Tracking Issue for const_vec_string_slice

@rustbot

Feature gate: #![feature(const_vec_string_slice)]

This is a tracking issue for making a bunch of String and Vec methods const.

Public API

The following methods are now const:

impl String {
    pub const fn into_bytes(self) -> Vec<u8>;
    pub const fn as_str(&self) -> &str;
    pub const fn capacity(&self) -> usize;
    pub const fn as_bytes(&self) -> &[u8];
    pub const fn len(&self) -> usize;
    pub const fn is_empty(&self) -> bool;
}

impl Vec<T> {
    pub const fn capacity(&self) -> usize;
    pub const fn as_slice(&self) -> &[T];
    pub const fn as_ptr(&self) -> *const T;
    pub const fn len(&self) -> usize;
    pub const fn is_empty(&self) -> bool;
}

A few additional methods are under this feature gate but were not included in the FCP at #129041 (comment); they got FCP'd in #137319 (comment):

impl String {
    pub const fn as_mut_str(&mut self) -> &mut str;
    pub const unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8>;
}

impl Vec<T> {
    pub const fn as_mut_slice(&mut self) -> &mut [T];
    pub const fn as_mut_ptr(&mut self) -> *mut T;
}

Steps / History

Implementation: liballoc: introduce String, Vec const-slicing #128399
Final comment period (FCP)¹
Stabilization PR: Stabilize const_vec_string_slice #137319

Unresolved Questions

None yet.

@rustbot label A-str

https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

This would be very useful for const fns that sometimes need to handle runtime data too, such as a const fn method that needs to use a Cow<'static, [T]> field.

For example:

struct Foo(Cow<'static, [u32]>);

impl Foo {
    // Initialize with heap-allocated data at runtime.
    fn heap(data: Vec<u32>) -> Self {
        Foo(Cow::Owned(data))
    }

    // Initialize with static data at compile-time.
    const fn static(data: &'static [u32]) -> Self {
        Foo(Cow::Borrowed(data))
    }

    // Access the data, possibly at compile-time, possibly at runtime.
    const fn data(&self) -> &[u32] {
        match self.0 {
            Cow::Borrowed(slice) => slice,
            Cow::Owned(vec) => vec.as_slice(),
        }
    }
}

liballoc: introduce String, Vec const-slicing

@rust-lang/libs-api from a const-eval perspective, these functions all look totally harmless. One can't actually construct non-trivial Vec/String in const, but according to the examples above it could still be useful to have these as const fn. So, I propose we move ahead with stabilizing these.

I started a new FCP on #137319.

Stabilize `const_vec_string_slice`

Rollup merge of rust-lang#137319 - Kixunil:stabilize-const-vec-string…

Unrolled build for rust-lang#137319

Stabilize `const_vec_string_slice`

Rollup merge of rust-lang#141467 - cyrgani:const-empty-stringlikes, r…

Unrolled build for #141467

Rollup merge of rust-lang#141467 - cyrgani:const-empty-stringlikes, r…

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Tracking Issue for `const_vec_string_slice` #129041

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Tracking Issue for const_vec_string_slice #129041

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Esper89 commented on Aug 13, 2024

RalfJung commented on Nov 30, 2024

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Amanieu commented on Feb 25, 2025

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Tracking Issue for `const_vec_string_slice` #129041