Rollup merge of #155564 - tshakalekholoane:tshaka/const_default/cstr, r=jhpratt

Implement const Default for &CStr

Implements `const Default` for `&CStr`.

Tracking issue: rust-lang/rust#143894.

library/core/src/ffi/c_str.rs

@@ -176,7 +176,8 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 }
 
 #[stable(feature = "cstr_default", since = "1.10.0")]
-impl Default for &CStr {
+#[rustc_const_unstable(feature = "const_default", issue = "143894")]
+impl const Default for &CStr {
     #[inline]
     fn default() -> Self {
         c""

library/coretests/tests/ffi/cstr.rs

@@ -1,5 +1,11 @@
 use core::ffi::CStr;
 
+#[test]
+fn const_default() {
+    const S: &CStr = <_>::default();
+    assert_eq!(S, c"");
+}
+
 #[test]
 fn compares_as_u8s() {
     let a: &CStr = c"Hello!"; // Starts with ascii