src/language-js/embed/css.js/replacePlaceholders
function replacePlaceholders(quasisDoc, expressionDocs) {
  if (!expressionDocs || !expressionDocs.length) {
    return quasisDoc;
  }

  let replaceCounter = 0;
  const newDoc = mapDoc(quasisDoc, (doc) => {
    if (!doc || !doc.parts || !doc.parts.length) {
      return doc;
    }

    let { parts } = doc;
    const atIndex = parts.indexOf("@");
    const placeholderIndex = atIndex + 1;
    if (
      atIndex > -1 &&
      typeof parts[placeholderIndex] === "string" &&
      parts[placeholderIndex].startsWith("prettier-placeholder")
    ) {
      // If placeholder is split, join it
      const at = parts[atIndex];
      const placeholder = parts[placeholderIndex];
      const rest = parts.slice(placeholderIndex + 1);
      parts = parts
        .slice(0, atIndex)
        .concat([at + placeholder])
        .concat(rest);
    }

    const replacedParts = [];
    parts.forEach((part) => {
      if (typeof part !== "string" || !part.includes("@prettier-placeholder")) {
        replacedParts.push(part);
        return;
      }

      // When we have multiple placeholders in one line, like:
      // ${Child}${Child2}:not(:first-child)
      part.split(/@prettier-placeholder-(\d+)-id/).forEach((component, idx) => {
        // The placeholder is always at odd indices
        if (idx % 2 === 0) {
          replacedParts.push(replaceNewlinesWithLiterallines(component));
          return;
        }

        // The component will always be a number at odd index
        replacedParts.push(expressionDocs[component]);
        replaceCounter++;
      });
    });
    return { ...doc, parts: replacedParts };
  });
  return expressionDocs.length === replaceCounter ? newDoc : null;
}