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Define a TryClone trait for cloning with fallible allocation #12519

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Merged
fitzgen merged 2 commits into from
Feb 4, 2026
+117 −3
Merged

Define a TryClone trait for cloning with fallible allocation #12519

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ce58523
Define a `TryClone` trait for cloning with fallible allocation
fitzgen Feb 4, 2026
b09c3f7
add reason to `allow` annotation
fitzgen Feb 4, 2026
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2 changes: 2 additions & 0 deletions crates/core/src/alloc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@

mod arc;
mod boxed;
mod try_clone;
mod try_collect;
mod try_new;
mod vec;
Expand All @@ -11,6 +12,7 @@ pub use boxed::{
new_boxed_slice_from_fallible_iter, new_boxed_slice_from_iter,
new_boxed_slice_from_iter_with_len, new_uninit_boxed_slice,
};
pub use try_clone::TryClone;
pub use try_collect::{TryCollect, TryExtend, TryFromIterator};
pub use try_new::{TryNew, try_new};
pub use vec::Vec;
Expand Down
27 changes: 26 additions & 1 deletion crates/core/src/alloc/boxed.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
use super::{TryNew, Vec, try_alloc};
use super::{TryClone, TryNew, Vec, try_alloc};
use crate::error::OutOfMemory;
use core::{
alloc::Layout,
Expand Down Expand Up @@ -42,6 +42,31 @@ impl<T> TryNew for Box<T> {
}
}

impl<T> TryClone for Box<T>
where
T: TryClone,
{
fn try_clone(&self) -> Result<Self, OutOfMemory> {
let b = new_uninit_box::<T>()?;
let v = (**self).try_clone()?;
Ok(Box::write(b, v))
}
}

impl<T> TryClone for Box<[T]>
where
T: TryClone,
{
fn try_clone(&self) -> Result<Self, OutOfMemory> {
let mut builder = BoxedSliceBuilder::new(self.len())?;
for v in &*self {
builder.push(v.try_clone()?).expect("reserved capacity");
}
debug_assert_eq!(builder.init_len(), builder.capacity());
Ok(builder.finish())
}
}

/// Allocate a new `Box<[MaybeUninit<T>]>` of the given length with
/// uninitialized contents, returning `Err(OutOfMemory)` on allocation failure.
///
Expand Down
74 changes: 74 additions & 0 deletions crates/core/src/alloc/try_clone.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,74 @@
use crate::error::OutOfMemory;

/// A trait for values that can be cloned, but contain owned, heap-allocated
/// values whose allocations may fail during cloning.
pub trait TryClone: Sized {
/// Attempt to clone `self`, returning an error if any allocation fails
/// during cloning.
fn try_clone(&self) -> Result<Self, OutOfMemory>;
}

impl<T> TryClone for *mut T {
#[inline]
fn try_clone(&self) -> Result<Self, OutOfMemory> {
Ok(*self)
}
}

impl<T> TryClone for core::ptr::NonNull<T> {
#[inline]
fn try_clone(&self) -> Result<Self, OutOfMemory> {
Ok(*self)
}
}

macro_rules! impl_try_clone_via_clone {
( $( $ty:ty ),* $(,)? ) => {
$(
impl TryClone for $ty {
#[inline]
fn try_clone(&self) -> Result<Self, OutOfMemory> {
Ok(self.clone())
}
}
)*
};
}

impl_try_clone_via_clone! {
bool, char,
u8, u16, u32, u64, u128, usize,
i8, i16, i32, i64, i128, isize,
f32, f64,
}

macro_rules! tuples {
( $( $( $t:ident ),* );*) => {
$(
impl<$($t),*> TryClone for ( $($t,)* )
where
$( $t: TryClone ),*
{
#[inline]
fn try_clone(&self) -> Result<Self, OutOfMemory> {
#[allow(non_snake_case, reason = "macro code")]
let ( $($t,)* ) = self;
Ok(( $( $t.try_clone()?, )* ))
}
}
)*
};
}

tuples! {
A;
A, B;
A, B, C;
A, B, C, D;
A, B, C, D, E;
A, B, C, D, E, F;
A, B, C, D, E, F, G;
A, B, C, D, E, F, G, H;
A, B, C, D, E, F, G, H, I;
A, B, C, D, E, F, G, H, I, J;
}
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@alexcrichton alexcrichton Feb 4, 2026

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Could this perhaps be impl<T: Copy> TryClone for T?

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@fitzgen fitzgen Feb 4, 2026

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FWIW, we would still need these implementations for tuples even with that blanket impl, so that something like (Box<usize>, Vec<u8>), which is not Copy, could still implement TryClone.

I did try to implement TryClone for all T: Copy and ran into conflicting trait impls. I guess because (T0, T1) might be Copy and Rust doesn't know whether to use the blanket Copy impl or the where T0: TryClone, T1: TryClone impl?

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@fitzgen fitzgen Feb 4, 2026

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(Enqueuing to merge now, if you know of a way to resolve this issue, we can do it in a follow up.)

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@alexcrichton alexcrichton Feb 4, 2026

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Ah yeah I thought coherence wouldn't trip up here but it's definitely tripping up, so definitely not possible

15 changes: 14 additions & 1 deletion crates/core/src/alloc/vec.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
use crate::alloc::try_realloc;
use crate::alloc::{TryClone, try_realloc};
use crate::error::OutOfMemory;
use core::{
fmt, mem,
Expand Down Expand Up @@ -29,6 +29,19 @@ impl<T: fmt::Debug> fmt::Debug for Vec<T> {
}
}

impl<T> TryClone for Vec<T>
where
T: TryClone,
{
fn try_clone(&self) -> Result<Self, OutOfMemory> {
let mut v = Vec::with_capacity(self.len())?;
for x in self {
v.push(x.try_clone()?).expect("reserved capacity");
}
Ok(v)
}
}

impl<T> Vec<T> {
/// Same as [`std::vec::Vec::new`].
pub fn new() -> Self {
Expand Down
2 changes: 1 addition & 1 deletion crates/environ/src/collections.rs
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Expand Up @@ -9,7 +9,7 @@ pub use entity_set::EntitySet;
pub use hash_set::HashSet;
pub use primary_map::PrimaryMap;
pub use secondary_map::SecondaryMap;
pub use wasmtime_core::alloc::{TryNew, Vec, try_new};
pub use wasmtime_core::alloc::{TryClone, TryNew, Vec, try_new};

/// Collections which abort on OOM.
//
Expand Down