Rollup merge of #89726 - jkugelman:must-use-alloc-constructors, r=joshtriplett

Add #[must_use] to alloc constructors

Added `#[must_use]`. to the various forms of `new`, `pin`, and `with_capacity` in the `alloc` crate. No extra explanations given as I couldn't think of anything useful to add.

I figure this deserves extra scrutiny compared to the other PRs I've done so far. In particular:

* The 4 `pin`/`pin_in` methods I touched. Are there legitimate use cases for pinning and not using the result? Pinning's a difficult concept I'm not very comfortable with.
* `Box`'s constructors. Do people ever create boxes just for the side effects... allocating or zeroing out memory?

Parent issue: #89692

r? ``@joshtriplett``

library/alloc/src/boxed.rs

@@ -187,6 +187,7 @@ impl<T> Box<T> {
     #[cfg(not(no_global_oom_handling))]
     #[inline(always)]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn new(x: T) -> Self {
         box x
     }
@@ -211,6 +212,7 @@ impl<T> Box<T> {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     #[inline]
     pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
         Self::new_uninit_in(Global)
@@ -237,6 +239,7 @@ impl<T> Box<T> {
     #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
         Self::new_zeroed_in(Global)
     }
@@ -245,6 +248,7 @@ impl<T> Box<T> {
     /// `x` will be pinned in memory and unable to be moved.
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "pin", since = "1.33.0")]
+    #[must_use]
     #[inline(always)]
     pub fn pin(x: T) -> Pin<Box<T>> {
         (box x).into()
@@ -339,6 +343,7 @@ impl<T, A: Allocator> Box<T, A> {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[must_use]
     #[inline]
     pub fn new_in(x: T, alloc: A) -> Self {
         let mut boxed = Self::new_uninit_in(alloc);
@@ -395,6 +400,7 @@ impl<T, A: Allocator> Box<T, A> {
     /// ```
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[cfg(not(no_global_oom_handling))]
+    #[must_use]
     // #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
@@ -459,6 +465,7 @@ impl<T, A: Allocator> Box<T, A> {
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[cfg(not(no_global_oom_handling))]
     // #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
@@ -503,6 +510,7 @@ impl<T, A: Allocator> Box<T, A> {
     /// `x` will be pinned in memory and unable to be moved.
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[must_use]
     #[inline(always)]
     pub fn pin_in(x: T, alloc: A) -> Pin<Self>
     where
@@ -561,6 +569,7 @@ impl<T> Box<[T]> {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
         unsafe { RawVec::with_capacity(len).into_box(len) }
     }
@@ -585,6 +594,7 @@ impl<T> Box<[T]> {
     /// [zeroed]: mem::MaybeUninit::zeroed
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
         unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
     }
@@ -681,6 +691,7 @@ impl<T, A: Allocator> Box<[T], A> {
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
         unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
     }
@@ -708,6 +719,7 @@ impl<T, A: Allocator> Box<[T], A> {
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
         unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
     }

library/alloc/src/collections/binary_heap.rs

@@ -364,6 +364,7 @@ impl<T: Ord> BinaryHeap<T> {
     /// heap.push(4);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn new() -> BinaryHeap<T> {
         BinaryHeap { data: vec![] }
     }
@@ -383,6 +384,7 @@ impl<T: Ord> BinaryHeap<T> {
     /// heap.push(4);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn with_capacity(capacity: usize) -> BinaryHeap<T> {
         BinaryHeap { data: Vec::with_capacity(capacity) }
     }

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

@@ -502,6 +502,7 @@ impl<K, V> BTreeMap<K, V> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
+    #[must_use]
     pub const fn new() -> BTreeMap<K, V> {
         BTreeMap { root: None, length: 0 }
     }

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

@@ -248,6 +248,7 @@ impl<T> BTreeSet<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
+    #[must_use]
     pub const fn new() -> BTreeSet<T> {
         BTreeSet { map: BTreeMap::new() }
     }

library/alloc/src/collections/linked_list.rs

@@ -417,6 +417,7 @@ impl<T> LinkedList<T> {
     #[inline]
     #[rustc_const_stable(feature = "const_linked_list_new", since = "1.32.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub const fn new() -> Self {
         LinkedList { head: None, tail: None, len: 0, marker: PhantomData }
     }

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

@@ -475,6 +475,7 @@ impl<T> VecDeque<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn new() -> VecDeque<T> {
         VecDeque::new_in(Global)
     }
@@ -490,6 +491,7 @@ impl<T> VecDeque<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn with_capacity(capacity: usize) -> VecDeque<T> {
         Self::with_capacity_in(capacity, Global)
     }

library/alloc/src/raw_vec.rs

@@ -69,6 +69,7 @@ impl<T> RawVec<T, Global> {
     /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
     /// `RawVec` with capacity `usize::MAX`. Useful for implementing
     /// delayed allocation.
+    #[must_use]
     pub const fn new() -> Self {
         Self::new_in(Global)
     }
@@ -87,6 +88,7 @@ impl<T> RawVec<T, Global> {
     ///
     /// Aborts on OOM.
     #[cfg(not(no_global_oom_handling))]
+    #[must_use]
     #[inline]
     pub fn with_capacity(capacity: usize) -> Self {
         Self::with_capacity_in(capacity, Global)
@@ -94,6 +96,7 @@ impl<T> RawVec<T, Global> {
 
