Downcasting from a trait object to a generic struct that implements the trait.

DowncastGenericStruct.rs

trait A {
	fn as_any_mut(&mut self) -> &mut dyn std::any::Any;
}
struct B<T> {
	_unused: T
}
impl<T: 'static> A for B<T> { // static, else: "the parameter type `T` may not live long enough"
	fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
		self
	}
}

let mut inst = Box::new(B{_unused: true});
let virt: &mut dyn A = &mut (*inst);

let _cast = virt.as_any_mut().downcast_mut::<B<bool>>().unwrap();