Fix to show vfile message causes nicely

lib/index.js

@@ -236,10 +236,11 @@ function createByline(state, stats) {
  *   Info passed around.
  * @param {unknown} cause
  *   Cause.
- * @returns {Array<string>}
+ * @returns {Array<Array<string> | string>}
  *   Lines.
  */
 function createCauseLines(state, cause) {
+  /** @type {Array<Array<string> | string>} */
   const lines = ['  ' + state.bold + '[cause]' + state.normalIntensity + ':']
   let foundReasonableCause = false
 
@@ -250,15 +251,34 @@ function createCauseLines(state, cause) {
 
     if (typeof stackValue === 'string') {
       foundReasonableCause = true
+      /** @type {Array<Array<string> | string>} */
+      let causeLines
+
+      // Looks like a message.
+      if ('file' in cause && 'fatal' in cause) {
+        causeLines = createMessageLine(
+          state,
+          /** @type {VFileMessage} */ (cause)
+        )
+      }
+      // Regular error.
+      else {
+        causeLines = stackValue.split(eol)
 
-      const stackLines = stackValue.split(eol)
-      stackLines[0] = '    ' + stackLines[0]
-
-      lines.push(...stackLines)
+        // Recurse.
+        if ('cause' in cause && cause.cause) {
+          causeLines.push(...createCauseLines(state, cause.cause))
+        }
+      }
 
-      if ('cause' in cause && cause.cause) {
-        lines.push(...createCauseLines(state, cause.cause))
+      const head = causeLines[0]
+      if (typeof head === 'string') {
+        causeLines[0] = '    ' + head
+      } else {
+        head[0] = '    ' + head[0]
       }
+
+      lines.push(...causeLines)
     }
   }
 
@@ -345,7 +365,7 @@ function createMessageLine(state, message) {
   let reason = message.stack || message.message
 
   const match = eol.exec(reason)
-  /** @type {Array<string>} */
+  /** @type {Array<Array<string> | string>} */
   let rest = []
 
   if (match) {

test.js

@@ -10,6 +10,7 @@ import assert from 'node:assert/strict'
 import test from 'node:test'
 import strip from 'strip-ansi'
 import {VFile} from 'vfile'
+import {VFileMessage} from 'vfile-message'
 import {reporter} from 'vfile-reporter'
 
 /* eslint-disable no-undef */
@@ -441,6 +442,24 @@ test('reporter', async function () {
     'should support a `message.cause`, w/ another cause'
   )
 
+  file = new VFile()
+
+  file.message('Something failed terribly', {
+    cause: new VFileMessage('Boom!', {ruleId: 'foo', source: 'bar'})
+  })
+
+  assert.equal(
+    strip(reporter(file)),
+    [
+      '     warning Something failed terribly',
+      '  [cause]:',
+      '     info    Boom!                     foo bar',
+      '',
+      '⚠ 1 warning'
+    ].join('\n'),
+    'should support a `message.cause` w/ a message'
+  )
+
   file = new VFile()
   let message = file.message('Something failed terribly')
   message.cause = 'Boom!'