Auto merge of rust-lang#89144 - sexxi-goose:insig_stdlib, r=nikomatsakis

2229: Mark insignificant dtor in stdlib

I looked at all public [stdlib Drop implementations](https://doc.rust-lang.org/stable/std/ops/trait.Drop.html#implementors) and categorized them into Insigificant/Maybe/Significant Drop.

Reasons are noted here: https://docs.google.com/spreadsheets/d/19edb9r5lo2UqMrCOVjV0fwcSdS-R7qvKNL76q7tO8VA/edit#gid=1838773501

One thing missing from this PR is tagging HashMap as insigificant destructor as that needs some discussion.

r? `@Mark-Simulacrum`

cc `@nikomatsakis`

Author: bors

alloc/src/collections/btree/map.rs

@@ -155,6 +155,7 @@ pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "BTreeMap")]
+#[rustc_insignificant_dtor]
 pub struct BTreeMap<K, V> {
     root: Option<Root<K, V>>,
     length: usize,
@@ -331,6 +332,7 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for IterMut<'_, K, V> {
 /// [`into_iter`]: IntoIterator::into_iter
 /// [`IntoIterator`]: core::iter::IntoIterator
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_insignificant_dtor]
 pub struct IntoIter<K, V> {
     range: LazyLeafRange<marker::Dying, K, V>,
     length: usize,

alloc/src/collections/linked_list.rs

@@ -46,6 +46,7 @@ mod tests;
 /// [`VecDeque`]: super::vec_deque::VecDeque
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "LinkedList")]
+#[rustc_insignificant_dtor]
 pub struct LinkedList<T> {
     head: Option<NonNull<Node<T>>>,
     tail: Option<NonNull<Node<T>>>,

alloc/src/collections/vec_deque/mod.rs

@@ -90,6 +90,7 @@ const MAXIMUM_ZST_CAPACITY: usize = 1 << (usize::BITS - 1); // Largest possible
 /// [`make_contiguous`]: VecDeque::make_contiguous
 #[cfg_attr(not(test), rustc_diagnostic_item = "vecdeque_type")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_insignificant_dtor]
 pub struct VecDeque<
     T,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,

alloc/src/rc.rs

@@ -305,6 +305,7 @@ struct RcBox<T: ?Sized> {
 /// [get_mut]: Rc::get_mut
 #[cfg_attr(not(test), rustc_diagnostic_item = "Rc")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_insignificant_dtor]
 pub struct Rc<T: ?Sized> {
     ptr: NonNull<RcBox<T>>,
     phantom: PhantomData<RcBox<T>>,

alloc/src/vec/into_iter.rs

@@ -22,6 +22,7 @@ use core::slice::{self};
 /// let iter: std::vec::IntoIter<_> = v.into_iter();
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_insignificant_dtor]
 pub struct IntoIter<
     T,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,

alloc/src/vec/mod.rs

@@ -396,6 +396,7 @@ mod spec_extend;
 /// [owned slice]: Box
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "vec_type")]
+#[rustc_insignificant_dtor]
 pub struct Vec<T, #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
     buf: RawVec<T, A>,
     len: usize,

core/src/array/iter.rs

@@ -10,6 +10,7 @@ use crate::{
 
 /// A by-value [array] iterator.
 #[stable(feature = "array_value_iter", since = "1.51.0")]
+#[rustc_insignificant_dtor]
 pub struct IntoIter<T, const N: usize> {
     /// This is the array we are iterating over.
     ///

std/src/collections/hash/map.rs

@@ -205,6 +205,7 @@ use crate::sys;
 
 #[cfg_attr(not(test), rustc_diagnostic_item = "hashmap_type")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_insignificant_dtor]
 pub struct HashMap<K, V, S = RandomState> {
     base: base::HashMap<K, V, S>,
 }