Can bypass repr(packed) checking by hiding it in a proc-macro-attribute #32
Description
Given a proc-macro crate like this:
extern crate proc_macro;
use proc_macro::TokenStream;
#[proc_macro_attribute]
pub fn hidden_repr(attr: TokenStream, item: TokenStream) -> TokenStream {
format!("#[repr({})] {}", attr, item).parse().unwrap()
}Using it along with pin-project in a project can cause a value to be pinned in one location and dropped in another:
use core::pin::Pin;
use hidden_repr::hidden_repr;
use pin_project::pin_projectable;
#[derive(Debug)]
struct Foo(u16);
impl Foo {
fn foo(self: Pin<&mut Self>) {
println!("{:?} at {:p}", self, self);
}
}
impl Drop for Foo {
fn drop(&mut self) {
println!("{:?} dropped at {:p}", self, self);
}
}
#[pin_projectable]
#[hidden_repr(packed)]
struct Bar {
_bar: u8,
#[pin]
foo: Foo,
}
fn main() {
let mut bar = Box::pin(Bar { _bar: 0, foo: Foo(0) });
bar.as_mut().project().foo.foo();
}output:
Foo(0) at 0x560220317a41
Foo(0) dropped at 0x7fff2b8510b6
(it appears proc-macro-attribute application order is not defined, so not guaranteed to reproduce, but this seems like the sane order for rustc to apply them in so it's likely to continue working).