Add internal-only rustc_serialize_exclude_null attribute for making the field only exist in the json if the flag is passed

1 files changed, 14 insertions, 2 deletions

diff --git a/src/libsyntax_ext/deriving/encodable.rs b/src/libsyntax_ext/deriving/encodable.rs
index 0e6e96438d8..743f22b6b31 100644
--- a/src/libsyntax_ext/deriving/encodable.rs
+++ b/src/libsyntax_ext/deriving/encodable.rs
@@ -190,7 +190,7 @@ fn encodable_substructure(cx: &mut ExtCtxt,
         Struct(_, ref fields) => {
             let emit_struct_field = cx.ident_of("emit_struct_field");
             let mut stmts = Vec::new();
-            for (i, &FieldInfo { name, ref self_, span, .. }) in fields.iter().enumerate() {
+            for (i, &FieldInfo { name, ref self_, span, attrs, .. }) in fields.iter().enumerate() {
                 let name = match name {
                     Some(id) => id.name,
                     None => Symbol::intern(&format!("_field{}", i)),
@@ -212,7 +212,19 @@ fn encodable_substructure(cx: &mut ExtCtxt,
                 } else {
                     cx.expr(span, ExprKind::Ret(Some(call)))
                 };
-                stmts.push(cx.stmt_expr(call));
+
+                // This exists for https://github.com/rust-lang/rust/pull/47540
+                //
+                // If we decide to stabilize that flag this can be removed
+                let expr = if attrs.iter().any(|a| a.check_name("rustc_serialize_exclude_null")) {
+                    let is_some = cx.ident_of("is_some");
+                    let condition = cx.expr_method_call(span, self_.clone(), is_some, vec![]);
+                    cx.expr_if(span, condition, call, None)
+                } else {
+                    call
+                };
+                let stmt = cx.stmt_expr(expr);
+                stmts.push(stmt);
             }
 
             // unit structs have no fields and need to return Ok()