ACP: Allow comparisons between CString and CStr.

[bjoernager]

Proposal

Problem statement

alloc::ffi::CString is very similar to alloc::string::String, and the same goes for core::ffi::CStr when compared to str. But the FFI types do differ a bit with regard to the relationship between the owned and borrowed types.

Currently, str, String, and alloc::borrow::Cow<'a, str> all implement PartialEq in the following variants:

• PartialEq<str>
• PartialEq<&'b str>
• PartialEq<String>
• PartialEq<Cow<'b, str>>

CString and CStr, on the other hand, only implement the default PartialEq<Self>.

Motivating examples or use cases

Comparing a CString with a CStr currently requires converting either of the objects to the other's type:

let s0: CString = todo!();
let s1: &CStr   = todo!();

assert_eq!(s0,          s1.to_owned());
assert_eq!(s0.as_ref(), s1);
// Etc.

Implementing this proposal would eliminate the need for the ToOwned::to_owned or AsRef::as_ref call, as well as increase the overall parity with String / str.

Solution sketch

Add the following implementations to the standard library:

// core::ffi

impl PartialEq<&Self> for CStr;

impl PartialEq<CString> for CStr;

impl PartialEq<Cow<'_, Self>> for CStr;

// alloc::ffi

impl PartialEq<CStr> for CString;

impl PartialEq<&CStr> for CString;

impl PartialEq<Cow<'_, Self>> for CString;

// alloc::borrow

impl PartialEq<CStr> for Cow<'_, CStr>;

impl<'a, 'b> PartialEq<&'b CStr> for Cow<'a, CStr>;

impl PartialEq<CString> for Cow<'_, CStr>;

[joshtriplett]

We discussed this in today's @rust-lang/libs-api meeting. We're happy to see these added, but they'll need a crater run to make sure they don't cause too many inference issues in the ecosystem.