Struct FMMCallbackData 

#[non_exhaustive]
pub struct FMMCallbackData<'src> {
    pub map: &'src [f64],
    pub next_pixels: &'src mut dyn Iterator<Item = GridPos>,
}

A type that will be given as the argument to the callback with crate::edt_fmm_cb.

It has non_exhaustive attribute so that the library can add more arguments in the future.

Fields (Non-exhaustive)

Non-exhaustive structs could have additional fields added in future. Therefore, non-exhaustive structs cannot be constructed in external crates using the traditional Struct { .. } syntax; cannot be matched against without a wildcard ..; and struct update syntax will not work.

map: &'src [f64]

The buffer for Fast Marching output in progress.

next_pixels: &'src mut dyn Iterator<Item = GridPos>

A dynamically dispatched iterator for positions of next pixels.

You can examine “expanding wavefront” by iterating this iterator.