Do not put if on the same line as match arm

src/expr.rs

@@ -1500,7 +1500,8 @@ fn flatten_arm_body<'a>(context: &'a RewriteContext, body: &'a ast::Expr) -> (bo
         {
             if let ast::StmtKind::Expr(ref expr) = block.stmts[0].node {
                 (
-                    !context.config.force_multiline_blocks() && expr.can_be_overflowed(context, 1),
+                    !context.config.force_multiline_blocks()
+                        && (can_extend_match_arm_body(expr), &**expr),
                     &**expr,
                 )
             } else {
@@ -1723,6 +1724,33 @@ fn rewrite_pat_expr(
         .map(|expr_rw| format!("\n{}{}", nested_indent_str, expr_rw))
 }
 
+fn can_extend_match_arm_body(body: &ast::Expr) -> bool {
+    match body.node {
+        // We do not allow `if` to stay on the same line, since we could easily mistake
+        // `pat => if cond { ... }` and `pat if cond => { ... }`.
+        ast::ExprKind::If(..) | ast::ExprKind::IfLet(..) => false,
+        ast::ExprKind::ForLoop(..) |
+        ast::ExprKind::Loop(..) |
+        ast::ExprKind::While(..) |
+        ast::ExprKind::WhileLet(..) |
+        ast::ExprKind::Match(..) |
+        ast::ExprKind::Block(..) |
+        ast::ExprKind::Closure(..) |
+        ast::ExprKind::Array(..) |
+        ast::ExprKind::Call(..) |
+        ast::ExprKind::MethodCall(..) |
+        ast::ExprKind::Mac(..) |
+        ast::ExprKind::Struct(..) |
+        ast::ExprKind::Tup(..) => true,
+        ast::ExprKind::AddrOf(_, ref expr) |
+        ast::ExprKind::Box(ref expr) |
+        ast::ExprKind::Try(ref expr) |
+        ast::ExprKind::Unary(_, ref expr) |
+        ast::ExprKind::Cast(ref expr, _) => can_extend_match_arm_body(expr),
+        _ => false,
+    }
+}
+
 pub fn rewrite_literal(context: &RewriteContext, l: &ast::Lit, shape: Shape) -> Option<String> {
     match l.node {
         ast::LitKind::Str(_, ast::StrStyle::Cooked) => rewrite_string_lit(context, l.span, shape),