remove trailing space inserted with empty dyn macro arg

src/macros.rs

@@ -303,9 +303,9 @@ fn rewrite_macro_inner(
 
     if DelimToken::Brace != style {
         loop {
-            if let Some(arg) = parse_macro_arg(&mut parser) {
+            if let Some(arg) = check_keyword(&mut parser) {
                 arg_vec.push(arg);
-            } else if let Some(arg) = check_keyword(&mut parser) {
+            } else if let Some(arg) = parse_macro_arg(&mut parser) {
                 arg_vec.push(arg);
             } else {
                 return return_macro_parse_failure_fallback(context, shape.indent, mac.span);

src/overflow.rs

@@ -122,6 +122,7 @@ impl<'a> OverflowableItem<'a> {
     pub(crate) fn is_simple(&self) -> bool {
         match self {
             OverflowableItem::Expr(expr) => is_simple_expr(expr),
+            OverflowableItem::MacroArg(MacroArg::Keyword(..)) => true,
             OverflowableItem::MacroArg(MacroArg::Expr(expr)) => is_simple_expr(expr),
             OverflowableItem::NestedMetaItem(nested_meta_item) => match nested_meta_item {
                 ast::NestedMetaItem::Literal(..) => true,

tests/source/issue-3709.rs

@@ -0,0 +1,10 @@
+// rustfmt-edition: 2018
+
+macro_rules! token {
+    ($t:tt) => {};
+}
+
+fn main() {
+    token!(dyn);
+    token!(dyn );
+}

tests/target/issue-3709.rs

@@ -0,0 +1,10 @@
+// rustfmt-edition: 2018
+
+macro_rules! token {
+    ($t:tt) => {};
+}
+
+fn main() {
+    token!(dyn);
+    token!(dyn);
+}