Can bypass #[repr(packed)] checking since rust 1.57

[macpp]

Since rust 1.57 it is possible to put proc macro attributes after derive (see rust-lang/rust#81119)
so given proc macro crate like that:

use proc_macro::TokenStream;
use syn::{parse_macro_input, parse_quote, ItemStruct};
use quote::quote;

#[proc_macro_attribute]
pub fn add_repr(_:TokenStream, item: TokenStream) -> TokenStream {
    let mut item = parse_macro_input!(item as ItemStruct);
    item.attrs.push(parse_quote!{#[add_repr_hidden]});
    quote!{#item}.into()
}

#[proc_macro_attribute]
pub fn add_repr_hidden(_:TokenStream, item: TokenStream) -> TokenStream {
    let mut item = parse_macro_input!(item as ItemStruct);
    item.attrs.push(parse_quote!{#[repr(packed)]});
    quote!{#item}.into()
}

and following usage :

#[pin_project]
#[add_repr]
pub struct Test {
    x: i32
}

__PinProjectInternalDerive won't notice that at the end of the expansion there is #[repr(packed)] on struct
Of course, --cap-lints=warn is still required to actually compile it
I'm not sure how probable it is to trigger this problem in real code base..

[taiki-e]

As well as taiki-e/pin-project-lite#26, it should eventually be fixed by unaligned_references becoming a hard error.

[taiki-e]

unaligned_references is now a hard error 🎉: rust-lang/rust#102513