support in-place collect for MapWhile adapters

library/alloc/tests/lib.rs

@@ -15,6 +15,7 @@
 #![feature(split_inclusive)]
 #![feature(binary_heap_retain)]
 #![feature(inplace_iteration)]
+#![feature(iter_map_while)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};

library/alloc/tests/vec.rs

@@ -819,6 +819,7 @@ fn test_from_iter_specialization_with_iterator_adapters() {
         .map(|i| i.0 + i.1)
         .zip(std::iter::repeat(1usize))
         .map(|(a, b)| a + b)
+        .map_while(Option::Some)
         .peekable()
         .skip(1)
         .map(|e| std::num::NonZeroUsize::new(e));

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

@@ -2191,6 +2191,27 @@ where
     }
 }
 
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<S: Iterator, B, I: Iterator, P> SourceIter for MapWhile<I, P>
+where
+    P: FnMut(I::Item) -> Option<B>,
+    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) }
+    }
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<B, I: InPlaceIterable, P> InPlaceIterable for MapWhile<I, P> where
+    P: FnMut(I::Item) -> Option<B>
+{
+}
+
 /// An iterator that skips over `n` elements of `iter`.
 ///
 /// This `struct` is created by the [`skip`] method on [`Iterator`]. See its