Tracking issue for string-like `AsRef` coverage

[thaliaarchi]

Almost all of the implementations of AsRef in core and std are for cheap conversions between string-like types. This tracks all of these such implementations and coordinates efforts to expand coverage.

Each column represents a type that can be converted to and each column represents a type that can be converted from. In the tables, ✅ marks an existing impl, ⚠️ marks an impl that I argue should be added, N/A marks an impl that does not make sense for that pair of types, and N/A? marks an impl that may make sense (via as_encoded_bytes). Impls that are from a more generic impl are marked with a footnote.

impl AsRef<T> for U	AsRef​<str>	AsRef​<OsStr>	AsRef​<Path>	AsRef​<ByteStr>	AsRef​<[u8]>
str	✅	✅	✅	✅	✅
String	✅	✅	✅	⚠️	✅
Cow<'_, str>	✅1	⚠️	⚠️	⚠️	⚠️
Box<str>	✅2	⚠️	⚠️	⚠️	⚠️
Rc<str>	✅3	⚠️	⚠️	⚠️	⚠️
Arc<str>	✅4	⚠️	⚠️	⚠️	⚠️
UniqueRc<str>	✅5	⚠️	⚠️	⚠️	⚠️
std::string::Drain<'_>	✅	⚠️	⚠️	⚠️	✅
OsStr	N/A	✅	✅	N/A?	N/A?
OsString	N/A	✅	✅	N/A?	N/A?
Cow<'_, OsStr>	N/A	✅1	✅	N/A?	N/A?
Box<OsStr>	N/A	✅2	⚠️	N/A?	N/A?
Rc<OsStr>	N/A	✅3	⚠️	N/A?	N/A?
Arc<OsStr>	N/A	✅4	⚠️	N/A?	N/A?
UniqueRc<OsStr>	N/A	✅5	⚠️	N/A?	N/A?
Path	N/A	✅	✅	N/A?	N/A?
PathBuf	N/A	✅	✅	N/A?	N/A?
Cow<'_, Path>	N/A	⚠️#139432	✅1	N/A?	N/A?
Box<Path>	N/A	⚠️	✅2	N/A?	N/A?
Rc<Path>	N/A	⚠️	✅3	N/A?	N/A?
Arc<Path>	N/A	⚠️	✅4	N/A?	N/A?
UniqueRc<Path>	N/A	⚠️	✅5	N/A?	N/A?
std::path::Component<'_>	N/A	✅	✅	N/A?	N/A?
std::path::Components<'_>	N/A	✅	✅	N/A?	N/A?
std::path::Iter<'_>	N/A	✅	✅	N/A?	N/A?
ByteStr	N/A	N/A	N/A	✅	✅
ByteString	N/A	N/A	N/A	✅	✅
Cow<'_, ByteStr>	N/A	N/A	N/A	✅1	⚠️
Box<ByteStr>	N/A	N/A	N/A	✅2	⚠️
Rc<ByteStr>	N/A	N/A	N/A	✅3	⚠️
Arc<ByteStr>	N/A	N/A	N/A	✅4	⚠️
UniqueRc<ByteStr>	N/A	N/A	N/A	✅5	⚠️
[u8]	N/A	N/A	N/A	⚠️#139441	✅6
[u8; N]	N/A	N/A	N/A	⚠️	✅7
Vec<u8>	N/A	N/A	N/A	⚠️	✅8
Cow<'_, [u8]>	N/A	N/A	N/A	⚠️	✅1
Box<[u8]>	N/A	N/A	N/A	⚠️	✅2
Rc<[u8]>	N/A	N/A	N/A	⚠️	✅3
Arc<[u8]>	N/A	N/A	N/A	⚠️	✅4
UniqueRc<[u8]>	N/A	N/A	N/A	⚠️	✅5
std::slice::Iter<'_, u8>	N/A	N/A	N/A	⚠️	✅9
std::slice::IterMut<'_, u8>	N/A	N/A	N/A	⚠️	✅10
std::vec::IntoIter<u8>	N/A	N/A	N/A	⚠️	✅11
std::vec::Drain<'_, u8>	N/A	N/A	N/A	⚠️	✅12
Simd<u8, N>	N/A	N/A	N/A	⚠️	✅13

Although CStr is string-like, its AsStr impls do not interact with other string-like types:

impl AsRef<CStr> for T	AsRef<CStr>
CStr	✅
CString	✅
Cow<'_, CStr>	✅1
Box<CStr>	✅2
Rc<CStr>	✅3
Arc<CStr>	✅4
UniqueRc<CStr>	✅5

Additionally, all of the above impls may be used with any number of references:

impl<T: AsRef<U> + ?Sized, U: ?Sized> AsRef<U> for &T
impl<T: AsRef<U> + ?Sized, U: ?Sized> AsRef<U> for &mut T

Steps

These are the steps I plan to break this effort into:

• Implement AsRef<OsStr> for Cow<'_, Path> #139432
• Implement AsRef<ByteStr> for [u8] #139441
• Implement AsRef<{OsStr, Path}> for std::string::Drain<'_>.
• Bring AsRef<ByteStr> up to parity with AsRef<[u8]> and implement AsRef<[u8]> for Cow<'_, ByteStr>.
• Implement AsRef for Box, Rc, Arc, and UniqueRc containers on non-self types, as marked in the table.
  • Add some AsRef implementations for smart pointers  #139318

@rustbot claim

@rustbot label +T-libs-api

Footnotes

1. Implemented via impl<T: ToOwned + ?Sized> AsRef<T> for Cow<'_, T> ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

2. Implemented via impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

3. Implemented via impl<T: ?Sized, A: Allocator> AsRef<T> for Rc<T, A> ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

4. Implemented via impl<T: ?Sized, A: Allocator> AsRef<T> for Arc<T, A> ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

5. Implemented via impl<T: ?Sized, A: Allocator> AsRef<T> for UniqueRc<T, A> ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

6. Implemented via impl<T> AsRef<[T]> for [T] ↩

7. Implemented via impl<T, const N: usize> AsRef<[T]> for [T; N] ↩

8. Implemented via impl<T, A: Allocator> AsRef<[T]> for Vec<T, A> ↩

9. Implemented via impl<T> AsRef<[T]> for std::slice::Iter<'_, T> ↩

10. Implemented via impl<T> AsRef<[T]> for std::slice::IterMut<'_, T> ↩

11. Implemented via impl<T, A: Allocator> AsRef<[T]> for std::vec::IntoIter<T, A> ↩

12. Implemented via impl<T, A: Allocator> AsRef<[T]> for std::vec::Drain<'_, T, A> ↩

13. Implemented via impl<T: SimdElement, const N: usize> AsRef<[T]> for Simd<T, N> where LaneCount<N>: SupportedLaneCount ↩

[MatthijsKok]

Nice work on the tracking issue!
It's super helpful that this information is nicely compiled in one place.

I've recently opened a PR that covers some of these, please check out #139318
Specifically:

• impl AsRef<[u8]> for Box<str>
• impl AsRef<[u8]> for Rc<str>
• impl AsRef<OsStr> for Box<str>
• impl AsRef<OsStr> for Rc<str>
• impl AsRef<Path> for Box<str>
• impl AsRef<Path> for Rc<str>

[thaliaarchi]

@MatthijsKok Thanks for the PR. Let’s pause at these three PRs and wait for decisions on them, before pursuing more. I think, due to the high number of inference failures, they are unlikely to be merged, although I wish that wasn’t the case.