do not in-place-iterate over flatmap/flatten

The implementation is unsound when a partially consumed iterator has
some elements buffered in the front/back parts and cloning the Iterator
removes the capacity from the backing vec::IntoIter.

library/alloc/tests/vec.rs

@@ -1204,22 +1204,16 @@ fn test_from_iter_specialization_with_iterator_adapters() {
 #[test]
 fn test_in_place_specialization_step_up_down() {
     fn assert_in_place_trait<T: InPlaceIterable>(_: &T) {}
-    let src = vec![[0u8; 4]; 256];
+
+    let src = vec![0u8; 1024];
     let srcptr = src.as_ptr();
-    let src_cap = src.capacity();
-    let iter = src.into_iter().flatten();
+    let src_bytes = src.capacity();
+    let iter = src.into_iter().array_chunks::<4>();
     assert_in_place_trait(&iter);
     let sink = iter.collect::<Vec<_>>();
     let sinkptr = sink.as_ptr();
-    assert_eq!(srcptr as *const u8, sinkptr);
-    assert_eq!(src_cap * 4, sink.capacity());
-
-    let iter = sink.into_iter().array_chunks::<4>();
-    assert_in_place_trait(&iter);
-    let sink = iter.collect::<Vec<_>>();
-    let sinkptr = sink.as_ptr();
-    assert_eq!(srcptr, sinkptr);
-    assert_eq!(src_cap, sink.capacity());
+    assert_eq!(srcptr.addr(), sinkptr.addr());
+    assert_eq!(src_bytes, sink.capacity() * 4);
 
     let mut src: Vec<u8> = Vec::with_capacity(17);
     let src_bytes = src.capacity();
@@ -1236,13 +1230,6 @@ fn test_in_place_specialization_step_up_down() {
     let sink: Vec<[u8; 2]> = iter.collect();
     assert_eq!(sink.len(), 8);
     assert!(sink.capacity() <= 25);
-
-    let src = vec![[0u8; 4]; 256];
-    let srcptr = src.as_ptr();
-    let iter = src.into_iter().flat_map(|a| a.into_iter().map(|b| b.wrapping_add(1)));
-    assert_in_place_trait(&iter);
-    let sink = iter.collect::<Vec<_>>();
-    assert_eq!(srcptr as *const u8, sink.as_ptr());
 }
 
 #[test]

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

@@ -1,7 +1,7 @@
 use crate::iter::adapters::SourceIter;
 use crate::iter::{
-    Cloned, Copied, Empty, Filter, FilterMap, Fuse, FusedIterator, InPlaceIterable, Map, Once,
-    OnceWith, TrustedFused, TrustedLen,
+    Cloned, Copied, Empty, Filter, FilterMap, Fuse, FusedIterator, Map, Once, OnceWith,
+    TrustedFused, TrustedLen,
 };
 use crate::num::NonZero;
 use crate::ops::{ControlFlow, Try};
@@ -157,21 +157,6 @@ where
 {
 }
 
-#[unstable(issue = "none", feature = "inplace_iteration")]
-unsafe impl<I, U, F> InPlaceIterable for FlatMap<I, U, F>
-where
-    I: InPlaceIterable,
-    U: BoundedSize + IntoIterator,
-{
-    const EXPAND_BY: Option<NonZero<usize>> = const {
-        match (I::EXPAND_BY, U::UPPER_BOUND) {
-            (Some(m), Some(n)) => m.checked_mul(n),
-            _ => None,
-        }
-    };
-    const MERGE_BY: Option<NonZero<usize>> = I::MERGE_BY;
-}
-
 #[unstable(issue = "none", feature = "inplace_iteration")]
 unsafe impl<I, U, F> SourceIter for FlatMap<I, U, F>
 where
@@ -386,21 +371,6 @@ where
 {
 }
 
-#[unstable(issue = "none", feature = "inplace_iteration")]
-unsafe impl<I> InPlaceIterable for Flatten<I>
-where
-    I: InPlaceIterable + Iterator,
-    <I as Iterator>::Item: IntoIterator + BoundedSize,
-{
-    const EXPAND_BY: Option<NonZero<usize>> = const {
-        match (I::EXPAND_BY, I::Item::UPPER_BOUND) {
-            (Some(m), Some(n)) => m.checked_mul(n),
-            _ => None,
-        }
-    };
-    const MERGE_BY: Option<NonZero<usize>> = I::MERGE_BY;
-}
-
 #[unstable(issue = "none", feature = "inplace_iteration")]
 unsafe impl<I> SourceIter for Flatten<I>
 where