Add comments around code where ordering is important due for panic-safety

Iterators contain arbitrary code which may panic. Unsafe code has to be
careful to do its state updates at the right point between calls
that may panic.

3 files changed, 8 insertions, 0 deletions

diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index b59d2977add..f3a47cba759 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -2568,6 +2568,8 @@ impl<T, A: Allocator> Vec<T, A> {
             }
             unsafe {
                 ptr::write(self.as_mut_ptr().add(len), element);
+                // Since next() executes user code which can panic we have to bump the length
+                // after each step.
                 // NB can't overflow since we would have had to alloc the address space
                 self.set_len(len + 1);
             }
diff --git a/library/alloc/src/vec/source_iter_marker.rs b/library/alloc/src/vec/source_iter_marker.rs
index e857d284d3a..d814d4ae355 100644
--- a/library/alloc/src/vec/source_iter_marker.rs
+++ b/library/alloc/src/vec/source_iter_marker.rs
@@ -89,6 +89,8 @@ fn write_in_place_with_drop<T>(
             // all we can do is check if it's still in range
             debug_assert!(sink.dst as *const _ <= src_end, "InPlaceIterable contract violation");
             ptr::write(sink.dst, item);
+            // Since this executes user code which can panic we have to bump the pointer
+            // after each step.
             sink.dst = sink.dst.add(1);
         }
         Ok(sink)
@@ -136,6 +138,8 @@ where
                 let dst = dst_buf.offset(i as isize);
                 debug_assert!(dst as *const _ <= end, "InPlaceIterable contract violation");
                 ptr::write(dst, self.__iterator_get_unchecked(i));
+                // Since this executes user code which can panic we have to bump the pointer
+                // after each step.
                 drop_guard.dst = dst.add(1);
             }
         }
diff --git a/library/alloc/src/vec/spec_extend.rs b/library/alloc/src/vec/spec_extend.rs
index c6f4f22a01f..c3b4534096d 100644
--- a/library/alloc/src/vec/spec_extend.rs
+++ b/library/alloc/src/vec/spec_extend.rs
@@ -40,6 +40,8 @@ where
                 iterator.for_each(move |element| {
                     ptr::write(ptr, element);
                     ptr = ptr.offset(1);
+                    // Since the loop executes user code which can panic we have to bump the pointer
+                    // after each step.
                     // NB can't overflow since we would have had to alloc the address space
                     local_len.increment_len(1);
                 });