diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index 23ded42fa6679..59732cdfbea53 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -2449,7 +2449,7 @@ extern "rust-intrinsic" {
     ///     const fn compiletime() -> i32 { 2 }
     ///
     ///     unsafe {
-    //          // ⚠ This code violates the required equivalence of `compiletime`
+    ///         // ⚠ This code violates the required equivalence of `compiletime`
     ///         // and `runtime`.
     ///         const_eval_select((), compiletime, runtime)
     ///     }
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index 26c51e8403522..cf793996944c7 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -146,6 +146,7 @@
 #![feature(const_replace)]
 #![feature(const_size_of_val)]
 #![feature(const_size_of_val_raw)]
+#![feature(const_slice_eq_ignore_ascii_case)]
 #![feature(const_slice_from_raw_parts_mut)]
 #![feature(const_slice_from_ref)]
 #![feature(const_slice_index)]
diff --git a/library/core/src/slice/ascii.rs b/library/core/src/slice/ascii.rs
index f3311f76a7f06..5716a16ca0429 100644
--- a/library/core/src/slice/ascii.rs
+++ b/library/core/src/slice/ascii.rs
@@ -2,6 +2,7 @@
 
 use crate::ascii;
 use crate::fmt::{self, Write};
+use crate::intrinsics;
 use crate::iter;
 use crate::mem;
 use crate::ops;
@@ -52,10 +53,33 @@ impl [u8] {
     /// Same as `to_ascii_lowercase(a) == to_ascii_lowercase(b)`,
     /// but without allocating and copying temporaries.
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_unstable(feature = "const_slice_eq_ignore_ascii_case", issue = "none")]
     #[must_use]
     #[inline]
-    pub fn eq_ignore_ascii_case(&self, other: &[u8]) -> bool {
-        self.len() == other.len() && iter::zip(self, other).all(|(a, b)| a.eq_ignore_ascii_case(b))
+    pub const fn eq_ignore_ascii_case(&self, other: &[u8]) -> bool {
+        const fn eq_ignore_ascii_case_ct(a: &[u8], b: &[u8]) -> bool {
+            a.len() == b.len() && {
+                let mut i = 0;
+                while i < a.len() {
+                    if !u8::eq_ignore_ascii_case(&a[i], &b[i]) {
+                        return false;
+                    }
+                    i += 1;
+                }
+                true
+            }
+        }
+        fn eq_ignore_ascii_case_rt(a: &[u8], b: &[u8]) -> bool {
+            a.len() == b.len() && iter::zip(a, b).all(|(a, b)| u8::eq_ignore_ascii_case(a, b))
+        }
+        // SAFETY: both branches compute the same result, the runtime one is just more optimized
+        unsafe {
+            intrinsics::const_eval_select(
+                (self, other),
+                eq_ignore_ascii_case_ct,
+                eq_ignore_ascii_case_rt,
+            )
+        }
     }
 
     /// Converts this slice to its ASCII upper case equivalent in-place.
diff --git a/library/core/src/str/mod.rs b/library/core/src/str/mod.rs
index ef05b25fdd06c..946c8ae8b0a84 100644
--- a/library/core/src/str/mod.rs
+++ b/library/core/src/str/mod.rs
@@ -2391,9 +2391,10 @@ impl str {
     /// assert!(!"Ferrös".eq_ignore_ascii_case("FERRÖS"));
     /// ```
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_unstable(feature = "const_slice_eq_ignore_ascii_case", issue = "none")]
     #[must_use]
     #[inline]
-    pub fn eq_ignore_ascii_case(&self, other: &str) -> bool {
+    pub const fn eq_ignore_ascii_case(&self, other: &str) -> bool {
         self.as_bytes().eq_ignore_ascii_case(other.as_bytes())
     }
 
diff --git a/library/core/tests/ascii.rs b/library/core/tests/ascii.rs
index f5f2dd0477885..47d924da95daf 100644
--- a/library/core/tests/ascii.rs
+++ b/library/core/tests/ascii.rs
@@ -16,6 +16,19 @@ fn test_is_ascii() {
     assert!(!"ประเทศไทย中华ệ ".chars().any(|c| c.is_ascii()));
 }
 
+#[test]
+fn test_is_ascii_const() {
+    const _: () = {
+        assert!(b"".is_ascii());
+        assert!(b"banana\0\x7F".is_ascii());
+        assert!(!b"Vi\xe1\xbb\x87t Nam".is_ascii());
+
+        assert!("".is_ascii());
+        assert!("banana\0\u{7F}".is_ascii());
+        assert!(!"ประเทศไทย中华Việt Nam".is_ascii());
+    };
+}
+
 #[test]
 fn test_to_ascii_uppercase() {
     assert_eq!("url()URL()uRl()ürl".to_ascii_uppercase(), "URL()URL()URL()üRL");
@@ -112,6 +125,19 @@ fn test_eq_ignore_ascii_case() {
     }
 }
 
+#[test]
+fn test_eq_ignore_ascii_case_const() {
+    const _: () = {
+        assert!("url()URL()uRl()Ürl".eq_ignore_ascii_case("url()url()url()Ürl"));
+        assert!(!"Ürl".eq_ignore_ascii_case("ürl"));
+        // Dotted capital I, Kelvin sign, Sharp S.
+        assert!("HİKß".eq_ignore_ascii_case("hİKß"));
+        assert!(!"İ".eq_ignore_ascii_case("i"));
+        assert!(!"K".eq_ignore_ascii_case("k"));
+        assert!(!"ß".eq_ignore_ascii_case("s"));
+    };
+}
+
 #[test]
 fn inference_works() {
     let x = "a".to_string();
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index 3c49d1705e5ca..85df486c57b53 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -19,6 +19,7 @@
 #![feature(const_pointer_is_aligned)]
 #![feature(const_ptr_as_ref)]
 #![feature(const_ptr_write)]
+#![feature(const_slice_eq_ignore_ascii_case)]
 #![feature(const_trait_impl)]
 #![feature(const_likely)]
 #![feature(const_location_fields)]