Do not flatten match arm block with leading attributes

Author: topecongiro

rustfmt-core/rustfmt-lib/src/matches.rs

@@ -2,7 +2,7 @@
 
 use std::iter::repeat;
 
-use rustc_ast::{ast, ptr};
+use rustc_ast::{ast, attr::HasAttrs, ptr};
 use rustc_span::{BytePos, Span};
 
 use crate::comment::{combine_strs_with_missing_comments, rewrite_comment};
@@ -298,8 +298,16 @@ fn flatten_arm_body<'a>(
 
     if let Some(ref block) = block_can_be_flattened(context, body) {
         if let ast::StmtKind::Expr(ref expr) = block.stmts[0].kind {
-            if let ast::ExprKind::Block(..) = expr.kind {
-                flatten_arm_body(context, expr, None)
+            if let ast::ExprKind::Block(ref b, _) = expr.kind {
+                if expr.attrs.is_empty()
+                    && b.stmts
+                        .first()
+                        .map_or(false, |stmt| stmt.attrs().is_empty())
+                {
+                    flatten_arm_body(context, expr, None)
+                } else {
+                    (true, body)
+                }
             } else {
                 let cond_becomes_muti_line = opt_shape
                     .and_then(|shape| rewrite_cond(context, expr, shape))