async function loader(content, sourceMap, meta) {
  const options = this.getOptions(schema);
  const callback = this.async();
  const configOption =
    typeof options.postcssOptions === "undefined" ||
    typeof options.postcssOptions.config === "undefined"
      ? true
      : options.postcssOptions.config;

  let implementation;

  try {
    implementation = getPostcssImplementation(this, options.implementation);
  } catch (error) {
    callback(error);

    return;
  }

  if (!implementation) {
    callback(
      new Error(
        `The Postcss implementation "${options.implementation}" not found`,
      ),
    );

    return;
  }

  let loadedConfig;

  if (configOption) {
    try {
      loadedConfig = await loadConfig(
        this,
        configOption,
        options.postcssOptions,
      );
    } catch (error) {
      callback(error);

      return;
    }
  }

  const { plugins, processOptions } = await getPostcssOptions(
    this,
    loadedConfig,
    options.postcssOptions,
  );

  const useSourceMap =
    typeof options.sourceMap !== "undefined"
      ? options.sourceMap
      : this.sourceMap;

  if (useSourceMap) {
    processOptions.map = {
      inline: false,
      annotation: false,
      ...processOptions.map,
    };
  }

  if (sourceMap && processOptions.map) {
    processOptions.map.prev = normalizeSourceMap(sourceMap, this.context);
  }

  let root;

  // Reuse PostCSS AST from other loaders
  if (
    meta &&
    meta.ast &&
    meta.ast.type === "postcss" &&
    // eslint-disable-next-line global-require
    require("semver").satisfies(meta.ast.version, `^${postcssPackage.version}`)
  ) {
    ({ root } = meta.ast);
  }

  if (!root && options.execute) {
    // eslint-disable-next-line no-param-reassign
    content = exec(content, this);
  }

  let result;
  let processor;

  try {
    processor = implementation(plugins);
    result = await processor.process(root || content, processOptions);
  } catch (error) {
    // Check postcss versions to avoid using PostCSS 7.
    // For caching reasons, we use the readFileSync and existsSync functions from the context,
    // not the functions from the `fs` module.
    if (
      !hasExplicitDependencyOnPostCSS &&
      processor &&
      processor.version &&
      processor.version.startsWith("7.")
    ) {
      // The `findPackageJsonDir` function returns `string` or `null`.
      // This is used to do for caching, that is, an explicit comparison with `undefined`
      // is used to make the condition body run once.
      const packageJSONDir = findPackageJSONDir(
        process.cwd(),
        this.fs.statSync,
      );

      if (packageJSONDir) {
        let bufferOfPackageJSON;

        try {
          bufferOfPackageJSON = this.fs.readFileSync(
            path.resolve(packageJSONDir, "package.json"),
            "utf8",
          );
        } catch (_error) {
          // Nothing
        }

        if (bufferOfPackageJSON) {
          let pkg;

          try {
            pkg = JSON.parse(bufferOfPackageJSON);
          } catch (_error) {
            // Nothing
          }

          if (pkg) {
            const { dependencies = {}, devDependencies = {} } = pkg;

            if (!dependencies.postcss && !devDependencies.postcss) {
              this.emitWarning(
                new Error(
                  "Add postcss as project dependency. postcss is not a peer dependency for postcss-loader. " +
                    "Use `npm install postcss` or `yarn add postcss`",
                ),
              );
            } else {
              hasExplicitDependencyOnPostCSS = true;
            }
          }
        }
      }
    }

    reportError(this, callback, error);

    return;
  }

  for (const warning of result.warnings()) {
    this.emitWarning(warningFactory(warning));
  }

  for (const message of result.messages) {
    // eslint-disable-next-line default-case
    switch (message.type) {
      case "dependency":
        this.addDependency(message.file);
        break;
      case "build-dependency":
        this.addBuildDependency(message.file);
        break;
      case "missing-dependency":
        this.addMissingDependency(message.file);
        break;
      case "context-dependency":
        this.addContextDependency(message.file);
        break;
      case "dir-dependency":
        this.addContextDependency(message.dir);
        break;
      case "asset":
        if (message.content && message.file) {
          this.emitFile(
            message.file,
            message.content,
            message.sourceMap,
            message.info,
          );
        }
    }
  }

  // eslint-disable-next-line no-undefined
  let map = result.map ? result.map.toJSON() : undefined;

  if (map && useSourceMap) {
    map = normalizeSourceMapAfterPostcss(map, this.context);
  }

  let ast;

  try {
    ast = {
      type: "postcss",
      version: result.processor.version,
      root: result.root,
    };
  } catch (error) {
    reportError(this, callback, error);

    return;
  }

  callback(null, result.css, map, { ast });
}
**PostCSS Loader**

Loads && processes CSS with [PostCSS](https://github.com/postcss/postcss)

@method loader

@param {String} content Source
@param {Object} sourceMap Source Map
@param {Object} meta Meta

@return {callback} callback Result