diff --git a/library/alloc/src/slice.rs b/library/alloc/src/slice.rs
index da9b40c2c3ce0..e6b540f093ba5 100644
--- a/library/alloc/src/slice.rs
+++ b/library/alloc/src/slice.rs
@@ -639,9 +639,7 @@ impl [u8] {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_uppercase(&self) -> Vec<u8> {
-        let mut me = self.to_vec();
-        me.make_ascii_uppercase();
-        me
+        self.iter().map(|b| b.to_ascii_uppercase()).collect()
     }
 
     /// Returns a vector containing a copy of this slice where each byte
@@ -660,9 +658,7 @@ impl [u8] {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_lowercase(&self) -> Vec<u8> {
-        let mut me = self.to_vec();
-        me.make_ascii_lowercase();
-        me
+        self.iter().map(|b| b.to_ascii_lowercase()).collect()
     }
 }
 
diff --git a/library/alloc/src/str.rs b/library/alloc/src/str.rs
index fd0d66cf028b3..ee9e82368899f 100644
--- a/library/alloc/src/str.rs
+++ b/library/alloc/src/str.rs
@@ -844,9 +844,10 @@ impl str {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_uppercase(&self) -> String {
-        let mut s = self.to_owned();
-        s.make_ascii_uppercase();
-        s
+        let bytes = self.as_bytes().to_ascii_uppercase();
+        // SAFETY: ASCII case conversion only maps a-z to A-Z and leaves
+        // all other bytes unchanged as valid UTF-8
+        unsafe { String::from_utf8_unchecked(bytes) }
     }
 
     /// Returns a copy of this string where each character is mapped to its
@@ -876,9 +877,10 @@ impl str {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_lowercase(&self) -> String {
-        let mut s = self.to_owned();
-        s.make_ascii_lowercase();
-        s
+        let bytes = self.as_bytes().to_ascii_lowercase();
+        // SAFETY: ASCII case conversion only maps A-Z to a-z and leaves
+        // all other bytes unchanged as valid UTF-8
+        unsafe { String::from_utf8_unchecked(bytes) }
     }
 }