Macro map_none 

macro_rules! map_none {
    ($option:expr, $variant:ident$(,)? $($arg:ident$(: $value:expr)?),*) => { ... };
}

Converts None into an error variant without returning early.

map_none should be used only when the error variant doesn’t capture any owned variables (which is very rare), or exactly at the end of the block (in the position of returned expression).