Add processor cache in webpack loader (#1912)

Backports: wooorm/xdm@decafe9.

Related-to: GH-1468.

packages/loader/lib/index.js

@@ -1,12 +1,18 @@
 /**
+ * @typedef {import('vfile').VFileCompatible} VFileCompatible
+ * @typedef {import('vfile').VFile} VFile
  * @typedef {import('@mdx-js/mdx').CompileOptions} CompileOptions
  * @typedef {Pick<CompileOptions, 'SourceMapGenerator'>} Defaults
  * @typedef {Omit<CompileOptions, 'SourceMapGenerator'>} Options
  * @typedef {import('webpack').LoaderContext<unknown>} LoaderContext
+ * @typedef {(vfileCompatible: VFileCompatible) => Promise<VFile>} Process
  */
 
 import {SourceMapGenerator} from 'source-map'
-import {compile} from '@mdx-js/mdx'
+import {createFormatAwareProcessors} from '@mdx-js/mdx/lib/util/create-format-aware-processors.js'
+
+/** @type {WeakMap<CompileOptions, Process>} */
+const cache = new WeakMap()
 
 /**
  * A Webpack (5+) loader for MDX.
@@ -21,20 +27,25 @@ export function loader(value, callback) {
   /** @type {Defaults} */
   const defaults = this.sourceMap ? {SourceMapGenerator} : {}
   const options = /** @type {CompileOptions} */ (this.getOptions())
+  const config = {...defaults, ...options}
 
   /* Removed option. */
   /* c8 ignore next 5 */
-  if ('renderer' in options) {
+  if ('renderer' in config) {
     throw new Error(
       '`options.renderer` is no longer supported. Please see <https://mdxjs.com/migrating/v2/> for more information'
     )
   }
 
-  compile({value, path: this.resourcePath}, {...defaults, ...options}).then(
-    (file) => {
-      callback(null, file.value, file.map)
-      return file
-    },
-    callback
-  )
+  let process = cache.get(config)
+
+  if (!process) {
+    process = createFormatAwareProcessors(config).process
+    cache.set(config, process)
+  }
+
+  process({value, path: this.resourcePath}).then((file) => {
+    callback(null, file.value, file.map)
+    return file
+  }, callback)
 }