Function render 

pub fn render(
    language: &str,
    code: &str,
    height: Option<u32>,
    window: &mut Window,
    cx: &mut App,
) -> Option<AnyElement>

Paint the block a fence names, or None to leave it to the ordinary code block. height is the fence’s stated height in whole pixels, for a block that takes one.

One answer for a tag no enabled block claims, a block turned off at compile time, and a block that read the source and declined.