Rollup merge of #135104 - the8472:disable-in-place-iter-for-flatten, r=Mark-Simulacrum

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.

This is a fix for #135103 that removes the specialization trait impls without removing some supporting parts. I've kept it small so it can be easily backported. I'll either remove the remaining parts or think of a way to recover the optimization in a separate PR.

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 srcptr = src.as_ptr();
-    let src_cap = src.capacity();
-    let iter = src.into_iter().flatten();
-    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>();
+    let src = vec![0u8; 1024];
+    let srcptr = src.as_ptr();
+    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, 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]
@@ -1350,6 +1337,20 @@ fn test_collect_after_iterator_clone() {
     assert_eq!(v, [1, 1, 1, 1, 1]);
     assert!(v.len() <= v.capacity());
 }
+
+// regression test for #135103, similar to the one above Flatten/FlatMap had an unsound InPlaceIterable
+// implementation.
+#[test]
+fn test_flatten_clone() {
+    const S: String = String::new();
+
+    let v = vec![[S, "Hello World!".into()], [S, S]];
+    let mut i = v.into_iter().flatten();
+    let _ = i.next();
+    let result: Vec<String> = i.clone().collect();
+    assert_eq!(result, ["Hello World!", "", ""]);
+}
+
 #[test]
 fn test_cow_from() {
     let borrowed: &[_] = &["borrowed", "(slice)"];

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