Improve formatting of empty macro_rules! definitions

Fixes 5882

src/macros.rs

@@ -379,6 +379,23 @@ fn handle_vec_semi(
     }
 }
 
+fn rewrite_empty_macro_def_body(
+    context: &RewriteContext<'_>,
+    span: Span,
+    shape: Shape,
+) -> Option<String> {
+    // Create an empty, dummy `ast::Block` representing an empty macro body
+    let block = ast::Block {
+        stmts: vec![].into(),
+        id: rustc_ast::node_id::DUMMY_NODE_ID,
+        rules: ast::BlockCheckMode::Default,
+        span: span,
+        tokens: None,
+        could_be_bare_literal: false,
+    };
+    block.rewrite(context, shape)
+}
+
 pub(crate) fn rewrite_macro_def(
     context: &RewriteContext<'_>,
     shape: Shape,
@@ -419,6 +436,13 @@ pub(crate) fn rewrite_macro_def(
         shape
     };
 
+    if parsed_def.branches.len() == 0 {
+        let lo = context.snippet_provider.span_before(span, "{");
+        result += " ";
+        result += &rewrite_empty_macro_def_body(context, span.with_lo(lo), shape)?;
+        return Some(result);
+    }
+
     let branch_items = itemize_list(
         context.snippet_provider,
         parsed_def.branches.iter(),

tests/source/issue_5882.rs

@@ -0,0 +1,7 @@
+macro_rules!foo{}
+macro_rules!bar{/*comment*/}
+macro_rules!baz{//comment
+}
+macro_rules!foobar{
+//comment
+}

tests/target/issue_5882.rs

@@ -0,0 +1,7 @@
+macro_rules! foo {}
+macro_rules! bar { /*comment*/ }
+macro_rules! baz { //comment
+}
+macro_rules! foobar {
+    //comment
+}