How to create a boxed slice without using type ascription?

Question

It is easy to create a boxed slice:

fn main() {
    let _ = Box::new([42, 0]);
}

But if I want to add specify a type:

fn main() {
    let _ = Box::<[i32]>::new([42, 0]);
}

I get:

error: no associated item named `new` found for type `std::boxed::Box<[i32]>` in the current scope
  --> src/main.rs:2:13
   |
 2 |     let _ = Box::<[i32]>::new([42, 0]);
   |             ^^^^^^^^^^^^^^^^^
   |
   = note: the method `new` exists but the following trait bounds were not satisfied: `[i32] : std::marker::Sized`

This is really strange, because it works with type ascription:

fn main() {
    let _: Box<[i32]> = Box::new([42, 0]);
}

Shepmaster · Accepted Answer · 2017-07-17T14:20:28.650

6

It is easy to create a boxed slice

Except you didn't do that. If you print the type, you can see that Box::new([1, 2]) creates a boxed array, not a slice. On the other hand, let _: Box<[i32]> = Box::new([1, 2]) creates a boxed array and the converts it to a boxed slice. These have different types. Notably, the first type has a length known at compile time, the second no longer does.

You can use as as well:

fn main() {
    let _ = Box::new([1, 2]) as Box<[i32]>;
}

Box::<[i32]>::new([1, 2]) doesn't work for the reason the error message says: T is not Sized. T is directly mapped to the type being boxed.

Box::new([1, 2]) as Box<[i32]> works because a sized item is passed to Box::new and then it is converted to a type with an unsized inner bit using unsized coercions.

It appears that unsized coercions don't apply for actual type ascription, an unstable feature in nightly Rust:

#![feature(type_ascription)]

fn main() {
    let foo = Box::new([1, 2]): Box<[i32]>;
}

For this specific case, you can use into_boxed_slice, which should avoid the need for any stack allocation.

vec![1, 2].into_boxed_slice();

edited Jul 17 '17 at 14:20

answered Jul 17 '17 at 13:58

Shepmaster

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Is `::::` not type ascription? – Boiethios Jul 17 '17 at 13:59
@Boiethios no, that's a separate feature that is not stabilized. – Shepmaster Jul 17 '17 at 14:04
About the last line, I tried to use the opposite: https://doc.rust-lang.org/std/primitive.slice.html#method.into_vec with an empty slice, *e.g.*: `Box::<[i32]>::new([]).into_vec()` – Boiethios Jul 17 '17 at 14:08
2

Just a terminological point: it seems to be an unsizing coercion and not a `Deref` coercion. – Masaki Hara Jul 17 '17 at 14:11
@MasakiHara thank you! I always screw up the various coercions. – Shepmaster Jul 17 '17 at 14:20
1

@Shepmaster, `as` casts are [superset of coercions](https://doc.rust-lang.org/nomicon/casts.html) and `Box<[T; N]>` can be coerced to `Box<[T]>` because `[T; N]` is `Unsize<[T]>` ([Unsize trait](https://doc.rust-lang.org/std/marker/trait.Unsize.html)) and there exists blanket implementation [`impl, U> CoerceUnsized> for Box`](https://doc.rust-lang.org/std/ops/trait.CoerceUnsized.html) – red75prime Jul 17 '17 at 14:22
Anyway, for now it can be described as compiler magic. [DST coercion RFC](https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md) isn't stable yet. – red75prime Jul 17 '17 at 14:34

score 2 · Answer 2 · answered Jul 17 '17 at 13:49

2

This is an experimental feature:

#![feature(type_ascription)]

fn main() {
    let _ = Box::new([42, 0]: [i32; 2]);
}

Also, in that case you need to be explicit about the length.

answered Jul 17 '17 at 13:49

ljedrz

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1

I was going to answer similarly, but OP is likely just misusing the term "type ascription". They really just mean to use a turbofish. They also want to have a `Box<[i32]>`, not a `Box<[i32; 2]>` — these are different types. – Shepmaster Jul 17 '17 at 13:50
@Shepmaster in that case I don't know if it's possible :) - especially if you want a boxed slice without a specific length. – ljedrz Jul 17 '17 at 13:52
Hum, I think I understand now from your comments. – Boiethios Jul 17 '17 at 13:54

How to create a boxed slice without using type ascription?

2 Answers2