Ignore dead_code warning in regression test

    warning: field `0` is never read
      --> test_suite/tests/regression/issue2846.rs:8:45
       |
    8  |         pub struct S(#[serde(with = $with)] i32);
       |                    - field in this struct   ^^^
    ...
    12 | declare_in_macro!("with");
       | ------------------------- in this macro invocation
       |
       = help: consider removing this field
       = note: `#[warn(dead_code)]` on by default
       = note: this warning originates in the macro `declare_in_macro` (in Nightly builds, run with -Z macro-backtrace for more info)

test_suite/tests/regression/issue2846.rs

@@ -5,7 +5,11 @@ use serde_derive::Deserialize;
 macro_rules! declare_in_macro {
     ($with:literal) => {
         #[derive(Deserialize)]
-        pub struct S(#[serde(with = $with)] i32);
+        pub struct S(
+            #[serde(with = $with)]
+            #[allow(dead_code)]
+            i32,
+        );
     };
 }