Use post-expansion crate to let other custom derives see serde attrs

serde_derive/Cargo.toml

@@ -14,6 +14,9 @@ include = ["Cargo.toml", "src/**/*.rs"]
 name = "serde_derive"
 proc-macro = true
 
+[dependencies]
+post-expansion = "0.0.2"
+
 [dependencies.serde_codegen]
 version = "=0.8.13"
 path = "../serde_codegen"

serde_derive/src/lib.rs

@@ -4,6 +4,9 @@
 extern crate proc_macro;
 extern crate serde_codegen;
 
+#[macro_use]
+extern crate post_expansion;
+
 use proc_macro::TokenStream;
 
 #[proc_macro_derive(Serialize)]
@@ -23,3 +26,5 @@ pub fn derive_deserialize(input: TokenStream) -> TokenStream {
         Err(msg) => panic!(msg),
     }
 }
+
+register_post_expansion!(PostExpansion_serde);