Function expand_deriving_from 

pub(crate) fn expand_deriving_from(
    cx: &ExtCtxt<'_>,
    span: Span,
    mitem: &MetaItem,
    annotatable: &Annotatable,
    push: &mut dyn FnMut(Annotatable),
    is_const: bool,
)

Generate an implementation of the From trait, provided that item is a struct or a tuple struct with exactly one field.