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Add custom allocators to (try_)map on Box, Rc, Arc #161617

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rust-bors merged 1 commit into from
Aug 25, 2026
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Add custom allocators to (try_)map on Box, Rc, Arc #161617

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147 changes: 71 additions & 76 deletions library/alloc/src/boxed.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -426,82 +426,6 @@ impl<T> Box<T> {
pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
Box::try_new_zeroed_in(Global)
}

/// Maps the value in a box, reusing the allocation if possible.
///
/// `f` is called on the value in the box, and the result is returned, also boxed.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::map(b, f)` instead of `b.map(f)`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// ```
/// #![feature(smart_pointer_try_map)]
///
/// let b = Box::new(7);
/// let new = Box::map(b, |i| i + 7);
/// assert_eq!(*new, 14);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "smart_pointer_try_map", issue = "144419")]
pub fn map<U>(this: Self, f: impl FnOnce(T) -> U) -> Box<U> {
if size_of::<T>() == size_of::<U>() && align_of::<T>() == align_of::<U>() {
let (value, allocation) = Box::take(this);
Box::write(
unsafe { mem::transmute::<Box<MaybeUninit<T>>, Box<MaybeUninit<U>>>(allocation) },
f(value),
)
} else {
Box::new(f(*this))
}
}

/// Attempts to map the value in a box, reusing the allocation if possible.
///
/// `f` is called on the value in the box, and if the operation succeeds, the result is
/// returned, also boxed.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::try_map(b, f)` instead of `b.try_map(f)`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// ```
/// #![feature(smart_pointer_try_map)]
///
/// let b = Box::new(7);
/// let new = Box::try_map(b, u32::try_from).unwrap();
/// assert_eq!(*new, 7);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "smart_pointer_try_map", issue = "144419")]
pub fn try_map<R>(
this: Self,
f: impl FnOnce(T) -> R,
) -> <R::Residual as Residual<Box<R::Output>>>::TryType
where
R: Try,
R::Residual: Residual<Box<R::Output>>,
{
if size_of::<T>() == size_of::<R::Output>() && align_of::<T>() == align_of::<R::Output>() {
let (value, allocation) = Box::take(this);
try {
Box::write(
unsafe {
mem::transmute::<Box<MaybeUninit<T>>, Box<MaybeUninit<R::Output>>>(
allocation,
)
},
f(value)?,
)
}
} else {
try { Box::new(f(*this)?) }
}
}
}

impl<T, A: Allocator> Box<T, A> {
Expand Down Expand Up @@ -776,6 +700,77 @@ impl<T, A: Allocator> Box<T, A> {
(value, uninit)
}
}

/// Maps the value in a box, reusing the allocation if possible.
///
/// `f` is called on the value in the box, and the result is returned, also boxed.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::map(b, f)` instead of `b.map(f)`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// ```
/// #![feature(smart_pointer_try_map)]
///
/// let b = Box::new(7);
/// let new = Box::map(b, |i| i + 7);
/// assert_eq!(*new, 14);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "smart_pointer_try_map", issue = "144419")]
pub fn map<U>(this: Self, f: impl FnOnce(T) -> U) -> Box<U, A> {
let (value, allocation) = Box::take(this);
let (raw, alloc) = Box::into_non_null_with_allocator(allocation);
if size_of::<T>() == size_of::<U>() && align_of::<T>() == align_of::<U>() {
let allocation = unsafe { Box::from_non_null_in(raw.cast::<MaybeUninit<U>>(), alloc) };
Box::write(allocation, f(value))
} else {
unsafe { alloc.deallocate(raw.cast(), Layout::for_value(&value)) }
Box::new_in(f(value), alloc)
}
}

/// Attempts to map the value in a box, reusing the allocation if possible.
///
/// `f` is called on the value in the box, and if the operation succeeds, the result is
/// returned, also boxed.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::try_map(b, f)` instead of `b.try_map(f)`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// ```
/// #![feature(smart_pointer_try_map)]
///
/// let b = Box::new(7);
/// let new = Box::try_map(b, u32::try_from).unwrap();
/// assert_eq!(*new, 7);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "smart_pointer_try_map", issue = "144419")]
pub fn try_map<R>(
this: Self,
f: impl FnOnce(T) -> R,
) -> <R::Residual as Residual<Box<R::Output, A>>>::TryType
where
R: Try,
R::Residual: Residual<Box<R::Output, A>>,
{
let (value, allocation) = Box::take(this);
let (raw, alloc) = Box::into_non_null_with_allocator(allocation);
if size_of::<T>() == size_of::<R::Output>() && align_of::<T>() == align_of::<R::Output>() {
let allocation =
unsafe { Box::from_non_null_in(raw.cast::<MaybeUninit<R::Output>>(), alloc) };
try { Box::write(allocation, f(value)?) }
} else {
unsafe { alloc.deallocate(raw.cast(), Layout::for_value(&value)) }
try { Box::new_in(f(value)?, alloc) }
}
}
}

impl<T: ?Sized + CloneToUninit> Box<T> {
Expand Down
183 changes: 96 additions & 87 deletions library/alloc/src/rc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -651,93 +651,6 @@ impl<T> Rc<T> {
pub fn pin(value: T) -> Pin<Rc<T>> {
unsafe { Pin::new_unchecked(Rc::new(value)) }
}

