style: Fix clippy warnings

Depends on D96634

Differential Revision: https://phabricator.services.mozilla.com/D96636

components/derive_common/cg.rs

@@ -368,7 +368,7 @@ pub fn to_css_identifier(mut camel_case: &str) -> String {
             }
         }
         if !first {
-            result.push_str("-");
+            result.push('-');
         }
         first = false;
         result.push_str(&segment.to_lowercase());

components/servo_arc/lib.rs

@@ -770,7 +770,7 @@ impl<H, T> Arc<HeaderSlice<H, [T]>> {
                 // We should have consumed the buffer exactly, maybe accounting
                 // for some padding from the alignment.
                 debug_assert!(
-                    (buffer.offset(size as isize) as usize - current as *mut u8 as usize) <
+                    (buffer.add(size) as usize - current as *mut u8 as usize) <
                         inner_align
                 );
             }
@@ -858,8 +858,8 @@ impl<H> HeaderWithLength<H> {
     /// Creates a new HeaderWithLength.
     pub fn new(header: H, length: usize) -> Self {
         HeaderWithLength {
-            header: header,
-            length: length,
+            header,
+            length,
         }
     }
 }

components/to_shmem/lib.rs

@@ -170,7 +170,7 @@ impl SharedMemoryBuilder {
         assert!(end <= self.capacity);
 
         self.index = end;
-        unsafe { self.buffer.offset(start as isize) as *mut T }
+        unsafe { self.buffer.add(start) as *mut T }
     }
 }