optimize str::iter::Chars::advance_by

this avoids part of the char decoding work by not looking at utf8 continuation bytes

Author: the8472

library/core/src/str/iter.rs

@@ -8,6 +8,7 @@ use crate::iter::{TrustedRandomAccess, TrustedRandomAccessNoCoerce};
 use crate::ops::Try;
 use crate::option;
 use crate::slice::{self, Split as SliceSplit};
+use core::num::NonZeroUsize;
 
 use super::from_utf8_unchecked;
 use super::pattern::Pattern;
@@ -49,6 +50,22 @@ impl<'a> Iterator for Chars<'a> {
         super::count::count_chars(self.as_str())
     }
 
+    #[inline]
+    fn advance_by(&mut self, mut remainder: usize) -> Result<(), NonZeroUsize> {
+        let bytes = &mut self.iter;
+
+        while (remainder > 0) && (bytes.len() > 0) {
+            remainder -= 1;
+            let b = bytes.as_slice()[0];
+            let slurped = super::validations::utf8_char_width(b);
+            // SAFETY: utf8-validity guarantees that the continuation bytes must be present
+            // and therefore we can skip this amount of bytes.
+            unsafe { bytes.advance_by(slurped).unwrap_unchecked() };
+        }
+
+        NonZeroUsize::new(remainder).map_or(Ok(()), Err)
+    }
+
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         let len = self.iter.len();