Rollup merge of rust-lang#137268 - bjoernager:c-string-eq-c-str, r=Amanieu

Allow comparisons between `CStr`, `CString`, and `Cow<CStr>`.

Closes: rust-lang#137265

This PR adds the trait implementations proposed in the [ACP](rust-lang/libs-team#517) under the `c_string_eq_c_str` feature gate:

```rust
// 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<'_, CStr>> for CString;

// alloc::borrow

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

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

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

As I understand it, stable traits cannot be unstably implemented for stable types, and we would thereby be forced to skip the FCP and directly stabilise these implementations (as is done in this PR).

(`@joshtriplett` mentioned that Crater may have to be run).

Author: GuillaumeGomez

library/alloc/src/ffi/c_str.rs

@@ -1099,6 +1099,46 @@ impl From<&CStr> for CString {
     }
 }
 
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<CStr> for CString {
+    #[inline]
+    fn eq(&self, other: &CStr) -> bool {
+        **self == *other
+    }
+
+    #[inline]
+    fn ne(&self, other: &CStr) -> bool {
+        **self != *other
+    }
+}
+
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<&CStr> for CString {
+    #[inline]
+    fn eq(&self, other: &&CStr) -> bool {
+        **self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &&CStr) -> bool {
+        **self != **other
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<Cow<'_, CStr>> for CString {
+    #[inline]
+    fn eq(&self, other: &Cow<'_, CStr>) -> bool {
+        **self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &Cow<'_, CStr>) -> bool {
+        **self != **other
+    }
+}
+
 #[stable(feature = "cstring_asref", since = "1.7.0")]
 impl ops::Index<ops::RangeFull> for CString {
     type Output = CStr;
@@ -1181,6 +1221,75 @@ impl CStr {
     }
 }
 
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<CString> for CStr {
+    #[inline]
+    fn eq(&self, other: &CString) -> bool {
+        *self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &CString) -> bool {
+        *self != **other
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<Cow<'_, Self>> for CStr {
+    #[inline]
+    fn eq(&self, other: &Cow<'_, Self>) -> bool {
+        *self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &Cow<'_, Self>) -> bool {
+        *self != **other
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<CStr> for Cow<'_, CStr> {
+    #[inline]
+    fn eq(&self, other: &CStr) -> bool {
+        **self == *other
+    }
+
+    #[inline]
+    fn ne(&self, other: &CStr) -> bool {
+        **self != *other
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<&CStr> for Cow<'_, CStr> {
+    #[inline]
+    fn eq(&self, other: &&CStr) -> bool {
+        **self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &&CStr) -> bool {
+        **self != **other
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<CString> for Cow<'_, CStr> {
+    #[inline]
+    fn eq(&self, other: &CString) -> bool {
+        **self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &CString) -> bool {
+        **self != **other
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl core::error::Error for NulError {
     #[allow(deprecated)]

library/core/src/ffi/c_str.rs

@@ -660,6 +660,19 @@ impl CStr {
     }
 }
 
+#[stable(feature = "c_string_eq_c_str", since = "CURRENT_RUSTC_VERSION")]
+impl PartialEq<&Self> for CStr {
+    #[inline]
+    fn eq(&self, other: &&Self) -> bool {
+        *self == **other
+    }
+
+    #[inline]
+    fn ne(&self, other: &&Self) -> bool {
+        *self != **other
+    }
+}
+
 // `.to_bytes()` representations are compared instead of the inner `[c_char]`s,
 // because `c_char` is `i8` (not `u8`) on some platforms.
 // That is why this is implemented manually and not derived.
@@ -670,6 +683,7 @@ impl PartialOrd for CStr {
         self.to_bytes().partial_cmp(&other.to_bytes())
     }
 }
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Ord for CStr {
     #[inline]