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WIP: smart pointer (try_)map #144420

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80 changes: 78 additions & 2 deletions library/alloc/src/boxed.rs
View file Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -192,10 +192,10 @@ use core::fmt;
use core::future::Future;
use core::hash::{Hash, Hasher};
use core::marker::{Tuple, Unsize};
use core::mem::{self, SizedTypeProperties};
use core::mem::{self, MaybeUninit, SizedTypeProperties};
use core::ops::{
AsyncFn, AsyncFnMut, AsyncFnOnce, CoerceUnsized, Coroutine, CoroutineState, Deref, DerefMut,
DerefPure, DispatchFromDyn, LegacyReceiver,
DerefPure, DispatchFromDyn, LegacyReceiver, Residual, Try,
};
use core::pin::{Pin, PinCoerceUnsized};
use core::ptr::{self, NonNull, Unique};
Expand Down Expand Up @@ -389,6 +389,82 @@ 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);
/// ```
#[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 (allocation, value) = unsafe {
let ptr = Box::into_raw(this);
let value = ptr.read();
let allocation = Box::from_raw(ptr.cast::<MaybeUninit<U>>());
(allocation, value)
};

Box::write(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);
/// ```
#[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 (allocation, value) = unsafe {
let ptr = Box::into_raw(this);
let value = ptr.read();
let allocation = Box::from_raw(ptr.cast::<MaybeUninit<R::Output>>());
(allocation, value)
};
<R::Residual as Residual<Box<R::Output>>>::TryType::from_output(Box::write(
allocation,
f(value)?,
))
} else {
<R::Residual as Residual<Box<R::Output>>>::TryType::from_output(Box::new(f(*this)?))
}
}
}

impl<T, A: Allocator> Box<T, A> {
Expand Down
1 change: 1 addition & 0 deletions library/alloc/src/lib.rs
View file Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -153,6 +153,7 @@
#![feature(trusted_len)]
#![feature(trusted_random_access)]
#![feature(try_trait_v2)]
#![feature(try_trait_v2_residual)]
#![feature(try_with_capacity)]
#![feature(tuple_trait)]
#![feature(ub_checks)]
Expand Down
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