Add regression test for issue 99938

That issue was a dupe of 99852, but it's always better to
have multiple regression tests rather than one.

Author: est31

src/test/ui/rfc-2497-if-let-chains/issue-99938.rs

@@ -0,0 +1,31 @@
+// compile-flags: -Zvalidate-mir -C opt-level=3
+// build-pass
+#![feature(let_chains)]
+struct TupleIter<T, I: Iterator<Item = T>> {
+    inner: I,
+}
+
+impl<T, I: Iterator<Item = T>> Iterator for TupleIter<T, I> {
+    type Item = (T, T, T);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let inner = &mut self.inner;
+
+        if let Some(first) = inner.next()
+            && let Some(second) = inner.next()
+            && let Some(third) = inner.next()
+        {
+            Some((first, second, third))
+        } else {
+            None
+        }
+    }
+}
+
+fn main() {
+    let vec: Vec<u8> = Vec::new();
+    let mut tup_iter = TupleIter {
+        inner: vec.into_iter(),
+    };
+    tup_iter.next();
+}