diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index 4c620bef6b329..01cadd78cc09d 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -132,6 +132,7 @@
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]
 #![feature(const_likely)]
+#![feature(const_make_ascii)]
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_nonnull_new)]
 #![feature(const_num_midpoint)]
@@ -150,6 +151,7 @@
 #![feature(const_slice_from_raw_parts_mut)]
 #![feature(const_slice_from_ref)]
 #![feature(const_slice_split_at_mut)]
+#![feature(const_str_as_mut)]
 #![feature(const_str_from_utf8_unchecked_mut)]
 #![feature(const_strict_overflow_ops)]
 #![feature(const_swap)]
diff --git a/library/core/src/slice/ascii.rs b/library/core/src/slice/ascii.rs
index d1ea52fab6b87..8f8050fdc3aaf 100644
--- a/library/core/src/slice/ascii.rs
+++ b/library/core/src/slice/ascii.rs
@@ -67,10 +67,15 @@ impl [u8] {
     ///
     /// [`to_ascii_uppercase`]: #method.to_ascii_uppercase
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_unstable(feature = "const_make_ascii", issue = "130698")]
     #[inline]
-    pub fn make_ascii_uppercase(&mut self) {
-        for byte in self {
+    pub const fn make_ascii_uppercase(&mut self) {
+        // FIXME(const-hack): We would like to simply iterate using `for` loops but this isn't currently allowed in constant expressions.
+        let mut i = 0;
+        while i < self.len() {
+            let byte = &mut self[i];
             byte.make_ascii_uppercase();
+            i += 1;
         }
     }
 
@@ -84,10 +89,15 @@ impl [u8] {
     ///
     /// [`to_ascii_lowercase`]: #method.to_ascii_lowercase
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_unstable(feature = "const_make_ascii", issue = "130698")]
     #[inline]
-    pub fn make_ascii_lowercase(&mut self) {
-        for byte in self {
+    pub const fn make_ascii_lowercase(&mut self) {
+        // FIXME(const-hack): We would like to simply iterate using `for` loops but this isn't currently allowed in constant expressions.
+        let mut i = 0;
+        while i < self.len() {
+            let byte = &mut self[i];
             byte.make_ascii_lowercase();
+            i += 1;
         }
     }
 
diff --git a/library/core/src/str/mod.rs b/library/core/src/str/mod.rs
index 300dde3dd4396..3d535214637f6 100644
--- a/library/core/src/str/mod.rs
+++ b/library/core/src/str/mod.rs
@@ -2473,8 +2473,9 @@ impl str {
     /// assert_eq!("GRüßE, JüRGEN ❤", s);
     /// ```
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_unstable(feature = "const_make_ascii", issue = "130698")]
     #[inline]
-    pub fn make_ascii_uppercase(&mut self) {
+    pub const fn make_ascii_uppercase(&mut self) {
         // SAFETY: changing ASCII letters only does not invalidate UTF-8.
         let me = unsafe { self.as_bytes_mut() };
         me.make_ascii_uppercase()
@@ -2500,8 +2501,9 @@ impl str {
     /// assert_eq!("grÜße, jÜrgen ❤", s);
     /// ```
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_unstable(feature = "const_make_ascii", issue = "130698")]
     #[inline]
-    pub fn make_ascii_lowercase(&mut self) {
+    pub const fn make_ascii_lowercase(&mut self) {
         // SAFETY: changing ASCII letters only does not invalidate UTF-8.
         let me = unsafe { self.as_bytes_mut() };
         me.make_ascii_lowercase()