diff --git a/library/alloc/src/slice.rs b/library/alloc/src/slice.rs
index 0c23d7cc0b071..40287e9fa1040 100644
--- a/library/alloc/src/slice.rs
+++ b/library/alloc/src/slice.rs
@@ -672,32 +672,28 @@ impl [u8] {
     #[unstable(issue = "none", feature = "std_internals")]
     #[allow(dead_code)]
     /// Safety:
-    ///    - Must be UTF-8
+    ///    - Must be valid UTF-8
     pub unsafe fn make_utf8_uppercase(&mut self) -> Result<usize, VecDeque<u8>> {
         let mut queue = VecDeque::new();
 
         let mut read_offset = 0;
         let mut write_offset = 0;
 
-        let mut buffer = [0; 4];
         while let Some((codepoint, width)) =
             unsafe { core::str::next_code_point_with_width(&mut self[read_offset..].iter()) }
         {
             read_offset += width;
+            // Queue must be flushed before encode_to_slice_or_else_to_queue is
+            // called to ensure proper order of bytes
+            dump_queue(&mut queue, &mut self[..read_offset], &mut write_offset);
             let lowercase_char = unsafe { char::from_u32_unchecked(codepoint) };
             for c in lowercase_char.to_uppercase() {
-                let l = c.len_utf8();
-                c.encode_utf8(&mut buffer);
-                queue.extend(&buffer[..l]);
-            }
-            while write_offset < read_offset {
-                match queue.pop_front() {
-                    Some(b) => {
-                        self[write_offset] = b;
-                        write_offset += 1;
-                    }
-                    None => break,
-                }
+                encode_to_slice_or_else_to_queue(
+                    c,
+                    &mut queue,
+                    &mut self[..read_offset],
+                    &mut write_offset,
+                );
             }
         }
         assert_eq!(read_offset, self.len());
@@ -708,19 +704,21 @@ impl [u8] {
     #[unstable(issue = "none", feature = "std_internals")]
     #[allow(dead_code)]
     /// Safety:
-    ///    - Must be UTF-8
+    ///    - Must be valid UTF-8
     pub unsafe fn make_utf8_lowercase(&mut self) -> Result<usize, VecDeque<u8>> {
         let mut queue = VecDeque::new();
 
         let mut read_offset = 0;
         let mut write_offset = 0;
 
-        let mut buffer = [0; 4];
         let mut final_sigma_automata = FinalSigmaAutomata::new();
         while let Some((codepoint, width)) =
             unsafe { core::str::next_code_point_with_width(&mut self[read_offset..].iter()) }
         {
             read_offset += width;
+            // Queue must be flushed before encode_to_slice_or_else_to_queue is
+            // called to ensure proper order of bytes
+            dump_queue(&mut queue, &mut self[..read_offset], &mut write_offset);
             let uppercase_char = unsafe { char::from_u32_unchecked(codepoint) };
             if uppercase_char == 'Σ' {
                 // Σ maps to σ, except at the end of a word where it maps to ς.
@@ -729,26 +727,23 @@ impl [u8] {
                 let is_word_final =
                     final_sigma_automata.is_accepting() && !case_ignorable_then_cased(rest.chars());
                 let sigma_lowercase = if is_word_final { 'ς' } else { 'σ' };
-                let l = sigma_lowercase.len_utf8();
-                sigma_lowercase.encode_utf8(&mut buffer);
-                queue.extend(&buffer[..l]);
+                encode_to_slice_or_else_to_queue(
+                    sigma_lowercase,
+                    &mut queue,
+                    &mut self[..read_offset],
+                    &mut write_offset,
+                );
             } else {
                 for c in uppercase_char.to_lowercase() {
-                    let l = c.len_utf8();
-                    c.encode_utf8(&mut buffer);
-                    queue.extend(&buffer[..l]);
+                    encode_to_slice_or_else_to_queue(
+                        c,
+                        &mut queue,
+                        &mut self[..read_offset],
+                        &mut write_offset,
+                    );
                 }
             }
             final_sigma_automata.step(uppercase_char);
-            while write_offset < read_offset {
-                match queue.pop_front() {
-                    Some(b) => {
-                        self[write_offset] = b;
-                        write_offset += 1;
-                    }
-                    None => break,
-                }
-            }
         }
         assert_eq!(read_offset, self.len());
         return if write_offset < read_offset { Ok(write_offset) } else { Err(queue) };
@@ -764,6 +759,33 @@ impl [u8] {
     }
 }
 
+fn encode_to_slice_or_else_to_queue(
+    c: char,
+    queue: &mut VecDeque<u8>,
+    slice: &mut [u8],
+    write_offset: &mut usize,
+) {
+    let mut buffer = [0; 4];
+    let len = c.encode_utf8(&mut buffer).len();
+    let writable_slice = &mut slice[*write_offset..];
+    let direct_copy_length = core::cmp::min(len, writable_slice.len());
+    writable_slice[..direct_copy_length].copy_from_slice(&buffer[..direct_copy_length]);
+    *write_offset += direct_copy_length;
+    queue.extend(&buffer[direct_copy_length..len]);
+}
+
+fn dump_queue(queue: &mut VecDeque<u8>, slice: &mut [u8], write_offset: &mut usize) {
+    while *write_offset < slice.len() {
+        match queue.pop_front() {
+            Some(b) => {
+                slice[*write_offset] = b;
+                *write_offset += 1;
+            }
+            None => break,
+        }
+    }
+}
+
 #[derive(Clone)]
 enum FinalSigmaAutomata {
     Init,