Android data binding attribute not found

I am trying to replicate this answer: Setting attribute of child element of included layout

I have a simple custom_edit_text.xml:

<?xml version="1.0" encoding="utf-8"?>
<layout
    xmlns:android="http://schemas.android.com/apk/res/android">
    <data>
        <variable name="hint123" type="String" />
    </data>
    <android.support.design.widget.TextInputLayout
        android:id="@+id/emailInputLayout"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content">
        <android.support.v7.widget.AppCompatEditText
            android:id="@+id/emailField"
            android:layout_width="275dp"
            android:layout_height="wrap_content"
            android:paddingBottom="16dp"
            android:paddingTop="14dp"
            android:hint="@{hint123}"
            android:textCursorDrawable="@null"
            android:background="@drawable/edit_text_background"
            android:fontFamily="@font/eina_regular"
            android:textColor="@color/edit_text_color"
            android:textColorHint="@color/edit_text_color"
            android:textSize="15sp"
            />
    </android.support.design.widget.TextInputLayout>
</layout>

And I include it in another file:

<?xml version="1.0" encoding="utf-8"?>
<layout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <include
            layout="@layout/custom_edit_text"
            app:hint123="Email"/>
</layout>

However the project refuses to compile after a clean & rebuild with the error:

AAPT: error: attribute hint123 (aka inc.company.appname:hint123) not found.

Any ideas?

I also have

dataBinding {
    enabled = true
}

enabled in the app level build.gradle

I think I've hit upon the solution. To activate data binding, you need to use a @{} expression, and what's in the braces must be valid Java code. So a literal string must be enclosed in quotes... which must be encoded as " inside an XML attribute value. Put it all together and you get:

<include
    layout="@layout/custom_edit_text"
    app:hint123="@{&quot;Email&quot;}"/>

Data binding does work for include files, it's just that the syntax for a literal is a bit convoluted. I had the same issue and this form is working in my project.