),
+    ]
+    let visited: string[] = []
+    walk(ast, (node, { replaceWith }) => {
+      visited.push(id(node))
+      if (node.kind === 'declaration' && node.value === '2') {
+        replaceWith([])
+        return WalkAction.Skip
+      }
+    })
+
+    expect(visited).toMatchInlineSnapshot(`
+      [
+        "--index: 0",
+        "--index: 1",
+        "--index: 2",
+        "--index: 3",
+        "--index: 4",
+      ]
+    `)
+  }
+
+  {
+    let ast = [
+      decl('--index', '0'),
+      decl('--index', '1'),
+      decl('--index', '2'),
+      decl('--index', '3'),
+      decl('--index', '4'),
+    ]
+    let visited: string[] = []
+    walk(ast, (node, { replaceWith }) => {
+      visited.push(id(node))
+      if (node.kind === 'declaration' && node.value === '2') {
+        replaceWith([])
+        return WalkAction.Continue
+      }
+    })
+
+    expect(visited).toMatchInlineSnapshot(`
+      [
+        "--index: 0",
+        "--index: 1",
+        "--index: 2",
+        "--index: 3",
+        "--index: 4",
+      ]
+    `)
+  }
+
+  {
+    let ast = [
+      decl('--index', '0'),
+      decl('--index', '1'),
+      decl('--index', '2'),
+      decl('--index', '3'),
+      decl('--index', '4'),
+    ]
+    let visited: string[] = []
+    walk(ast, (node, { replaceWith }) => {
+      visited.push(id(node))
+      if (node.kind === 'declaration' && node.value === '2') {
+        replaceWith([decl('--index', '2.1')])
+        return WalkAction.Skip
+      }
+    })
+
+    expect(visited).toMatchInlineSnapshot(`
+      [
+        "--index: 0",
+        "--index: 1",
+        "--index: 2",
+        "--index: 3",
+        "--index: 4",
+      ]
+    `)
+  }
+
+  {
+    let ast = [
+      decl('--index', '0'),
+      decl('--index', '1'),
+      decl('--index', '2'),
+      decl('--index', '3'),
+      decl('--index', '4'),
+    ]
+    let visited: string[] = []
+    walk(ast, (node, { replaceWith }) => {
+      visited.push(id(node))
+      if (node.kind === 'declaration' && node.value === '2') {
+        replaceWith([decl('--index', '2.1')])
+        return WalkAction.Continue
+      }
+    })
+
+    expect(visited).toMatchInlineSnapshot(`
+      [
+        "--index: 0",
+        "--index: 1",
+        "--index: 2",
+        "--index: 2.1",
+        "--index: 3",
+        "--index: 4",
+      ]
+    `)
+  }
+
+  {
+    let ast = [
+      decl('--index', '0'),
+      decl('--index', '1'),
+      decl('--index', '2'),
+      decl('--index', '3'),
+      decl('--index', '4'),
+    ]
+    let visited: string[] = []
+    walk(ast, (node, { replaceWith }) => {
+      visited.push(id(node))
+      if (node.kind === 'declaration' && node.value === '2') {
+        replaceWith([decl('--index', '2.1'), decl('--index', '2.2')])
+        return WalkAction.Skip
+      }
+    })
+
+    expect(visited).toMatchInlineSnapshot(`
+      [
+        "--index: 0",
+        "--index: 1",
+        "--index: 2",
+        "--index: 3",
+        "--index: 4",
+      ]
+    `)
+  }
+
+  {
+    let ast = [
+      decl('--index', '0'),
+      decl('--index', '1'),
+      decl('--index', '2'),
+      decl('--index', '3'),
+      decl('--index', '4'),
+    ]
+    let visited: string[] = []
+    walk(ast, (node, { replaceWith }) => {
+      visited.push(id(node))
+      if (node.kind === 'declaration' && node.value === '2') {
+        replaceWith([decl('--index', '2.1'), decl('--index', '2.2')])
+        return WalkAction.Continue
+      }
+    })
+
+    expect(visited).toMatchInlineSnapshot(`
+      [
+        "--index: 0",
+        "--index: 1",
+        "--index: 2",
+        "--index: 2.1",
+        "--index: 2.2",
+        "--index: 3",
+        "--index: 4",
+      ]
+    `)
+  }
+})
+
+function id(node: AstNode) {
+  switch (node.kind) {
+    case 'at-rule':
+      return `${node.name} ${node.params}`
+    case 'rule':
+      return node.selector
+    case 'context':
+      return '<context>'
+    case 'at-root':
+      return '<at-root>'
+    case 'declaration':
+      return `${node.property}: ${node.value}`
+    case 'comment':
+      return `// ${node.value}`
+    default:
+      node satisfies never
+      throw new Error('Unknown node kind')
+  }
+}

packages/tailwindcss/src/ast.ts

@@ -137,39 +137,54 @@ export function walk(
     }
 
     path.push(node)
+    let replacedNode = false
+    let replacedNodeOffset = 0
     let status =
       visit(node, {
         parent,
         context,
         path,
         replaceWith(newNode) {
+          replacedNode = true
+
           if (Array.isArray(newNode)) {
             if (newNode.length === 0) {
               ast.splice(i, 1)
+              replacedNodeOffset = 0
             } else if (newNode.length === 1) {
               ast[i] = newNode[0]
+              replacedNodeOffset = 1
             } else {
               ast.splice(i, 1, ...newNode)
+              replacedNodeOffset = newNode.length
             }
           } else {
             ast[i] = newNode
+            replacedNodeOffset = 1
           }
-
-          // We want to visit the newly replaced node(s), which start at the
-          // current index (i). By decrementing the index here, the next loop
-          // will process this position (containing the replaced node) again.
-          i--
         },
       }) ?? WalkAction.Continue
     path.pop()
 
+    // We want to visit or skip the newly replaced node(s), which start at the
+    // current index (i). By decrementing the index here, the next loop will
+    // process this position (containing the replaced node) again.
+    if (replacedNode) {
+      if (status === WalkAction.Continue) {
+        i--
+      } else {
+        i += replacedNodeOffset - 1
+      }
+      continue
+    }
+
     // Stop the walk entirely
     if (status === WalkAction.Stop) return WalkAction.Stop
 
     // Skip visiting the children of this node
     if (status === WalkAction.Skip) continue
 
-    if (node.kind === 'rule' || node.kind === 'at-rule') {
+    if ('nodes' in node) {
       path.push(node)
       let result = walk(node.nodes, visit, path, context)
       path.pop()