Fix using uninitialized variable when formatting config_path. (#4129)

Close: #4128

src/cli.zig

@@ -313,7 +313,7 @@ pub const Arguments = struct {
         defer ctx.debug.loaded_bunfig = true;
         var config_path: [:0]u8 = undefined;
         if (config_path_[0] == '/') {
-            @memcpy(config_buf[0..config_path.len], config_path);
+            @memcpy(config_buf[0..config_path_.len], config_path_);
             config_buf[config_path_.len] = 0;
             config_path = config_buf[0..config_path_.len :0];
         } else {

test/cli/bun.test.ts

@@ -1,6 +1,8 @@
 import { describe, test, expect } from "bun:test";
 import { spawnSync } from "bun";
 import { bunExe } from "harness";
+import { tmpdir } from "node:os";
+import fs from "node:fs";
 
 describe("bun", () => {
   describe("NO_COLOR", () => {
@@ -59,4 +61,22 @@ describe("bun", () => {
       );
     });
   });
+
+  describe("test command line arguments", () => {
+    test("test --config, issue #4128", () => {
+      const path = `${tmpdir()}/bunfig-${Date.now()}.toml`;
+      fs.writeFileSync(path, "[debug]");
+
+      const p = Bun.spawnSync({
+        cmd: [bunExe(), "--config", path],
+        env: {},
+        stderr: "inherit",
+      });
+      try {
+        expect(p.exitCode).toBe(0);
+      } finally {
+        fs.unlinkSync(path);
+      }
+    });
+  });
 });