diff --git a/library/core/src/ffi/c_str.rs b/library/core/src/ffi/c_str.rs
index e3a3aed4a794..97831c41dc0f 100644
--- a/library/core/src/ffi/c_str.rs
+++ b/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""
diff --git a/library/coretests/tests/ffi/cstr.rs b/library/coretests/tests/ffi/cstr.rs
index 7d669cc1c3ff..0fb98036b267 100644
--- a/library/coretests/tests/ffi/cstr.rs
+++ b/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