2229: Annotate stdlib with insignficant dtors

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

@@ -162,6 +162,7 @@ pub struct BTreeMap<K, V> {
 
 #[stable(feature = "btree_drop", since = "1.7.0")]
 unsafe impl<#[may_dangle] K, #[may_dangle] V> Drop for BTreeMap<K, V> {
+    #[rustc_insignificant_dtor]
     fn drop(&mut self) {
         drop(unsafe { ptr::read(self) }.into_iter())
     }
@@ -1459,6 +1460,7 @@ impl<K, V> IntoIterator for BTreeMap<K, V> {
 
 #[stable(feature = "btree_drop", since = "1.7.0")]
 impl<K, V> Drop for IntoIter<K, V> {
+    #[rustc_insignificant_dtor]
     fn drop(&mut self) {
         struct DropGuard<'a, K, V>(&'a mut IntoIter<K, V>);
 

library/alloc/src/collections/linked_list.rs

@@ -975,6 +975,7 @@ impl<T> LinkedList<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<#[may_dangle] T> Drop for LinkedList<T> {
+    #[rustc_insignificant_dtor]
     fn drop(&mut self) {
         struct DropGuard<'a, T>(&'a mut LinkedList<T>);
 

library/alloc/src/collections/vec_deque/mod.rs

@@ -130,6 +130,7 @@ impl<T: Clone, A: Allocator + Clone> Clone for VecDeque<T, A> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<#[may_dangle] T, A: Allocator> Drop for VecDeque<T, A> {
+    #[rustc_insignificant_dtor]
     fn drop(&mut self) {
         /// Runs the destructor for all items in the slice when it gets dropped (normally or
         /// during unwinding).

library/alloc/src/rc.rs

@@ -1441,6 +1441,7 @@ unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
     /// drop(foo);    // Doesn't print anything
     /// drop(foo2);   // Prints "dropped!"
     /// ```
+    #[rustc_insignificant_dtor]
     fn drop(&mut self) {
         unsafe {
             self.inner().dec_strong();

library/alloc/src/vec/into_iter.rs

@@ -246,6 +246,7 @@ impl<T: Clone, A: Allocator + Clone> Clone for IntoIter<T, A> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<#[may_dangle] T, A: Allocator> Drop for IntoIter<T, A> {
+    #[rustc_insignificant_dtor]
     fn drop(&mut self) {
         struct DropGuard<'a, T, A: Allocator>(&'a mut IntoIter<T, A>);
 

library/alloc/src/vec/mod.rs

@@ -2746,6 +2746,7 @@ impl<T: Ord, A: Allocator> Ord for Vec<T, A> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<#[may_dangle] T, A: Allocator> Drop for Vec<T, A> {
+    #[rustc_insignificant_dtor]
     fn drop(&mut self) {
         unsafe {
             // use drop for [T]

library/core/src/array/iter.rs

@@ -180,6 +180,7 @@ impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
 
 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 impl<T, const N: usize> Drop for IntoIter<T, N> {
+    #[rustc_insignificant_dtor]
     fn drop(&mut self) {
         // SAFETY: This is safe: `as_mut_slice` returns exactly the sub-slice
         // of elements that have not been moved out yet and that remain

library/std/src/lazy.rs

@@ -492,6 +492,7 @@ impl<T> SyncOnceCell<T> {
 }
 
 unsafe impl<#[may_dangle] T> Drop for SyncOnceCell<T> {
+    #[rustc_insignificant_dtor]
     fn drop(&mut self) {
         if self.is_initialized() {
             // SAFETY: The cell is initialized and being dropped, so it can't