Geometry for geometry generators in canvas units

Question

How to I convert @geometry to canvas units, for use in a geometry generator?

Let’s say that I want to draw a 14.1 mm long diagonal line for each feature in a point layer, using a geometry generator. With the units set to “millimeters”, this works:

make_line($geometry, make_point(x($geometry)+10, y($geometry)-10))

Now, the $geometry function is deprecated, and the docs tell me to use @geometry instead. However, while $geometry apparently gives me the coordinates in canvas units, @geometry seems to give them in the original layer units. Is there a function somewhere that can convert @geometry to canvas units? If there is, I can’t find it...