pub fn truncate(&self, builder: &mut Builder<C>, len: Usize<C::N>) {
        match self {
            Self::Fixed(_) => {
                todo!()
            }
            Self::Dyn(_, old_len) => {
                builder.assign(*old_len, len);
            }
        };
    }
Truncates the array to `len` elements.
crates/recursion/compiler/src/ir/collections.rs