fix panic on closure with empty block expr (#3846)

src/closures.rs

@@ -91,8 +91,9 @@ fn get_inner_expr<'a>(
 ) -> &'a ast::Expr {
     if let ast::ExprKind::Block(ref block, _) = expr.kind {
         if !needs_block(block, prefix, context) {
-            // block.stmts.len() == 1
-            if let Some(expr) = stmt_expr(&block.stmts[0]) {
+            // block.stmts.len() == 1 except with `|| {{}}`;
+            // https://github.com/rust-lang/rustfmt/issues/3844
+            if let Some(expr) = block.stmts.first().and_then(stmt_expr) {
                 return get_inner_expr(expr, prefix, context);
             }
         }

tests/source/issue_3844.rs

@@ -0,0 +1,3 @@
+fn main() {
+|| {{}};
+}

tests/target/issue_3844.rs

@@ -0,0 +1,3 @@
+fn main() {
+    || {};
+}