/// Maps the value in an `Rc`, reusing the allocation if possible.
///
/// `f` is called on a reference to the value in the `Rc`, and the result is returned, also in
/// an `Rc`.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Rc::map(r, f)` instead of `r.map(f)`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// ```
/// #![feature(smart_pointer_try_map)]
///
/// use std::rc::Rc;
///
/// let r = Rc::new(7);
/// let new = Rc::map(r, |i| i + 7);
/// assert_eq!(*new, 14);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "smart_pointer_try_map", issue = "144419")]
pub fn map<U>(this: Self, f: impl FnOnce(&T) -> U) -> Rc<U> {
if size_of::<T>() == size_of::<U>()
&& align_of::<T>() == align_of::<U>()
&& Rc::is_unique(&this)
{
unsafe {
let ptr = Rc::into_raw(this);
let value = ptr.read();
let mut allocation = Rc::from_raw(ptr.cast::<mem::MaybeUninit<U>>());

Rc::get_mut_unchecked(&mut allocation).write(f(&value));
allocation.assume_init()
}
} else {
Rc::new(f(&*this))
}
}

/// Attempts to map the value in an `Rc`, reusing the allocation if possible.
///
/// `f` is called on a reference to the value in the `Rc`, and if the operation succeeds, the
/// result is returned, also in an `Rc`.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Rc::try_map(r, f)` instead of `r.try_map(f)`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// ```
/// #![feature(smart_pointer_try_map)]
///
/// use std::rc::Rc;
///
/// let b = Rc::new(7);
/// let new = Rc::try_map(b, |&i| u32::try_from(i)).unwrap();
/// assert_eq!(*new, 7);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "smart_pointer_try_map", issue = "144419")]
pub fn try_map<R>(
this: Self,
f: impl FnOnce(&T) -> R,
) -> <R::Residual as Residual<Rc<R::Output>>>::TryType
where
R: Try,
R::Residual: Residual<Rc<R::Output>>,
{
if size_of::<T>() == size_of::<R::Output>()
&& align_of::<T>() == align_of::<R::Output>()
&& Rc::is_unique(&this)
{
unsafe {
let ptr = Rc::into_raw(this);
let value = ptr.read();
let mut allocation = Rc::from_raw(ptr.cast::<mem::MaybeUninit<R::Output>>());

Rc::get_mut_unchecked(&mut allocation).write(f(&value)?);
try { allocation.assume_init() }
}
} else {
try { Rc::new(f(&*this)?) }
}
}
}

impl<T, A: Allocator> Rc<T, A> {
Expand Down Expand Up @@ -1107,6 +1020,102 @@ impl<T, A: Allocator> Rc<T, A> {
pub fn into_inner(this: Self) -> Option<T> {
Rc::try_unwrap(this).ok()
}

/// Maps the value in an `Rc`, reusing the allocation if possible.
///
/// `f` is called on a reference to the value in the `Rc`, and the result is returned, also in
/// an `Rc`.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Rc::map(r, f)` instead of `r.map(f)`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// ```
/// #![feature(smart_pointer_try_map)]
///
/// use std::rc::Rc;
///
/// let r = Rc::new(7);
/// let new = Rc::map(r, |i| i + 7);
/// assert_eq!(*new, 14);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "smart_pointer_try_map", issue = "144419")]
pub fn map<U>(this: Self, f: impl FnOnce(&T) -> U) -> Rc<U, A> {
if size_of::<T>() == size_of::<U>()
&& align_of::<T>() == align_of::<U>()
&& Rc::is_unique(&this)
{
unsafe {
let (ptr, alloc) = Rc::into_raw_with_allocator(this);
let value = ptr.read();
let mut allocation = Rc::from_raw_in(ptr.cast::<mem::MaybeUninit<U>>(), alloc);

Rc::get_mut_unchecked(&mut allocation).write(f(&value));
allocation.assume_init()
}
} else {
let output = f(&*this);
let (ptr, alloc) = Rc::into_raw_with_allocator(this);
unsafe { Rc::decrement_strong_count_in(ptr, &alloc) }

Rc::new_in(output, alloc)
}
}

/// Attempts to map the value in an `Rc`, reusing the allocation if possible.
///
/// `f` is called on a reference to the value in the `Rc`, and if the operation succeeds, the
/// result is returned, also in an `Rc`.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Rc::try_map(r, f)` instead of `r.try_map(f)`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// ```
/// #![feature(smart_pointer_try_map)]
///
/// use std::rc::Rc;
///
/// let b = Rc::new(7);
/// let new = Rc::try_map(b, |&i| u32::try_from(i)).unwrap();
/// assert_eq!(*new, 7);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "smart_pointer_try_map", issue = "144419")]
pub fn try_map<R>(
this: Self,
f: impl FnOnce(&T) -> R,
) -> <R::Residual as Residual<Rc<R::Output, A>>>::TryType
where
R: Try,
R::Residual: Residual<Rc<R::Output, A>>,
{
if size_of::<T>() == size_of::<R::Output>()
&& align_of::<T>() == align_of::<R::Output>()
&& Rc::is_unique(&this)
{
unsafe {
let (ptr, alloc) = Rc::into_raw_with_allocator(this);
let value = ptr.read();
let mut allocation =
Rc::from_raw_in(ptr.cast::<mem::MaybeUninit<R::Output>>(), alloc);

Rc::get_mut_unchecked(&mut allocation).write(f(&value)?);
try { allocation.assume_init() }
}
} else {
let output = f(&*this)?;
let (ptr, alloc) = Rc::into_raw_with_allocator(this);
unsafe { Rc::decrement_strong_count_in(ptr, &alloc) }

try { Rc::new_in(output, alloc) }
}
}
}

impl<T> Rc<[T]> {
Expand Down
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