diff --git a/library/core/src/ffi/c_str.rs b/library/core/src/ffi/c_str.rs
index 288bac67f75..82e5fa75ded 100644
--- a/library/core/src/ffi/c_str.rs
+++ b/library/core/src/ffi/c_str.rs
@@ -320,8 +320,9 @@ fn strlen_rt(s: *const c_char) -> usize {
     /// assert_eq!(c_str.to_str().unwrap(), "AAAAAAAA");
     /// ```
     ///
+    #[rustc_allow_const_fn_unstable(const_slice_index)]
     #[stable(feature = "cstr_from_bytes_until_nul", since = "CURRENT_RUSTC_VERSION")]
-    #[rustc_const_unstable(feature = "const_cstr_from_bytes_until_nul", issue = "95027")]
+    #[rustc_const_stable(feature = "cstr_from_bytes_until_nul", since = "CURRENT_RUSTC_VERSION")]
     pub const fn from_bytes_until_nul(bytes: &[u8]) -> Result<&CStr, FromBytesUntilNulError> {
         let nul_pos = memchr::memchr(0, bytes);
         match nul_pos {
diff --git a/library/core/src/slice/memchr.rs b/library/core/src/slice/memchr.rs
index c848c2e18e9..98c8349eb60 100644
--- a/library/core/src/slice/memchr.rs
+++ b/library/core/src/slice/memchr.rs
@@ -16,25 +16,29 @@
 /// bytes where the borrow propagated all the way to the most significant
 /// bit."
 #[inline]
+#[rustc_const_stable(feature = "const_memchr", since = "1.65.0")]
 const fn contains_zero_byte(x: usize) -> bool {
     x.wrapping_sub(LO_USIZE) & !x & HI_USIZE != 0
 }
 
-#[cfg(target_pointer_width = "16")]
 #[inline]
+#[cfg(target_pointer_width = "16")]
+#[rustc_const_stable(feature = "const_memchr", since = "1.65.0")]
 const fn repeat_byte(b: u8) -> usize {
     (b as usize) << 8 | b as usize
 }
 
-#[cfg(not(target_pointer_width = "16"))]
 #[inline]
+#[cfg(not(target_pointer_width = "16"))]
+#[rustc_const_stable(feature = "const_memchr", since = "1.65.0")]
 const fn repeat_byte(b: u8) -> usize {
     (b as usize) * (usize::MAX / 255)
 }
 
 /// Returns the first index matching the byte `x` in `text`.
-#[must_use]
 #[inline]
+#[must_use]
+#[rustc_const_stable(feature = "const_memchr", since = "1.65.0")]
 pub const fn memchr(x: u8, text: &[u8]) -> Option<usize> {
     // Fast path for small slices.
     if text.len() < 2 * USIZE_BYTES {
@@ -45,6 +49,7 @@ pub const fn memchr(x: u8, text: &[u8]) -> Option<usize> {
 }
 
 #[inline]
+#[rustc_const_stable(feature = "const_memchr", since = "1.65.0")]
 const fn memchr_naive(x: u8, text: &[u8]) -> Option<usize> {
     let mut i = 0;
 
@@ -60,6 +65,10 @@ const fn memchr_naive(x: u8, text: &[u8]) -> Option<usize> {
     None
 }
 
+#[rustc_allow_const_fn_unstable(const_cmp)]
+#[rustc_allow_const_fn_unstable(const_slice_index)]
+#[rustc_allow_const_fn_unstable(const_align_offset)]
+#[rustc_const_stable(feature = "const_memchr", since = "1.65.0")]
 const fn memchr_aligned(x: u8, text: &[u8]) -> Option<usize> {
     // Scan for a single byte value by reading two `usize` words at a time.
     //