fix: set dest_path for --no-bundle with --outdir

When using `bun build --no-bundle --outdir`, the output file's dest_path
was not being set, causing ENOENT error when trying to write to an empty
filename.

This fix computes dest_path by taking the input file's basename and
replacing the extension with the appropriate output extension (e.g.,
.ts -> .js).

Fixes #10370

https://claude.ai/code/session_01WTzMmxa1RL14NCnqw1scjn

src/transpiler.zig

@@ -616,6 +616,9 @@ pub const Transpiler = struct {
 
         file_path.pretty = Linker.relative_paths_list.append(string, transpiler.fs.relativeTo(file_path.text)) catch unreachable;
 
+        const out_ext = transpiler.options.out_extensions.get(file_path.name.ext) orelse file_path.name.ext;
+        const dest_path = std.fmt.allocPrint(transpiler.allocator, "{s}{s}", .{ file_path.name.base, out_ext }) catch unreachable;
+
         var output_file = options.OutputFile{
             .src_path = file_path,
             .loader = loader,
@@ -623,6 +626,7 @@ pub const Transpiler = struct {
             .side = null,
             .entry_point_index = null,
             .output_kind = .chunk,
+            .dest_path = dest_path,
         };
 
         switch (loader) {

test/regression/issue/10370.test.ts

@@ -0,0 +1,64 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+import { readdirSync, existsSync } from "fs";
+import { join } from "path";
+
+test("bun build --no-bundle --outdir should output files correctly - issue #10370", async () => {
+  using dir = tempDir("10370-no-bundle-outdir", {
+    "index.ts": `console.log("hello from typescript");`,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "build", "./index.ts", "--no-bundle", "--outdir", "./dist"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([
+    proc.stdout.text(),
+    proc.stderr.text(),
+    proc.exited,
+  ]);
+
+  expect(stderr).not.toContain("error");
+  expect(exitCode).toBe(0);
+  
+  // Verify the output file exists
+  const distDir = join(String(dir), "dist");
+  expect(existsSync(distDir)).toBe(true);
+  
+  const files = readdirSync(distDir);
+  expect(files).toContain("index.js");
+});
+
+test("bun build --no-bundle --outdir should handle .tsx extension - issue #10370", async () => {
+  using dir = tempDir("10370-no-bundle-outdir-tsx", {
+    "App.tsx": `export function App() { return <div>Hello</div>; }`,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "build", "./App.tsx", "--no-bundle", "--outdir", "./out"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([
+    proc.stdout.text(),
+    proc.stderr.text(),
+    proc.exited,
+  ]);
+
+  expect(stderr).not.toContain("error");
+  expect(exitCode).toBe(0);
+  
+  // Verify the output file exists
+  const outDir = join(String(dir), "out");
+  expect(existsSync(outDir)).toBe(true);
+  
+  const files = readdirSync(outDir);
+  expect(files).toContain("App.js");
+});