Dagger Hilt plugin doesn't work with `com.android.legacy-kapt` plugin

[hungvietnguyen]

Context

Built-in Kotlin is a new feature in the Android Gradle plugin which allows Android developers to use Kotlin without having to apply the kotlin-android plugin (https://issuetracker.google.com/259523353).

In AGP 9.0 (canaries coming soon), built-in Kotlin support will be enabled by default.

With built-in Kotlin support, users will no longer be able to use the kotlin-kapt plugin (the build will fail). Instead, to continue using Kapt, users will need to apply the new com.android.legacy-kapt plugin (introduced in this commit, available in AGP 8.10+).

Problem

com.google.dagger.hilt.android plugin currently works with the kotlin-kapt plugin, but doesn't work with AGP's built-in Kotlin and the com.android.legacy-kapt plugin.

To reproduce this issue:

1. Open the attached project: kapt-android-sample.zip (same project used in scenario 3 of https://issuetracker.google.com/421403256). This project uses AGP's built-in Kotlin + com.android.legacy-kapt plugin.

2. Run :app:kaptDebugKotlin. The build will fail with:

> Task :app:kaptDebugKotlin
kapt-android-sample/app/build/intermediates/built_in_kapt_stubs/debug/kaptGenerateStubsDebugKotlin/com/example/dependency_injection_with_hilt/views/FirstFragment.java:20: error: [Hilt] Expected @AndroidEntryPoint to have a value. Did you forget to apply the Gradle Plugin? (com.google.dagger.hilt.android)
public final class FirstFragment extends androidx.fragment.app.Fragment implements com.example.dependency_injection_with_hilt.adapters.BaseRecyclerClickListener<com.example.dependency_injection_with_hilt.models.ExampleData> {
             ^
  See https://dagger.dev/hilt/gradle-setup.html
  [Hilt] Processing did not complete. See error above for details.

3. Try using kotlin-android and kotlin-kapt plugins instead:
   • In <root project>/build.gradle.kts, replace alias(libs.plugins.android.kotlin) apply false with alias(libs.plugins.jetbrains.kotlin.android) apply false.
   • In app/build.gradle.kts, replace alias(libs.plugins.android.kotlin) with alias(libs.plugins.jetbrains.kotlin.android)
   • In app/build.gradle.kts, replace id("com.android.legacy-kapt") with kotlin("kapt").

4. Run `:app:kaptDebugKotlin` again. The build will succeed.

Potential Root Cause

Hilt currently has a logic to run if the kotlin-kapt plugin is applied (this code).

To fix this issue, Hilt should probably expand that check to cover com.android.legacy-kapt plugin as well.