Fix Windows crash in bun build with --outdir

Fixes #23647

The issue was caused by calling std.posix.toPosixPath unconditionally
before opening a directory. This function doesn't handle Windows paths
properly (backslashes, drive letters, UNC paths, etc.).

The fix replaces the manual path conversion + openDirForPath call with
bun.sys.openA, which handles path conversion internally for both Windows
and POSIX systems. This is simpler and more correct.

Added a regression test to prevent this from happening again.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/cli/build_command.zig

@@ -164,10 +164,17 @@ pub const BuildCommand = struct {
                 break :brk2 resolve_path.getIfExistsLongestCommonPath(this_transpiler.options.entry_points) orelse ".";
             };
 
-            var dir = bun.FD.fromStdDir(bun.openDirForPath(&(try std.posix.toPosixPath(path))) catch |err| {
-                Output.prettyErrorln("<r><red>{s}<r> opening root directory {}", .{ @errorName(err), bun.fmt.quote(path) });
-                Global.exit(1);
-            });
+            // Use bun.sys.openA which handles path conversion internally for both Windows and POSIX
+            const O_PATH = if (comptime bun.Environment.isLinux) bun.O.PATH else bun.O.RDONLY;
+            const flags = bun.O.CLOEXEC | bun.O.NOCTTY | bun.O.DIRECTORY | O_PATH;
+
+            var dir = switch (bun.sys.openA(path, flags, 0)) {
+                .result => |fd| fd,
+                .err => |err| {
+                    Output.prettyErrorln("<r><red>{}<r> opening root directory {}", .{ err, bun.fmt.quote(path) });
+                    Global.exit(1);
+                },
+            };
             defer dir.close();
 
             break :brk1 dir.getFdPath(&src_root_dir_buf) catch |err| {

test/regression/issue/23647.test.ts

@@ -0,0 +1,55 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+import * as fs from "node:fs";
+import * as path from "node:path";
+
+// https://github.com/oven-sh/bun/issues/23647
+// Test that `bun build --outdir=dist --target=browser` works without crashing
+// This was crashing on Windows due to incorrect path handling in build_command.zig
+test("23647 - build with outdir and browser target doesn't crash", async () => {
+  using dir = tempDir("23647", {
+    "index.ts": `
+      import { Hono } from "hono";
+      const app = new Hono();
+      app.get("/hello", (c) => c.text("Hello!"));
+      export default { fetch: app.fetch };
+    `,
+    "package.json": `{
+      "name": "test-23647",
+      "dependencies": {
+        "hono": "^4.9.12"
+      }
+    }`,
+  });
+
+  // First install dependencies
+  const install = Bun.spawn({
+    cmd: [bunExe(), "install"],
+    cwd: String(dir),
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+  await install.exited;
+
+  // Now run the build that was crashing on Windows
+  const result = Bun.spawn({
+    cmd: [bunExe(), "build", "--outdir=dist", "--target=browser", "--production", "./index.ts"],
+    cwd: String(dir),
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const exitCode = await result.exited;
+  const stdout = await result.stdout.text();
+  const stderr = await result.stderr.text();
+
+  // Should not crash - exit code should be 0
+  expect(exitCode).toBe(0);
+  // Should indicate successful bundling
+  expect(stdout).toContain("module");
+  // Should create the output directory
+  const outputPath = path.join(String(dir), "dist", "index.js");
+  expect(fs.existsSync(outputPath)).toBe(true);
+}, 30000);