Const Box<str>::new()

libs
1

How best to create an empty Box<str> in a const context? This is what I'm currently doing:

#![feature(allocator_api)]
#![feature(const_box)]
#![feature(ptr_internals)]

use std::{alloc::Global, ptr::Unique};

#[inline]
pub const fn box_str_new() -> Box<str> {
    // from Default for Box<str> https://github.com/rust-lang/rust/blob/ca9ff83f1be558d4ce0a9a49b3d4d25034e5bd1e/library/alloc/src/boxed.rs#L1272-L1280
    unsafe {
        let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
        Box::from_raw_in(bytes.as_ptr() as *mut str, Global)
    }
}

https://play.rust-lang.org/?version=nightly&edition=2021&gist=0e5795358efcd1d62be32e0670b14dac

The features have recently started raising "using it is strongly discouraged" warnings. If there's no better alternative, should <Box<str>>::new() or similar be added? It would align with String::new() and Vec::new() which are both const. Or is this workaround adequate until <Box<str>>::default() is callable in a const context?

2

You can use NonNull instead of Unique to create the dangling pointer, as documented in the Box memory layout section (paragraph 2).

3

An alternative could be using the good ol' transmute:

pub const fn box_str_new() -> Box<str> {
    unsafe { std::mem::transmute("") }
}

pub fn main() {
    const B: Box<str> = box_str_new();
    println!("empty string: '{}'", B.as_ref());
}

Quick testing with miri did not flag this as illegal, and this even works on stable.

4

Transmute is currently only documented as working for Sized types. str is not a sized type, so we are allowed to change the layout of Box<str> to be different from &str. Miri doesn't flag anything regarding layout guarantees. Only if there is currently a layout mismatch.

3 Likes
5

It would be possible for std to have some sort of const fn new_str() -> Box<str> associated function on Box that doesn't allocate, but I don't know of any tools that std gives you today to safely do this.

Depending on your use case, can you use an Option<Box<str>> instead? You'll be able to initialize that to None at compile time, and it looks like you might already be using empty string as an "uninitialized" state in the code you posted.

6

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