Apply suggestions from code review

Co-authored-by: Oli Scherer <github35764891676564198441@oli-obk.de>

Author: ZuseZ4

compiler/rustc_builtin_macros/src/autodiff.rs

@@ -45,8 +45,7 @@ mod llvm_enzyme {
         }
     }
     fn first_ident(x: &MetaItemInner) -> rustc_span::Ident {
-        if x.lit().is_some() {
-            let l = x.lit().unwrap();
+        if let Some(l) = x.lit() {
             match l.kind {
                 ast::LitKind::Int(val, _) => {
                     // get an Ident from a lit
@@ -89,8 +88,8 @@ mod llvm_enzyme {
         // If he doesn't specify an integer (=width), we default to scalar mode, thus width=1.
         let mut first_activity = 2;
 
-        let width = if meta_item.len() >= 3
-            && let Some(x) = width(&meta_item[2])
+        let width = if let [_, _, x, ..] = &meta_item[..]
+            && let Some(x) = width(x)
         {
             first_activity = 3;
             match x.try_into() {