Function parse 

pub fn parse<T: Parse>(
    dst: &mut T,
) -> impl '_ + FnMut(ParseStream<'_>) -> Result<()>

Parse a Parse-able, assigning its value to dst.

You should take care that the Parse does not advance into subsequent keys, else use parse_str instead.

let mut val = Ident::new("Default", Span::call_site());

Attrs::new()
    .once("key", with::eq(on::parse(&mut val)))
    .parse2(quote!(key = Override))?;

assert_eq!(val, Ident::new("Override", Span::call_site()));