Tracking Issue for const_vec_string_slice

[GrigorenkoPV]

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

Footnotes

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

[Esper89]

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(),
        }
    }
}

[RalfJung]

@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.

[dtolnay]

@rfcbot fcp merge

• https://doc.rust-lang.org/nightly/alloc/string/struct.String.html#method.into_bytes
• https://doc.rust-lang.org/nightly/alloc/string/struct.String.html#method.as_str
• https://doc.rust-lang.org/nightly/alloc/string/struct.String.html#method.capacity
• https://doc.rust-lang.org/nightly/alloc/string/struct.String.html#method.as_bytes
• https://doc.rust-lang.org/nightly/alloc/string/struct.String.html#method.len
• https://doc.rust-lang.org/nightly/alloc/string/struct.String.html#method.is_empty
• https://doc.rust-lang.org/nightly/alloc/vec/struct.Vec.html#method.capacity
• https://doc.rust-lang.org/nightly/alloc/vec/struct.Vec.html#method.as_slice
• https://doc.rust-lang.org/nightly/alloc/vec/struct.Vec.html#method.as_ptr
• https://doc.rust-lang.org/nightly/alloc/vec/struct.Vec.html#method.len
• https://doc.rust-lang.org/nightly/alloc/vec/struct.Vec.html#method.is_empty

[rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[Kixunil]

The issue description is missing checkmark in FCP.

[tgross35]

As @RalfJung mentioned on the PR, there are a few methods gated by this feature that were not mentioned in the tracking issue or FCP links:

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;
}

Libs-api could you take a look at these?

@rustbot label +I-libs-api-nominated

[Amanieu]

I started a new FCP on #137319.