Make #[max] an attribute in newtype_index

compiler/rustc_query_system/src/dep_graph/serialized.rs

@@ -27,8 +27,8 @@ use smallvec::SmallVec;
 // unused so that we can store multiple index types in `CompressedHybridIndex`,
 // and use those bits to encode which index type it contains.
 rustc_index::newtype_index! {
+    #[max = 0x7FFF_FFFF]
     pub struct SerializedDepNodeIndex {
-        MAX = 0x7FFF_FFFF
     }
 }