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Make Rc<T>::deref zero-cost #141348

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Make Rc<T>::deref zero-cost #141348

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6fdfc76
Make `Rc<T>::deref` zero-cost
EFanZh Oct 17, 2026
cbf6766
Try some optimizations
EFanZh Oct 17, 2026
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2 changes: 2 additions & 0 deletions library/alloc/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -240,6 +240,8 @@ mod raw_vec;
/// Implementations of reference-counted pointers.
#[cfg(not(no_rc))]
mod rcs {
mod common;

pub mod rc;

#[cfg(all(not(no_sync), target_has_atomic = "ptr"))]
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8 changes: 7 additions & 1 deletion library/alloc/src/rcs/arc.rs
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Expand Up @@ -29,7 +29,6 @@ use crate::alloc::{
};
use crate::borrow::{Cow, ToOwned};
use crate::boxed::Box;
use crate::rc::is_dangling;
#[cfg(not(no_global_oom_handling))]
use crate::string::String;
#[cfg(not(no_global_oom_handling))]
Expand All @@ -46,6 +45,13 @@ use crate::vec::Vec;
/// See comment in `Arc::clone`.
const MAX_REFCOUNT: usize = (isize::MAX) as usize;

fn is_dangling<T>(ptr: *const T) -> bool
where
T: ?Sized,
{
ptr.addr() == usize::MAX
}

#[cold]
#[cfg_attr(not(panic = "immediate-abort"), inline(never))]
#[cfg_attr(panic = "immediate-abort", inline)]
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