Fix new dead_code instances in compiler's tests

Author: GrigorenkoPV

compiler/rustc_errors/src/json/tests.rs

@@ -13,6 +13,7 @@ struct TestData {
 }
 
 #[derive(Deserialize, Debug, PartialEq, Eq)]
+#[allow(dead_code, reason = "Fields are only used for equality comparisons")]
 struct SpanTestData {
     pub byte_start: u32,
     pub byte_end: u32,

compiler/rustc_thread_pool/tests/scoped_threadpool.rs

@@ -4,7 +4,7 @@ use crossbeam_utils::thread;
 use rustc_thread_pool::ThreadPoolBuilder;
 
 #[derive(PartialEq, Eq, Debug)]
-struct Local(i32);
+struct Local(#[allow(dead_code, reason = "Only used for equality comparisons")] i32);
 
 scoped_tls::scoped_thread_local!(static LOCAL: Local);