Function optimizeAst

packages/tailwindcss/src/ast.ts:222–683

(
  ast: AstNode[],
  designSystem: DesignSystem,
  polyfills: Polyfills = Polyfills.All,
)

// Optimize the AST for printing where all the special nodes that require custom
// handling are handled such that the printing is a 1-to-1 transformation.
export function optimizeAst(
  ast: AstNode[],
  designSystem: DesignSystem,
  polyfills: Polyfills = Polyfills.All,
) {
  let atRoots: AstNode[] = []
  let seenAtProperties = new Set<string>()
  let cssThemeVariables = new DefaultMap<AstNode[], Set<Declaration>>(() => new Set())
  let colorMixDeclarations = new DefaultMap<AstNode[], Set<Declaration>>(() => new Set())
  let keyframes = new Set<AtRule>()
  let usedKeyframeNames = new Set()

  let propertyFallbacksRoot: Declaration[] = []
  let propertyFallbacksUniversal: Declaration[] = []

  let variableDependencies = new DefaultMap<string, Set<string>>(() => new Set())

  function transform(
    node: AstNode,
    parent: AstNode[],
    context: Record<string, string | boolean> = {},
    depth = 0,
  ) {
    // Declaration
    if (node.kind === 'declaration') {
      if (node.property === '--tw-sort' || node.value === undefined || node.value === null) {
        return
      }

      // Track variables defined in `@theme`
      if (context.theme && node.property[0] === '-' && node.property[1] === '-') {
        // Variables that resolve to `initial` should never be emitted. This can happen because of
        // the `--theme(…)` being used and evaluated lazily
        if (node.value === 'initial') {
          node.value = undefined
          return
        }

        if (!context.keyframes) {
          cssThemeVariables.get(parent).add(node)
        }
      }

      // Track used CSS variables
      if (node.value.includes('var(')) {
        // Declaring another variable does not count as usage. Instead, we mark
        // the relationship
        if (context.theme && node.property[0] === '-' && node.property[1] === '-') {
          for (let variable of extractUsedVariables(node.value)) {
            variableDependencies.get(variable).add(node.property)
          }
        } else {
          designSystem.trackUsedVariables(node.value)
        }
      }

      // Track used animation names
      if (node.property === 'animation') {