Function get_vtable 

pub(crate) fn get_vtable<'tcx, Cx: CodegenMethods<'tcx>>(
    cx: &Cx,
    ty: Ty<'tcx>,
    trait_ref: Option<ExistentialTraitRef<'tcx>>,
) -> Cx::Value

Creates a dynamic vtable for the given type and vtable origin. This is used only for objects.

The vtables are cached instead of created on every call.

The trait_ref encodes the erased self type. Hence if we are making an object Foo<dyn Trait> from a value of type Foo<T>, then trait_ref would map T: Trait.