Fix handling of Self keyword in type definition

serde_derive/src/internals/respan.rs

@@ -0,0 +1,22 @@
+use proc_macro2::{Span, TokenStream};
+use quote::ToTokens;
+use syn::parse::Parse;
+
+pub(crate) fn respan<T>(node: &T, span: Span) -> T
+where
+    T: ToTokens + Parse,
+{
+    let tokens = node.to_token_stream();
+    let respanned = respan_tokens(tokens, span);
+    syn::parse2(respanned).unwrap()
+}
+
+fn respan_tokens(tokens: TokenStream, span: Span) -> TokenStream {
+    tokens
+        .into_iter()
+        .map(|mut token| {
+            token.set_span(span);
+            token
+        })
+        .collect()
+}