Fix to improve support for unknown causes

lib/index.js

@@ -234,17 +234,29 @@ function createByline(state, stats) {
  *
  * @param {State} state
  *   Info passed around.
- * @param {Error} cause
+ * @param {unknown} cause
  *   Cause.
  * @returns {Array<string>}
  *   Lines.
  */
 function createCauseLines(state, cause) {
   const lines = ['  ' + state.bold + '[cause]' + state.normalIntensity + ':']
-  /* c8 ignore next -- stacks can be missing for weird reasons or in weird places. */
-  const stackLines = (cause.stack || cause.message).split(eol)
-  stackLines[0] = '    ' + stackLines[0]
-  lines.push(...stackLines)
+  /** @type {string | undefined} */
+  let stackValue
+
+  if (cause !== null && typeof cause === 'object') {
+    stackValue =
+      ('stack' in cause ? String(cause.stack) : undefined) ||
+      ('message' in cause ? String(cause.message) : undefined)
+  }
+
+  if (typeof stackValue === 'string') {
+    const stackLines = stackValue.split(eol)
+    stackLines[0] = '    ' + stackLines[0]
+    lines.push(...stackLines)
+  } else {
+    lines.push('    ' + cause)
+  }
 
   return lines
 }
@@ -346,7 +358,7 @@ function createMessageLine(state, message) {
   ]
 
   if (message.cause) {
-    rest.push(...createCauseLines(state, /** @type {Error} */ (message.cause)))
+    rest.push(...createCauseLines(state, message.cause))
   }
 
   if (state.verbose && message.url) {

test.js

@@ -413,6 +413,54 @@ test('reporter', async function () {
     'should support a `message.cause`'
   )
 
+  file = new VFile()
+  let message = file.message('Something failed terribly')
+  message.cause = 'Boom!'
+
+  assert.equal(
+    strip(reporter(file)),
+    [
+      ' warning Something failed terribly',
+      '  [cause]:',
+      '    Boom!',
+      '',
+      '⚠ 1 warning'
+    ].join('\n'),
+    'should support a `message.cause` set to a primitive'
+  )
+
+  file = new VFile()
+  message = file.message('Something failed terribly')
+  message.cause = {message: 'Boom!'}
+
+  assert.equal(
+    strip(reporter(file)),
+    [
+      ' warning Something failed terribly',
+      '  [cause]:',
+      '    Boom!',
+      '',
+      '⚠ 1 warning'
+    ].join('\n'),
+    'should support a `message.cause` set to a n object w/o stack'
+  )
+
+  file = new VFile()
+  message = file.message('Something failed terribly')
+  message.cause = {}
+
+  assert.equal(
+    strip(reporter(file)),
+    [
+      ' warning Something failed terribly',
+      '  [cause]:',
+      '    [object Object]',
+      '',
+      '⚠ 1 warning'
+    ].join('\n'),
+    'should support a `message.cause` set to a n object w/o stack or message'
+  )
+
   /** @type {Text} */
   const text = {type: 'text', value: 'a'}
   /** @type {MdxJsxTextElementHast} */