Record expansion info

serde_codegen/src/span.rs

@@ -0,0 +1,43 @@
+use syntax::ast;
+use syntax::codemap::{self, ExpnId, Span};
+use syntax::ext::base::{Annotatable, ExtCtxt};
+use syntax::fold::{self, Folder};
+use syntax::parse::token::intern;
+use syntax::ptr::P;
+
+pub fn record_expansion(
+    cx: &ExtCtxt,
+    item: P<ast::Item>,
+    derive: &str,
+) -> Annotatable {
+    let info = codemap::ExpnInfo {
+        call_site: codemap::DUMMY_SP,
+        callee: codemap::NameAndSpan {
+            format: codemap::MacroAttribute(intern(&format!("derive({})", derive))),
+            span: None,
+            allow_internal_unstable: false,
+        },
+    };
+    let expn_id = cx.codemap().record_expansion(info);
+
+    let mut respanner = Respanner { expn_id: expn_id };
+    let item = item.map(|item| respanner.fold_item_simple(item));
+    Annotatable::Item(item)
+}
+
+struct Respanner {
+    expn_id: ExpnId,
+}
+
+impl Folder for Respanner {
+    fn new_span(&mut self, span: Span) -> Span {
+        Span {
+            expn_id: self.expn_id,
+            .. span
+        }
+    }
+
+    fn fold_mac(&mut self, mac: ast::Mac) -> ast::Mac {
+        fold::noop_fold_mac(mac, self)
+    }
+}