feat: retained unclosed open brackets in link reference

__tests__/migration/brackets.test.ts

@@ -0,0 +1,12 @@
+import { migrate } from '../helpers';
+
+describe('migration of brackets', () => {
+  it('should retain opening bracket after a link reference brackets', () => {
+    const md = '[foo][bar';
+    const mdx = migrate(md);
+    expect(mdx).toMatchInlineSnapshot(`
+      "\\[foo]\\[bar
+      "
+    `);
+  });
+});

processor/migration/linkReference.ts

@@ -1,10 +1,13 @@
 import type { Definition, LinkReference, Root, Text } from 'mdast';
+import type { VFile } from 'vfile';
 
 import { visit } from 'unist-util-visit';
 
 const linkReferenceTransformer =
   () =>
-  (tree: Root): Root => {
+  (tree: Root, vfile: VFile): Root => {
+    const originalContent = vfile.toString();
+
     visit(tree, 'linkReference', (node: LinkReference, index, parent) => {
       const definitions = {};
 
@@ -14,9 +17,21 @@ const linkReferenceTransformer =
 
       if (node.label === node.identifier && parent) {
         if (!(node.identifier in definitions)) {
+          // Use offsets from the source file so we can slice out the exact string, including stray '[' characters.
+          const startOffset = node.position?.start.offset;
+          const endOffset = node.position?.end.offset;
+          const sourceValue =
+            typeof startOffset === 'number' && typeof endOffset === 'number'
+              ? originalContent.slice(startOffset, endOffset)
+              : null;
+          // If offsets aren't available we regenerate a safe `[label]` string instead.
+          const fallbackLabel = node.label || node.identifier || '';
+          // Preserve the sliced substring when it contains the dangling '[', otherwise synthesize `[label]`.
+          const value = sourceValue?.endsWith('[') ? sourceValue : `[${fallbackLabel}]`;
+
           parent.children[index] = {
             type: 'text',
-            value: `[${node.label}]`,
+            value,
             position: node.position,
           } as Text;
         }