Avoid unsafe code in to_ascii_[lower/upper]case()

Author: ChayimFriedman2

library/alloc/src/str.rs

@@ -559,10 +559,9 @@ impl str {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_uppercase(&self) -> String {
-        let mut bytes = self.as_bytes().to_vec();
-        bytes.make_ascii_uppercase();
-        // make_ascii_uppercase() preserves the UTF-8 invariant.
-        unsafe { String::from_utf8_unchecked(bytes) }
+        let mut s = self.to_owned();
+        s.make_ascii_uppercase();
+        s
     }
 
     /// Returns a copy of this string where each character is mapped to its
@@ -592,10 +591,9 @@ impl str {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_lowercase(&self) -> String {
-        let mut bytes = self.as_bytes().to_vec();
-        bytes.make_ascii_lowercase();
-        // make_ascii_lowercase() preserves the UTF-8 invariant.
-        unsafe { String::from_utf8_unchecked(bytes) }
+        let mut s = self.to_owned();
+        s.make_ascii_lowercase();
+        s
     }
 }