Incorrect Debug derive macro error

[carloabelli]

Code

[package]
name = "minimal"
version = "0.1.0"
edition = "2021"

[dependencies]
rkyv = "0.7"

use rkyv::{
    with::{AsOwned, With},
    Archive, Serialize,
};
use std::{any::type_name, borrow::Cow};

#[derive(Archive, Serialize)]
#[archive_attr(derive(Debug))]
struct Wrapper1(String);

#[derive(Archive, Serialize)]
//#[archive_attr(derive(Debug))] // <== error
struct Wrapper2<'a>(#[with(AsOwned)] Cow<'a, str>);

fn main() {
    println!("{}", type_name::<<String as Archive>::Archived>());
    println!(
        "{}",
        type_name::<<With<Cow<'_, str>, AsOwned> as Archive>::Archived>()
    );
}

Meta

rustc --version --verbose:

rustc 1.59.0 (9d1b2106e 2022-02-23)
binary: rustc
commit-hash: 9d1b2106e23b1abd32fce1f17267604a5102f57a
commit-date: 2022-02-23
host: x86_64-unknown-linux-gnu
release: 1.59.0
LLVM version: 13.0.0

Output

rkyv::string::ArchivedString
rkyv::string::ArchivedString

Error when the commented line is un-commented

error[E0277]: `<With<Cow<'_, str>, AsOwned> as Archive>::Archived` doesn't implement `Debug`
  --> src/main.rs:11:10
   |
11 | #[derive(Archive, Serialize)]
   |          ^^^^^^^ `<With<Cow<'_, str>, AsOwned> as Archive>::Archived` cannot be formatted using `{:?}` because it doesn't implement `Debug`
12 | #[archive_attr(derive(Debug))]
   |                       ----- in this derive macro expansion
   |
   = help: the trait `Debug` is not implemented for `<With<Cow<'_, str>, AsOwned> as Archive>::Archived`
   = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

Despite the fact that <With<Cow<'_, str>, AsOwned> as Archive>::Archived is rkyv::string::ArchivedString which implements Debug.

[carloabelli]

Maybe related to #94651?

[djkoloski]

One workaround for this for the time being is to add:

#[archive(bound(archive = "<With<Cow<'a, str>, AsOwned> as Archive>::Archived: ::core::fmt::Debug"))]

Which explicitly tells the compiler that the archived version of Cow<'a, str> implements Debug. It isn't great but it keeps things moving.