comments explaining why we have and don't have ManuallyDrop

Author: RalfJung

library/alloc/src/collections/btree/map.rs

@@ -175,6 +175,7 @@ pub struct BTreeMap<
 > {
     root: Option<Root<K, V>>,
     length: usize,
+    /// `ManuallyDrop` to control drop order (needs to be dropped after all the nodes).
     pub(super) alloc: ManuallyDrop<A>,
 }
 
@@ -384,6 +385,7 @@ pub struct IntoIter<
 > {
     range: LazyLeafRange<marker::Dying, K, V>,
     length: usize,
+    /// The BTreeMap will outlive this IntoIter so we don't care about drop order for `alloc`.
     alloc: A,
 }
 
@@ -1800,6 +1802,7 @@ pub struct DrainFilter<
 {
     pred: F,
     inner: DrainFilterInner<'a, K, V>,
+    /// The BTreeMap will outlive this IntoIter so we don't care about drop order for `alloc`.
     alloc: A,
 }
 /// Most of the implementation of DrainFilter are generic over the type

library/alloc/src/collections/btree/map/entry.rs

@@ -56,6 +56,7 @@ pub struct VacantEntry<
     pub(super) handle: Option<Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>>,
     pub(super) dormant_map: DormantMutRef<'a, BTreeMap<K, V, A>>,
 
+    /// The BTreeMap will outlive this IntoIter so we don't care about drop order for `alloc`.
     pub(super) alloc: A,
 
     // Be invariant in `K` and `V`
@@ -81,6 +82,7 @@ pub struct OccupiedEntry<
     pub(super) handle: Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, marker::KV>,
     pub(super) dormant_map: DormantMutRef<'a, BTreeMap<K, V, A>>,
 
+    /// The BTreeMap will outlive this IntoIter so we don't care about drop order for `alloc`.
     pub(super) alloc: A,
 
     // Be invariant in `K` and `V`

library/alloc/src/collections/btree/set.rs

@@ -1285,6 +1285,7 @@ pub struct DrainFilter<
 {
     pred: F,
     inner: super::map::DrainFilterInner<'a, T, ()>,
+    /// The BTreeMap will outlive this IntoIter so we don't care about drop order for `alloc`.
     alloc: A,
 }