Function set_hl_ns_fast 

pub fn set_hl_ns_fast(ns_id: u32) -> Result<(), Error>

Binding to nvim_set_hl_ns_fast().

Set the active namespace for the highlights defined with set_hl while redrawing.

This function is meant to be called while redrawing, primarily from set_decoration_provider’s on_win and on_lines callbacks, which are allowed to change the namespace during a redraw cycle.