Fix some clippy lints

compiler/rustc_hir/src/hir.rs

@@ -2401,7 +2401,7 @@ impl StructField<'_> {
     // Still necessary in couple of places
     pub fn is_positional(&self) -> bool {
         let first = self.ident.as_str().as_bytes()[0];
-        first >= b'0' && first <= b'9'
+        (b'0'..=b'9').contains(&first)
     }
 }