Add fast path for displaying pre-validated Wtf8Buf

Author: thaliaarchi

library/std/src/sys/os_str/wtf8.rs

@@ -36,13 +36,13 @@ impl AsInner<Wtf8> for Buf {
 
 impl fmt::Debug for Buf {
     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(self.as_slice(), formatter)
+        fmt::Debug::fmt(&self.inner, formatter)
     }
 }
 
 impl fmt::Display for Buf {
     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Display::fmt(self.as_slice(), formatter)
+        fmt::Display::fmt(&self.inner, formatter)
     }
 }
 

library/std/src/sys_common/wtf8.rs

@@ -169,6 +169,17 @@ impl fmt::Debug for Wtf8Buf {
     }
 }
 
+/// Formats the string with unpaired surrogates substituted with the replacement
+/// character, U+FFFD.
+impl fmt::Display for Wtf8Buf {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Some(s) = self.as_known_utf8() {
+            return fmt::Display::fmt(s, formatter);
+        }
+        fmt::Display::fmt(&**self, formatter)
+    }
+}
+
 impl Wtf8Buf {
     /// Creates a new, empty WTF-8 string.
     #[inline]
@@ -262,6 +273,18 @@ impl Wtf8Buf {
         unsafe { Wtf8::from_mut_bytes_unchecked(&mut self.bytes) }
     }
 
+    /// Converts the string to UTF-8 without validation, if it was created from
+    /// valid UTF-8.
+    #[inline]
+    fn as_known_utf8(&self) -> Option<&str> {
+        if self.is_known_utf8 {
+            // SAFETY: The buffer is known to be valid UTF-8.
+            Some(unsafe { str::from_utf8_unchecked(self.as_bytes()) })
+        } else {
+            None
+        }
+    }
+
     /// Reserves capacity for at least `additional` more bytes to be inserted
     /// in the given `Wtf8Buf`.
     /// The collection may reserve more space to avoid frequent reallocations.