Auto merge of #141808 - bend-n:faster_charsearcher_take2_with_memchr, r=<try>

faster charsearcher

attempt to do #141516 better
resolves #82471

r? `@workingjubilee`

Author: rust-bors[bot]

library/core/src/str/pattern.rs

@@ -429,8 +429,24 @@ unsafe impl<'a> Searcher<'a> for CharSearcher<'a> {
             SearchStep::Done
         }
     }
-    #[inline]
+    #[inline(always)]
     fn next_match(&mut self) -> Option<(usize, usize)> {
+        if self.utf8_size == 1 {
+            let find = |haystack: &[u8]| {
+                if haystack.len() < 32 {
+                    haystack.iter().position(|&x| x == self.utf8_encoded[0])
+                } else {
+                    memchr::memchr(self.utf8_encoded[0], haystack)
+                }
+            };
+            return match find(self.haystack.as_bytes().get(self.finger..self.finger_back)?) {
+                Some(x) => {
+                    self.finger += x + 1;
+                    Some((self.finger - 1, self.finger))
+                }
+                None => None,
+            };
+        }
         loop {
             // get the haystack after the last character found
             let bytes = self.haystack.as_bytes().get(self.finger..self.finger_back)?;
@@ -498,6 +514,22 @@ unsafe impl<'a> ReverseSearcher<'a> for CharSearcher<'a> {
     }
     #[inline]
     fn next_match_back(&mut self) -> Option<(usize, usize)> {
+        if self.utf8_size == 1 {
+            let find = |haystack: &[u8]| {
+                if haystack.len() < 32 {
+                    haystack.iter().rposition(|&x| x == self.utf8_encoded[0])
+                } else {
+                    memchr::memrchr(self.utf8_encoded[0], haystack)
+                }
+            };
+            return match find(self.haystack.as_bytes().get(self.finger..self.finger_back)?) {
+                Some(x) => {
+                    self.finger_back = self.finger + x;
+                    Some((self.finger_back, self.finger_back + 1))
+                }
+                None => None,
+            };
+        }
         let haystack = self.haystack.as_bytes();
         loop {
             // get the haystack up to but not including the last character searched