Auto merge of rust-lang#74289 - lzutao:unroll, r=LukasKalbertodt

Remove some redundant parts from `unrolled_find_u16s`

See each commit message for details.

r? @wesleywiser from old PR rust-lang#67705 .

library/std/src/sys/windows/mod.rs

@@ -99,43 +99,28 @@ pub fn decode_error_kind(errno: i32) -> ErrorKind {
 
 pub fn unrolled_find_u16s(needle: u16, haystack: &[u16]) -> Option<usize> {
     let ptr = haystack.as_ptr();
-    let mut len = haystack.len();
     let mut start = &haystack[..];
 
     // For performance reasons unfold the loop eight times.
-    while len >= 8 {
-        if start[0] == needle {
-            return Some((start.as_ptr() as usize - ptr as usize) / 2);
-        }
-        if start[1] == needle {
-            return Some((start[1..].as_ptr() as usize - ptr as usize) / 2);
-        }
-        if start[2] == needle {
-            return Some((start[2..].as_ptr() as usize - ptr as usize) / 2);
-        }
-        if start[3] == needle {
-            return Some((start[3..].as_ptr() as usize - ptr as usize) / 2);
-        }
-        if start[4] == needle {
-            return Some((start[4..].as_ptr() as usize - ptr as usize) / 2);
-        }
-        if start[5] == needle {
-            return Some((start[5..].as_ptr() as usize - ptr as usize) / 2);
-        }
-        if start[6] == needle {
-            return Some((start[6..].as_ptr() as usize - ptr as usize) / 2);
-        }
-        if start[7] == needle {
-            return Some((start[7..].as_ptr() as usize - ptr as usize) / 2);
+    while start.len() >= 8 {
+        macro_rules! if_return {
+            ($($n:literal,)+) => {
+                $(
+                    if start[$n] == needle {
+                        return Some((&start[$n] as *const u16 as usize - ptr as usize) / 2);
+                    }
+                )+
+            }
         }
 
+        if_return!(0, 1, 2, 3, 4, 5, 6, 7,);
+
         start = &start[8..];
-        len -= 8;
     }
 
-    for (i, c) in start.iter().enumerate() {
+    for c in start {
         if *c == needle {
-            return Some((start.as_ptr() as usize - ptr as usize) / 2 + i);
+            return Some((c as *const u16 as usize - ptr as usize) / 2);
         }
     }
     None