Rollup merge of #127091 - Sky9x:fused-error-sources-iter, r=dtolnay

impl FusedIterator and a size hint for the error sources iter

cc tracking issue #58520

library/core/src/error.rs

@@ -1008,7 +1008,14 @@ fn next(&mut self) -> Option<Self::Item> {
         self.current = self.current.and_then(Error::source);
         current
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.current.is_some() { (1, None) } else { (0, Some(0)) }
     }
+}
+
+#[unstable(feature = "error_iter", issue = "58520")]
+impl<'a> crate::iter::FusedIterator for Source<'a> {}
 
 #[stable(feature = "error_by_ref", since = "1.51.0")]
 impl<'a, T: Error + ?Sized> Error for &'a T {