Rollup merge of #81619 - SkiFire13:resultshunt-inplace, r=the8472

Implement `SourceIterator` and `InPlaceIterable` for `ResultShunt`

library/alloc/tests/vec.rs

@@ -1004,9 +1004,9 @@ fn test_from_iter_specialization_with_iterator_adapters() {
         .map_while(Option::Some)
         .peekable()
         .skip(1)
-        .map(|e| std::num::NonZeroUsize::new(e));
+        .map(|e| if e != usize::MAX { Ok(std::num::NonZeroUsize::new(e)) } else { Err(()) });
     assert_in_place_trait(&iter);
-    let sink = iter.collect::<Vec<_>>();
+    let sink = iter.collect::<Result<Vec<_>, _>>().unwrap();
     let sinkptr = sink.as_ptr();
     assert_eq!(srcptr, sinkptr as *const usize);
 }

library/core/src/iter/adapters/mod.rs

@@ -194,3 +194,26 @@ where
         self.try_fold(init, ok(fold)).unwrap()
     }
 }
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, I, E> SourceIter for ResultShunt<'_, I, E>
+where
+    I: SourceIter<Source = S>,
+{
+    type Source = S;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut S {
+        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
+        unsafe { SourceIter::as_inner(&mut self.iter) }
+    }
+}
+
+// SAFETY: ResultShunt::next calls I::find, which has to advance `iter` in order to
+// return `Some(_)`. Since `iter` has type `I: InPlaceIterable` it's guaranteed that
+// at least one item will be moved out from the underlying source.
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<I, T, E> InPlaceIterable for ResultShunt<'_, I, E> where
+    I: Iterator<Item = Result<T, E>> + InPlaceIterable
+{
+}