change replace_ascii to an unsafe fn

Author: LaihoE

library/alloc/src/str.rs

@@ -289,7 +289,9 @@ impl str {
             let to_utf8_len = to.as_bytes().len();
             // Fast path for ascii
             if from_utf8_len == 1 && to_utf8_len == 1 {
-                return replace_ascii(&self.as_bytes(), from_as_utf8[0], to.as_bytes()[0]);
+                return unsafe {
+                    replace_ascii(&self.as_bytes(), from_as_utf8[0], to.as_bytes()[0])
+                };
             }
             capacity = get_minimum_result_capacity(self.bytes().len(), from_utf8_len, to_utf8_len);
         }
@@ -690,7 +692,7 @@ fn convert_while_ascii(b: &[u8], convert: fn(&u8) -> u8) -> Vec<u8> {
 #[inline]
 #[cfg(not(test))]
 #[cfg(not(no_global_oom_handling))]
-fn replace_ascii(utf8_bytes: &[u8], from: u8, to: u8) -> String {
+unsafe fn replace_ascii(utf8_bytes: &[u8], from: u8, to: u8) -> String {
     let result: Vec<u8> = utf8_bytes.iter().map(|b| if *b == from { to } else { *b }).collect();
     // SAFETY: We replaced ascii with ascii on valid utf8 strings.
     unsafe { String::from_utf8_unchecked(result) }