     /// Like `with_capacity`, but guarantees the buffer is zeroed.
     #[cfg(not(no_global_oom_handling))]
+    #[must_use]
     #[inline]
     pub fn with_capacity_zeroed(capacity: usize) -> Self {
         Self::with_capacity_zeroed_in(capacity, Global)

library/alloc/src/rc.rs

@@ -452,6 +452,7 @@ impl<T> Rc<T> {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
         unsafe {
             Rc::from_ptr(Rc::allocate_for_layout(
@@ -484,6 +485,7 @@ impl<T> Rc<T> {
     /// [zeroed]: mem::MaybeUninit::zeroed
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_zeroed() -> Rc<mem::MaybeUninit<T>> {
         unsafe {
             Rc::from_ptr(Rc::allocate_for_layout(
@@ -587,6 +589,7 @@ impl<T> Rc<T> {
     /// `value` will be pinned in memory and unable to be moved.
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "pin", since = "1.33.0")]
+    #[must_use]
     pub fn pin(value: T) -> Pin<Rc<T>> {
         unsafe { Pin::new_unchecked(Rc::new(value)) }
     }
@@ -658,6 +661,7 @@ impl<T> Rc<[T]> {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
         unsafe { Rc::from_ptr(Rc::allocate_for_slice(len)) }
     }
@@ -684,6 +688,7 @@ impl<T> Rc<[T]> {
     /// [zeroed]: mem::MaybeUninit::zeroed
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_zeroed_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
         unsafe {
             Rc::from_ptr(Rc::allocate_for_layout(
@@ -2044,6 +2049,7 @@ impl<T> Weak<T> {
     /// assert!(empty.upgrade().is_none());
     /// ```
     #[stable(feature = "downgraded_weak", since = "1.10.0")]
+    #[must_use]
     pub fn new() -> Weak<T> {
         Weak { ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0") }
     }

library/alloc/src/string.rs

@@ -378,6 +378,7 @@ impl String {
     #[inline]
     #[rustc_const_stable(feature = "const_string_new", since = "1.39.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub const fn new() -> String {
         String { vec: Vec::new() }
     }
@@ -422,6 +423,7 @@ impl String {
     #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn with_capacity(capacity: usize) -> String {
         String { vec: Vec::with_capacity(capacity) }
     }

library/alloc/src/sync.rs

@@ -448,6 +448,7 @@ impl<T> Arc<T> {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
         unsafe {
             Arc::from_ptr(Arc::allocate_for_layout(
@@ -480,6 +481,7 @@ impl<T> Arc<T> {
     /// [zeroed]: mem::MaybeUninit::zeroed
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
         unsafe {
             Arc::from_ptr(Arc::allocate_for_layout(
@@ -494,6 +496,7 @@ impl<T> Arc<T> {
     /// `data` will be pinned in memory and unable to be moved.
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "pin", since = "1.33.0")]
+    #[must_use]
     pub fn pin(data: T) -> Pin<Arc<T>> {
         unsafe { Pin::new_unchecked(Arc::new(data)) }
     }
@@ -662,6 +665,7 @@ impl<T> Arc<[T]> {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
         unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
     }
@@ -688,6 +692,7 @@ impl<T> Arc<[T]> {
     /// [zeroed]: mem::MaybeUninit::zeroed
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use]
     pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
         unsafe {
             Arc::from_ptr(Arc::allocate_for_layout(
@@ -1680,6 +1685,7 @@ impl<T> Weak<T> {
     /// assert!(empty.upgrade().is_none());
     /// ```
     #[stable(feature = "downgraded_weak", since = "1.10.0")]
+    #[must_use]
     pub fn new() -> Weak<T> {
         Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
     }

library/alloc/src/vec/mod.rs

@@ -420,6 +420,7 @@ impl<T> Vec<T> {
     #[inline]
     #[rustc_const_stable(feature = "const_vec_new", since = "1.39.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub const fn new() -> Self {
         Vec { buf: RawVec::NEW, len: 0 }
     }
@@ -464,6 +465,7 @@ impl<T> Vec<T> {
     #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn with_capacity(capacity: usize) -> Self {
         Self::with_capacity_in(capacity, Global)
     }

src/test/ui/weak-new-uninhabited-issue-48493.rs

@@ -2,6 +2,6 @@
 
 fn main() {
     enum Void {}
-    std::rc::Weak::<Void>::new();
-    std::sync::Weak::<Void>::new();
+    let _ = std::rc::Weak::<Void>::new();
+    let _ = std::sync::Weak::<Void>::new();
 }