src/language-js/embed/css.js/replacePlaceholders
function replacePlaceholders(quasisDoc, expressionDocs) {
  if (!isNonEmptyArray(expressionDocs)) {
    return quasisDoc;
  }
  let replaceCounter = 0;
  const newDoc = mapDoc(cleanDoc(quasisDoc), (doc) => {
    if (typeof doc !== "string" || !doc.includes("@prettier-placeholder")) {
      return doc;
    }
    // When we have multiple placeholders in one line, like:
    // ${Child}${Child2}:not(:first-child)
    return doc.split(/@prettier-placeholder-(\d+)-id/).map((component, idx) => {
      // The placeholder is always at odd indices
      if (idx % 2 === 0) {
        return replaceEndOfLine(component);
      }

      // The component will always be a number at odd index
      replaceCounter++;
      return expressionDocs[component];
    });
  });
  return expressionDocs.length === replaceCounter ? newDoc : null;
}