Throw error while trying to set a dagger flag via annotationProcessor

RELNOTES=Added error while trying to set a dagger flag via annotationProcessor
PiperOrigin-RevId: 882008099

Author: trexem

dev-guide/compiler-options.md

@@ -65,7 +65,7 @@ broken by this change it can usually be fixed by just moving the module to the
 correct component. For example, we can fix the case above by installing the
 module into the `ActivityComponent` instead of the `SingletonComponent`:
 
-## fastInit mode
+## fastInit mode {#fast-init}
 
 You can choose to generate your Dagger [`@Component`]s in a mode that
 prioritizes fast initialization. Normally, the number of classes loaded when
@@ -95,9 +95,11 @@ leaks would always have the potential to have grow dependencies on the activity
 as the codebase changes (thereby leaking it in the default mode as well).
 
 To enable fastInit mode, pass the following option to javac when building your
-Dagger [`@Component`]: `-Adagger.fastInit=enabled`. Note that if you are using
-the Hilt Gradle Plugin in your project, fastInit mode will already be enabled by
-default.
+Dagger [`@Component`]: `-Adagger.fastInit=enabled`.
+
+**Note:** If you are using Hilt, fastInit mode is enabled by default but can be
+configured differently. See
+[Hilt Fast Init](https://dagger.dev/hilt/flags#fast-init) for more details.
 
 ## Turning on code formatting
 

hilt/flags.md

@@ -78,6 +78,23 @@ a migration. To turn off this check, this flag can be used:
 
 `-Adagger.hilt.disableCrossCompilationRootValidation=true`.
 
+## Fast Init {#fast-init}
+
+Hilt ["Fast Init"][fast-init-link] is an optimization that improves
+startup performance by avoiding class loading during Hilt initialization. It is
+the recommended mode for android applications.
+
+If you are not using the Hilt Gradle plugin, see Dagger's
+[compiler options][compiler-options-link] for instructions on how
+to enable Fast Init.
+
+If you are using the Hilt Gradle plugin:
+
+  * FastInit is enabled by default.
+  * FastInit can only be disabling via the `dagger.hilt.fastInit` Gradle
+    property, e.g. via command line: `-Pdagger.hilt.fastInit=false`.
+  * Hilt will fail if you try to set the flag via Annotation Processing options.
+
 ## Runtime flags
 
 Runtime flags to control Hilt behavior for rollout of changes. These flags are
@@ -123,3 +140,6 @@ This flag used to be paired with a compiler option flag
 `dagger.hilt.android.useFragmentGetContextFix`, however, as of Dagger 2.40 this
 compiler option has now been removed and this behavior is only controlled via
 the runtime flag.
+
+[fast-init-link]: https://dagger.dev/compiler-options.html#fastinit-mode
+[compiler-options-link]: https://dagger.dev/compiler-options.html#fastinit-mode