What is the reason for the lifetime argument in the Facet trait?

[mikeyhew]

When playing around with facet I noticed that the Facet trait takes a lifetime argument, but I didn't see an obvious reason for it since it's not used in any of the associated items, and I didn't see any documentation explaining its purpose.

pub unsafe trait Facet<'a>: 'a {
    const SHAPE: &'static Shape<'static>;
    const VTABLE: &'static ValueVTable;
}

What is the lifetime parameter 'a for? I'm guessing there's some sort of soundness reason, but not sure what that reason is.