ui/node_modules/tailwindcss/src/postcss-plugins/nesting/plugin.js

import postcss from 'postcss'
import postcssNested from 'postcss-nested'

export function nesting(opts = postcssNested) {
  return (root, result) => {
    root.walkAtRules('screen', (rule) => {
      rule.name = 'media'
      rule.params = `screen(${rule.params})`
    })

    root.walkAtRules('apply', (rule) => {
      rule.before(postcss.decl({ prop: '__apply', value: rule.params, source: rule.source }))
      rule.remove()
    })

    let plugin = (() => {
      if (
        typeof opts === 'function' ||
        (typeof opts === 'object' && opts?.hasOwnProperty?.('postcssPlugin'))
      ) {
        return opts
      }

      if (typeof opts === 'string') {
        return require(opts)
      }

      if (Object.keys(opts).length <= 0) {
        return postcssNested
      }

      throw new Error('tailwindcss/nesting should be loaded with a nesting plugin.')
    })()

    postcss([plugin]).process(root, result.opts).sync()

    root.walkDecls('__apply', (decl) => {
      decl.before(postcss.atRule({ name: 'apply', params: decl.value, source: decl.source }))
      decl.remove()
    })

    /**
     * Use a private PostCSS API to remove the "clean" flag from the entire AST.
     * This is done because running process() on the AST will set the "clean"
     * flag on all nodes, which we don't want.
     *
     * This causes downstream plugins using the visitor API to be skipped.
     *
     * This is guarded because the PostCSS API is not public
     * and may change in future versions of PostCSS.
     *
     * See https://github.com/postcss/postcss/issues/1712 for more details
     *
     * @param {import('postcss').Node} node
     */
    function markDirty(node) {
      if (!('markDirty' in node)) {
        return
      }

      // Traverse the tree down to the leaf nodes
      if (node.nodes) {
        node.nodes.forEach((n) => markDirty(n))
      }

      // If it's a leaf node mark it as dirty
      // We do this here because marking a node as dirty
      // will walk up the tree and mark all parents as dirty
      // resulting in a lot of unnecessary work if we did this
      // for every single node
      if (!node.nodes) {
        node.markDirty()
      }
    }

    markDirty(root)

    return root
  }
}