Auto merge of #95183 - ibraheemdev:arc-count-acquire, r=Amanieu

Weaken needlessly restrictive orderings on `Arc::*_count` There is no apparent reason for these to be `SeqCst`. For reference, [the Boost C++ implementation relies on acquire semantics](https://github.com/boostorg/smart_ptr/blob/f2cc84a23c64b8a73c9b72b34799d0854d7e0787/include/boost/smart_ptr/detail/sp_counted_base_std_atomic.hpp#L137-L140).

Author: bors

library/alloc/src/sync.rs

@@ -25,7 +25,7 @@ use core::ptr::{self, NonNull};
 #[cfg(not(no_global_oom_handling))]
 use core::slice::from_raw_parts_mut;
 use core::sync::atomic;
-use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
+use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
 
 #[cfg(not(no_global_oom_handling))]
 use crate::alloc::handle_alloc_error;
@@ -984,7 +984,7 @@ impl<T: ?Sized> Arc<T> {
  #[must_use]
  #[stable(feature = "arc_counts", since = "1.15.0")]
  pub fn weak_count(this: &Self) -> usize {
- let cnt = this.inner().weak.load(SeqCst);
+ let cnt = this.inner().weak.load(Acquire);
  // If the weak count is currently locked, the value of the
  // count was 0 just before taking the lock.
  if cnt == usize::MAX { 0 } else { cnt - 1 }
@@ -1014,7 +1014,7 @@ impl<T: ?Sized> Arc<T> {
  #[must_use]
  #[stable(feature = "arc_counts", since = "1.15.0")]
  pub fn strong_count(this: &Self) -> usize {
- this.inner().strong.load(SeqCst)
+ this.inner().strong.load(Acquire)
  }
 
  /// Increments the strong reference count on the `Arc<T>` associated with the
@@ -1976,7 +1976,7 @@ impl<T: ?Sized> Weak<T> {
  #[must_use]
  #[stable(feature = "weak_counts", since = "1.41.0")]
  pub fn strong_count(&self) -> usize {
- if let Some(inner) = self.inner() { inner.strong.load(SeqCst) } else { 0 }
+ if let Some(inner) = self.inner() { inner.strong.load(Acquire) } else { 0 }
  }
 
  /// Gets an approximation of the number of `Weak` pointers pointing to this
@@ -1995,8 +1995,8 @@ impl<T: ?Sized> Weak<T> {
  pub fn weak_count(&self) -> usize {
  self.inner()
  .map(|inner| {
- let weak = inner.weak.load(SeqCst);
- let strong = inner.strong.load(SeqCst);
+ let weak = inner.weak.load(Acquire);
+ let strong = inner.strong.load(Acquire);
  if strong == 0 {
  0
  } else {