Tracking Issue for `str::as_str()` & friends

[GrigorenkoPV]

Feature gate: #![feature(str_as_str)]

This is a tracking issue for the as_str() method on the str type itself, so that Box<str>, Rc<str>, Arc<str> & others can have the same things as String has.

Later also expanded to some other types (see below).

Public API

impl str {
    pub const fn as_str(&self) -> &str;
}
impl CStr {
    pub const fn as_c_str(&self) -> &CStr;
}
impl OsStr {
    pub const fn as_os_str(&self) -> &OsStr;
}
impl Path {
    pub const fn as_path(&self) -> &Path;
}
impl [T] {
    pub const fn as_slice(&self) -> &[T];
    pub const fn as_mut_slice(&mut self) -> &mut [T];
}
impl ByteStr {
    pub const fn as_byte_str(&self) -> &ByteStr;
    pub const fn as_mut_byte_str(&mut self) -> &mut ByteStr;
}

• Tracking issue for ByteStr: Tracking Issue for ByteStr/ByteString #134915

Steps / History

• Implementation:
  • for str: Add str.as_str() for easy Deref to string slices #129550
    • t-libs-api's approval: Add str.as_str() for easy Deref to string slices #129550 (comment)
  • constify: constify (the unstable) str::as_str #145930
  • more types: Expand str_as_str to more types #145933
    • ACP: str::as_str but for CStr, OsStr, Path, and [T] libs-team#643
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• None yet.

Footnotes

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

[Noratrieb]

i don't think this is the correct API from the PR

[GrigorenkoPV]

i don't think this is the correct API from the PR

No, but this is what it will presumably get changed to: #129550 (comment)

[kornelski]

While this method is unstable, it might trip people up. If this causes a compilation error:

error[E0658]: use of unstable library feature 'str_as_str'

The solution is to change string.as_str() to string.as_ref(), or to &**string(adjusting the number of *s as required).

[GrigorenkoPV]

The solution is to change string.as_str() to string.as_ref(), or to &**string(adjusting the number of *s as required).

I am firmly against recommending to use .as_ref() on anything other than T: AsRef<U>. To quote the docs,

Ideally, AsRef would be reflexive, i.e. there would be an impl<T: ?Sized> AsRef<T> for T with as_ref simply returning its argument unchanged. Such a blanket implementation is currently not provided due to technical restrictions of Rust's type system (it would be overlapping with another existing blanket implementation for &T where T: AsRef<U> which allows AsRef to auto-dereference, see "Generic Implementations" above).

To combat this limitation, some concrete impls were added, such as AsRef<str> for String>, AsRef<OsStr> for str, etc. But notably AsRef<[T; N]> for [T; N] was not one of them (presumably because const-generics were not a thing back in 1.0). So now [T; N].as_ref() desugars to <[T; N] as Deref<[T]>>.deref().as_ref() and functions identically to .as_slice().

Which means that now we cannot even add AsMut/AsRef<[T; N]> for [T; N] without repeating the whole IntoIterator for [T; N] story with edition-dependent method resolution.

I guess there already is quite a lot of code out there that will break in the face of AsRef<T> for T and it will be quite an effort to resolve this when it's time to add the impl, but for now let's at least not guide people towards the potential future breakage.

[aleexharris]

Worth noting that this is relevant if you want to use the stdlib to efficiently handle environment variables. I like to use the pattern:

use std::sync::{Arc, LazyLock}};

static VAR: LazyLock<Arc<str>> = LazyLock::new(|| std::env::var("VAR").expect("VAR must be set").into());

However the one downside of this pattern is that some libraries will only define From or Into for &str and String, thus requiring a tidy way for your to deference the Arc<str>. This method would be a nice, explicit way of doing that.

[clarfonthey]

FWIW, it would be nice to have slice::as_slice too. And for completeness, we could add:

• CStr::as_c_str
• OsStr::as_os_str
• Path::as_path

[neeko-cat]

Also useful for Cow<'_, str> :)

[clarfonthey]

(Cow<'_, str> automatically can use str::as_str since it implements Deref<Target = str>)