Use if-let chain

Author: Kobzol

compiler/rustc_builtin_macros/src/deriving/from.rs

@@ -28,12 +28,12 @@ pub(crate) fn expand_deriving_from(
     };
 
     // #[derive(From)] is currently usable only on structs with exactly one field.
-    let field = match &item.kind {
-        ItemKind::Struct(_, _, data) => match data.fields() {
-            [field] => Some(field.clone()),
-            _ => None,
-        },
-        _ => None,
+    let field = if let ItemKind::Struct(_, _, data) = &item.kind
+        && let [field] = data.fields()
+    {
+        Some(field.clone())
+    } else {
+        None
     };
 
     let from_type = match &field {