Don't eagerly scan for is_known_utf8 in to_ascii_lowercase/uppercase.

Author: sunfishcode

library/std/src/sys_common/wtf8.rs

@@ -762,18 +762,12 @@ impl Wtf8 {
 
     #[inline]
     pub fn to_ascii_lowercase(&self) -> Wtf8Buf {
-        Wtf8Buf {
-            bytes: self.bytes.to_ascii_lowercase(),
-            is_known_utf8: self.next_surrogate(0).is_none(),
-        }
+        Wtf8Buf { bytes: self.bytes.to_ascii_lowercase(), is_known_utf8: false }
     }
 
     #[inline]
     pub fn to_ascii_uppercase(&self) -> Wtf8Buf {
-        Wtf8Buf {
-            bytes: self.bytes.to_ascii_uppercase(),
-            is_known_utf8: self.next_surrogate(0).is_none(),
-        }
+        Wtf8Buf { bytes: self.bytes.to_ascii_uppercase(), is_known_utf8: false }
     }
 
     #[inline]

library/std/src/sys_common/wtf8/tests.rs

@@ -635,12 +635,10 @@ fn wtf8_make_ascii_lowercase() {
     let mut lowercase = Wtf8Buf::from_str("");
     lowercase.make_ascii_lowercase();
     assert_eq!(lowercase.bytes, b"");
-    assert!(lowercase.is_known_utf8);
 
     let mut lowercase = Wtf8Buf::from_str("GrEeN gRaPeS! 🍇");
     lowercase.make_ascii_lowercase();
     assert_eq!(lowercase.bytes, b"green grapes! \xf0\x9f\x8d\x87");
-    assert!(lowercase.is_known_utf8);
 
     let mut lowercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
     lowercase.make_ascii_lowercase();
@@ -653,12 +651,10 @@ fn wtf8_make_ascii_uppercase() {
     let mut uppercase = Wtf8Buf::from_str("");
     uppercase.make_ascii_uppercase();
     assert_eq!(uppercase.bytes, b"");
-    assert!(uppercase.is_known_utf8);
 
     let mut uppercase = Wtf8Buf::from_str("GrEeN gRaPeS! 🍇");
     uppercase.make_ascii_uppercase();
     assert_eq!(uppercase.bytes, b"GREEN GRAPES! \xf0\x9f\x8d\x87");
-    assert!(uppercase.is_known_utf8);
 
     let mut uppercase = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
     uppercase.make_ascii_uppercase();