Can you use a grayscale image with no alpha as a layer mask?

Normally if you install an image as the contents of a layer's mask layer, the alpha channel of the mask layer's image is what masks the other layer. I know how to do that.

I have a vague memory of reading that you can instead install a grayscale image with no alpha and use the brightness of the image as the mask. I've tried searching various ways, but can't find any documentation about that.

Am I imagining it?

It would be more memory efficient, and make your code cleaner, if you could draw into a mask image using a shade of gray.

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