fix: handle YAML front matter block (#3802)

src/language-css/parser-postcss.js

@@ -1,6 +1,7 @@
 "use strict";
 
 const createError = require("../common/parser-create-error");
+const grayMatter = require("gray-matter");
 
 function parseSelector(selector) {
   // If there's a comment inside of a selector, the parser tries to parse
@@ -189,7 +190,11 @@ function parseNestedCSS(node) {
         };
       }
     }
-    if (node.type && typeof node.value === "string") {
+    if (
+      node.type &&
+      node.type !== "css-comment-yaml" &&
+      typeof node.value === "string"
+    ) {
       try {
         if (node.value.endsWith(DEFAULT_SCSS_DIRECTIVE)) {
           node.default = true;
@@ -226,7 +231,7 @@ function parseNestedCSS(node) {
   return node;
 }
 
-function parseWithParser(parser, text) {
+function parseWithParser(parser, text, frontMatter) {
   let result;
   try {
     result = parser.parse(text);
@@ -236,6 +241,14 @@ function parseWithParser(parser, text) {
     }
     throw createError("(postcss) " + e.name + " " + e.reason, { start: e });
   }
+
+  if (Object.keys(frontMatter.data).length > 0) {
+    result.nodes.unshift({
+      type: "comment-yaml",
+      value: grayMatter.stringify("", frontMatter.data).replace(/\s$/, "")
+    });
+  }
+
   const prefixedResult = addTypePrefix(result, "css-");
   const parsedResult = parseNestedCSS(prefixedResult);
   return parsedResult;
@@ -269,15 +282,22 @@ function parse(text, parsers, opts) {
     ? opts.parser === "scss"
     : IS_POSSIBLY_SCSS.test(text);
 
+  const frontMatter = grayMatter(text);
+  const normalizedText = frontMatter.content;
+
   try {
-    return parseWithParser(requireParser(isSCSS), text);
+    return parseWithParser(requireParser(isSCSS), normalizedText, frontMatter);
   } catch (originalError) {
     if (hasExplicitParserChoice) {
       throw originalError;
     }
 
     try {
-      return parseWithParser(requireParser(!isSCSS), text);
+      return parseWithParser(
+        requireParser(!isSCSS),
+        normalizedText,
+        frontMatter
+      );
     } catch (_secondError) {
       throw originalError;
     }