debug-assert that the size_hint is well-formed in collect

library/core/src/iter/traits/iterator.rs

@@ -1968,6 +1968,15 @@ pub trait Iterator {
     where
         Self: Sized,
     {
+        // This is too aggressive to turn on for everything all the time, but PR#137908
+        // accidentally noticed that some rustc iterators had malformed `size_hint`s,
+        // so this will help catch such things in debug-assertions-std runners,
+        // even if users won't actually ever see it.
+        if cfg!(debug_assertions) {
+            let hint = self.size_hint();
+            assert!(hint.1.is_none_or(|high| high >= hint.0), "Malformed size_hint {hint:?}");
+        }
+
         FromIterator::from_iter(self)
     }