Ignore dead code lint in tests

New in nightly-2024-03-24 from rust-lang/rust#119552.

    warning: fields `nested` and `string` are never read
      --> test_suite/tests/regression/issue2371.rs:10:9
       |
    8  |     Flatten {
       |     ------- fields in this variant
    9  |         #[serde(flatten)]
    10 |         nested: Nested,
       |         ^^^^^^
    11 |         string: &'static str,
       |         ^^^^^^
       |
       = note: `#[warn(dead_code)]` on by default

    warning: fields `nested` and `string` are never read
      --> test_suite/tests/regression/issue2371.rs:20:9
       |
    18 |     Flatten {
       |     ------- fields in this variant
    19 |         #[serde(flatten)]
    20 |         nested: Nested,
       |         ^^^^^^
    21 |         string: &'static str,
       |         ^^^^^^

    warning: fields `nested` and `string` are never read
      --> test_suite/tests/regression/issue2371.rs:30:9
       |
    28 |     Flatten {
       |     ------- fields in this variant
    29 |         #[serde(flatten)]
    30 |         nested: Nested,
       |         ^^^^^^
    31 |         string: &'static str,
       |         ^^^^^^

    warning: fields `nested` and `string` are never read
      --> test_suite/tests/regression/issue2371.rs:40:9
       |
    38 |     Flatten {
       |     ------- fields in this variant
    39 |         #[serde(flatten)]
    40 |         nested: Nested,
       |         ^^^^^^
    41 |         string: &'static str,
       |         ^^^^^^

    warning: field `0` is never read
       --> test_suite/tests/test_gen.rs:690:33
        |
    690 |         Single(#[serde(borrow)] RelObject<'a>),
        |         ------                  ^^^^^^^^^^^^^
        |         |
        |         field in this variant
        |
        = note: `#[warn(dead_code)]` on by default
    help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
        |
    690 |         Single(#[serde(borrow)] ()),
        |                                 ~~

    warning: field `0` is never read
       --> test_suite/tests/test_gen.rs:691:31
        |
    691 |         Many(#[serde(borrow)] Vec<RelObject<'a>>),
        |         ----                  ^^^^^^^^^^^^^^^^^^
        |         |
        |         field in this variant
        |
    help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
        |
    691 |         Many(#[serde(borrow)] ()),
        |                               ~~

test_suite/tests/regression/issue2371.rs

@@ -7,7 +7,9 @@ pub struct Nested;
 pub enum ExternallyTagged {
     Flatten {
         #[serde(flatten)]
+        #[allow(dead_code)]
         nested: Nested,
+        #[allow(dead_code)]
         string: &'static str,
     },
 }
@@ -17,7 +19,9 @@ pub enum ExternallyTagged {
 pub enum InternallyTagged {
     Flatten {
         #[serde(flatten)]
+        #[allow(dead_code)]
         nested: Nested,
+        #[allow(dead_code)]
         string: &'static str,
     },
 }
@@ -27,7 +31,9 @@ pub enum InternallyTagged {
 pub enum AdjacentlyTagged {
     Flatten {
         #[serde(flatten)]
+        #[allow(dead_code)]
         nested: Nested,
+        #[allow(dead_code)]
         string: &'static str,
     },
 }
@@ -37,7 +43,9 @@ pub enum AdjacentlyTagged {
 pub enum UntaggedWorkaround {
     Flatten {
         #[serde(flatten)]
+        #[allow(dead_code)]
         nested: Nested,
+        #[allow(dead_code)]
         string: &'static str,
     },
 }

test_suite/tests/test_gen.rs

@@ -687,8 +687,16 @@ fn test_gen() {
     #[derive(Deserialize)]
     #[serde(untagged)]
     pub enum UntaggedWithBorrow<'a> {
-        Single(#[serde(borrow)] RelObject<'a>),
-        Many(#[serde(borrow)] Vec<RelObject<'a>>),
+        Single(
+            #[serde(borrow)]
+            #[allow(dead_code)]
+            RelObject<'a>,
+        ),
+        Many(
+            #[serde(borrow)]
+            #[allow(dead_code)]
+            Vec<RelObject<'a>>,
+        ),
     }
 
     #[derive(Deserialize)]