Apply suggestions from code review

Co-authored-by: Oli Scherer <[email protected]>

Author: ZuseZ4

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -823,11 +823,10 @@ fn autodiff_attrs(tcx: TyCtxt<'_>, id: DefId) -> Option<AutoDiffAttrs> {
         }
     };
 
-    let width: u32;
-    match width_meta {
+    let width: u32 = match width_meta {
         MetaItemInner::MetaItem(MetaItem { path: p1, .. }) => {
             let w = p1.segments.first().unwrap().ident;
-            width = match w.as_str().parse() {
+            match w.as_str().parse() {
                 Ok(val) => val,
                 Err(_) => {
                     span_bug!(w.span, "rustc_autodiff width should fit u32");
@@ -836,7 +835,7 @@ fn autodiff_attrs(tcx: TyCtxt<'_>, id: DefId) -> Option<AutoDiffAttrs> {
         }
         MetaItemInner::Lit(lit) => {
             if let LitKind::Int(val, _) = lit.kind {
-                width = match val.get().try_into() {
+                match val.get().try_into() {
                     Ok(val) => val,
                     Err(_) => {
                         span_bug!(lit.span, "rustc_autodiff width should fit u32");