Function substituteAtApply

packages/tailwindcss/src/apply.ts:10–339

(ast: AstNode[], designSystem: DesignSystem)

import { walk, WalkAction } from './walk'

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>()

  // Track all the dependencies of an `AstNode`
  let dependencies = new DefaultMap<AstNode, Set<string>>(() => new Set<string>())

  // Track all `@utility` definitions by its root (name)
  let definitions = new DefaultMap(() => new Set<AstNode>())

  // Collect all new `@utility` definitions and all `@apply` rules first
  walk([root], (node, ctx) => {
    if (node.kind !== 'at-rule') return

    // Do not allow `@apply` rules inside `@keyframes` rules.
    if (node.name === '@keyframes') {
      walk(node.nodes, (child) => {
        if (child.kind === 'at-rule' && child.name === '@apply') {
          throw new Error(`You cannot use \`@apply\` inside \`@keyframes\`.`)
        }
      })
      return WalkAction.Skip
    }

    // `@utility` defines a utility, which is important information in order to
    // do a correct topological sort later on.
    if (node.name === '@utility') {
      let name = node.params.replace(/-\*$/, '')
      definitions.get(name).add(node)

      // In case `@apply` rules are used inside `@utility` rules.
      walk(node.nodes, (child) => {
        if (child.kind !== 'at-rule' || child.name !== '@apply') return

        parents.add(node)

        for (let dependency of resolveApplyDependencies(child, designSystem)) {
          dependencies.get(node).add(dependency)
        }
      })
      return
    }

    // Any other `@apply` node.
    if (node.name === '@apply') {
      // `@apply` cannot be top-level, so we need to have a parent such that we
      // can replace the `@apply` node with the actual utility classes later.
      if (ctx.parent === null) return

      features |= Features.AtApply