impl Display for CStr{,ing}

library/alloc/src/ffi/c_str.rs

@@ -716,6 +716,15 @@ impl fmt::Debug for CString {
     }
 }
 
+/// Delegates to the [`CStr`] implementation of [`fmt::Display`],
+/// showing invalid UTF-8 as the Unicode replacement character.
+#[stable(feature = "cstr_display", since = "CURRENT_RUSTC_VERSION")]
+impl fmt::Display for CString {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self.as_c_str(), f)
+    }
+}
+
 #[stable(feature = "cstring_into", since = "1.7.0")]
 impl From<CString> for Vec<u8> {
     /// Converts a [`CString`] into a <code>[Vec]<[u8]></code>.

library/core/src/ffi/c_str.rs

@@ -168,6 +168,15 @@ impl fmt::Debug for CStr {
     }
 }
 
+/// Behaves as if `self` were first lossily converted to a `str`, with
+/// invalid UTF-8 presented as the Unicode replacement character: �.
+#[stable(feature = "cstr_display", since = "CURRENT_RUSTC_VERSION")]
+impl fmt::Display for CStr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(crate::bstr::ByteStr::from_bytes(self.to_bytes()), f)
+    }
+}
+
 #[stable(feature = "cstr_default", since = "1.10.0")]
 impl Default for &CStr {
     #[inline]

library/coretests/tests/ffi/cstr.rs

@@ -19,3 +19,9 @@ fn debug() {
     let s = c"abc\x01\x02\n\xE2\x80\xA6\xFF";
     assert_eq!(format!("{s:?}"), r#""abc\x01\x02\n\xe2\x80\xa6\xff""#);
 }
+
+#[test]
+fn display() {
+    let s = c"\xf0\x28\x8c\xbc";
+    assert_eq!(format!("{s}"), "�(��");
+}