Function assemble_with 

pub fn assemble_with(
    file: &str,
    defines: &[(&str, &str)],
    resolve: impl Fn(&str) -> Option<&'static str>,
) -> String

The same assembly against a caller-supplied resolver, which is how the device crate lets a hot-reload build prefer the checkout’s copy of a shader (and of every fragment it splices) over the embedded one. A resolver that returns None falls back to the embedded text, so a missing file loses the edit rather than the shader.