Add fallback language support for lazy loading in rehype-shiki; updat…

…e tests and documentation.

packages/rehype/src/core.ts

@@ -6,6 +6,7 @@ import type { Root } from 'hast'
 import type { Transformer } from 'unified'
 import type { RehypeShikiHandler } from './handlers'
 import type { RehypeShikiCoreOptions } from './types'
+import { bundledLanguages } from 'shiki'
 import { isSpecialLang } from 'shiki/core'
 import { visit } from 'unist-util-visit'
 import { InlineCodeHandlers, PreHandler } from './handlers'
@@ -97,6 +98,9 @@ function rehypeShikiFromHighlighter(
         return lang
 
       if (lazy) {
+        if (fallbackLanguage && !(Object.keys(bundledLanguages) as string[]).includes(lang)) {
+          lang = fallbackLanguage as string
+        }
         languageQueue.push(lang)
         return lang
       }

packages/rehype/src/types.ts

@@ -28,7 +28,6 @@ export interface RehypeShikiExtraOptions {
   /**
    * The fallback language to use when specified language is not loaded
    *
-   * Ignored if `lazy` is enabled
    */
   fallbackLanguage?: string
 

packages/rehype/test/core.test.ts

@@ -63,6 +63,30 @@ it('run with lazy', async () => {
   expect(file.toString()).toMatchFileSnapshot('./fixtures/a.core.out.html')
 })
 
+it('run with lazy and fallback', async () => {
+  const highlighter = await createHighlighter({
+    themes: [
+      'vitesse-light',
+    ],
+    langs: [],
+  })
+
+  const file = await unified()
+    .use(remarkParse)
+    .use(remarkRehype)
+    .use(rehypeShikiFromHighlighter, highlighter, {
+      lazy: true,
+      theme: 'vitesse-light',
+      fallbackLanguage: 'text',
+      transformers: [
+        transformerMetaHighlight(),
+      ],
+    })
+    .use(rehypeStringify)
+    .process(await fs.readFile(new URL('./fixtures/d.md', import.meta.url)))
+
+  expect(file.toString()).toMatchFileSnapshot('./fixtures/d.core.out.html')
+})
 it('run with rehype-raw', async () => {
   const highlighter = await createHighlighter({
     themes: [