replace placeholder regexp. 

alternate implementation of Ranting
during compile time create a HashMap that
add a vec<String> to the struct.
During compilation build an init function that populates a vec for each unique placeholder imputation
difficult because the path of the define site is required. Maybe via https://doc.rust-lang.org/std/any/fn.type_name_of_val.html
this requires per struct one subject, and the struct name as name, same as enum in a way.

// see https://github.com/rust-lang/rust/issues/44034:
// https://github.com/sam0x17/macro_state

#[proc_macro_derive(Foo)]
pub fn foo(input: TokenStream) -> TokenStream {
    ...
}
#[proc_macro_derive(Bar, depends_on(Foo))]
pub fn bar(input: TokenStream) -> TokenStream {
    ...
}

/////
derive with attribute subject for name and enum. Every instance of this struct/enum will have this subject, and structs their struct name, enums the variant name.

nope, impossible