Add support for await in MDX

This likely doesn’t work in your JSX runtime.
But if you type `await`, it would currently always be invalid.
Now it turns into an `async` function that doesn’t crash.
If your runtime supports promises, MDX will support it too.

Closes GH-2242.

packages/mdx/lib/plugin/recma-document.js

@@ -602,9 +602,33 @@ export function recmaDocument(options) {
       argument = argument.children[0]
     }
 
+    let awaitExpression = false
+
+    walk(argument, {
+      enter(node) {
+        if (
+          node.type === 'ArrowFunctionExpression' ||
+          node.type === 'FunctionDeclaration' ||
+          node.type === 'FunctionExpression'
+        ) {
+          return this.skip()
+        }
+
+        if (
+          node.type === 'AwaitExpression' ||
+          /* c8 ignore next 2 -- can only occur in a function (which then can
+           * only be async, so skipped it) */
+          (node.type === 'ForOfStatement' && node.await)
+        ) {
+          awaitExpression = true
+        }
+      }
+    })
+
     return [
       {
         type: 'FunctionDeclaration',
+        async: awaitExpression,
         id: {type: 'Identifier', name: '_createMdxContent'},
         params: [{type: 'Identifier', name: 'props'}],
         body: {

packages/mdx/test/compile.js

@@ -973,6 +973,42 @@ test('@mdx-js/mdx: compile', async function (t) {
     }
   )
 
+  await t.test(
+    'should support an `await` expression in content (GH-2242)',
+    async function () {
+      const element = React.createElement(
+        await run(await compile('{await Promise.resolve(42)}'))
+      )
+
+      try {
+        // Not supported yet by React, so it throws.
+        renderToStaticMarkup(element)
+        assert.fail()
+      } catch (error) {
+        assert.match(
+          String(error),
+          /Objects are not valid as a React child \(found: \[object Promise]\)/
+        )
+      }
+    }
+  )
+
+  await t.test(
+    'should not detect `await` inside functions (GH-2242)',
+    async function () {
+      const value = `{(function () {
+  return 21
+
+  async function unused() {
+    await Promise.resolve(42)
+  }
+})()}`
+      const element = React.createElement(await run(await compile(value)))
+
+      assert.equal(renderToStaticMarkup(element), '21')
+    }
+  )
+
   await t.test('should support source maps', async function () {
     const base = new URL('context/', import.meta.url)
     const url = new URL('sourcemap.js', base)