Regression in 1.93.0 `the type <XYZ as Archive>::Archived has an unknown layout`

[konnorandrews]

Code

I tried the following in a new cargo project. This is minimized from a real codebase.

[package]
name = "broken-nightly"
version = "0.1.0"
edition = "2024"

[dependencies]
cynic-codegen = { version = "3.12.0", features = [ "rkyv" ] }

[profile.dev.package.cynic-codegen]
opt-level = 1 # only 0 doesn't cause the issue

I expected to see this happen: compilation to succeed.

Instead, this happened: multiple errors that are basically impossible to track down the cause of.

error[E0080]: the type `<With<Cow<'_, str>, AsOwned> as Archive>::Archived` has an unknown layout
 --> /rustc/254b59607d4417e9dffbc307138ae5c86280fe4c/library/core/src/mem/mod.rs:1251:26
  |
  = note: evaluation of `<schema::names::ArchivedFieldName<'_> as std::mem::SizedTypeProperties>::ALIGN` failed here

error[E0080]: the type `<std::collections::HashMap<std::string::String, schema::types::Type<'_>> as Archive>::Archived` has an unknown layout
 --> /rustc/254b59607d4417e9dffbc307138ae5c86280fe4c/library/core/src/mem/mod.rs:1251:26
  |
  = note: evaluation of `<schema::type_index::optimised::ArchivedOptimisedTypes<'_> as std::mem::SizedTypeProperties>::ALIGN` failed here

error[E0080]: the type `<schema::types::ObjectType<'_> as Archive>::Archived` has an unknown layout
 --> /rustc/254b59607d4417e9dffbc307138ae5c86280fe4c/library/core/src/mem/mod.rs:1251:26
  |
  = note: evaluation of `<schema::types::ArchivedSchemaRoots<'_> as std::mem::SizedTypeProperties>::ALIGN` failed here

error[E0080]: the type `<schema::types::ScalarType<'_> as Archive>::Archived` has an unknown layout
 --> /rustc/254b59607d4417e9dffbc307138ae5c86280fe4c/library/core/src/mem/mod.rs:1251:26
  |
  = note: evaluation of `<schema::types::ArchivedType<'_> as std::mem::SizedTypeProperties>::ALIGN` failed here

...

For anyone that is encountering this in the wild, updating cynic to be the latest main that uses rkyv 0.8 resolves the issue.

It would seem specifically rkyv 0.7 is causing the issue when applied to the cynic-codegen types.

Version it worked on

It most recently worked on: 1.92.0

Version with regression

rustc --version --verbose:

rustc 1.93.0 (254b59607 2026-01-19)

Running a bisection I get the nightly that introduced the bug to be nightly-2025-10-24. I am unable to get the exact commit at the moment because of GitHub rate limiting.

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged

[zachs18]

searched nightlies: from nightly-2025-10-07 to nightly-2025-10-28
regressed nightly: nightly-2025-10-24
searched commit range: 1d23d06...6501e64
regressed commit: 4b3ba58

bisected with cargo-bisect-rustc v0.6.11

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --preserve --start=2025-10-07 --end=2025-10-28 

CC #147793 @cjgillot

[zachs18]

Minimized cynic-codegen out (with same bisection):

# Cargo.toml
[package]
name = "sdfasldfs"
version = "0.1.0"
edition = "2024"

[dependencies.rkyv]
version = "0.7"
features = ["validation"]

[profile.dev]
opt-level = 1

// src/lib.rs
use std::borrow::Cow;

#[derive(rkyv::Archive)]
#[archive(check_bytes)]
pub struct FieldName<'a> {
    #[with(rkyv::with::AsOwned)]
    pub graphql_name: Cow<'a, str>,
}

Further minimized (no derive macros):

// src/main.rs
use std::borrow::Cow;

struct ArchivedFieldName<'a> {
    pub graphql_name: ::rkyv::Archived<
        ::rkyv::with::With<Cow<'a, str>, rkyv::with::AsOwned>,
    >,
}

unsafe fn foo<'a>(x: *const ArchivedFieldName<'a>) -> &'a ArchivedFieldName<'a>
where
    ::rkyv::with::With<Cow<'a, str>, rkyv::with::AsOwned>: ::rkyv::Archive, // *
{
    unsafe { &*x }
}


fn main() {
    std::hint::black_box(foo as unsafe fn(_) -> _); // force codegenning `foo`
}

Removing the where clause marked // * causes the error to not reproduce (it compiles successfully, With<Cow<'a, str>, AsOwned> always implements Archive).
foo can be generic over 'a, or specifically mention 'static, either still reproduces the error. As does where for<'b> With<Cow<'b, str>, ....
foo needs to be codegenned for the error to reproduce.

Possibly of note:

• rkyv::Archived<T> is a type alias pub type Archived<T> = <T as Archive>::Archived;
• With<Cow<'a, str>, rkyv::with::AsOwned> implements Archive for all 'a (via this impl, via this blanket impl), with Archived = ArchivedString.
• Maybe having both an "ambient" impl and a where bound for With<Cow<'a, str>, AsOwned> is messing up trait solving somewhere?

[theemathas]

Minimized :

Cargo.toml

[package]
name = "repro"
version = "0.1.0"
edition = "2024"

[profile.dev]
opt-level = 1

src/main.rs

struct ArchivedString;

pub trait ArchiveWith<'a> {
    type Archived;
}

struct AsOwned;
impl ArchiveWith<'_> for AsOwned {
    type Archived = ArchivedString;
}

fn foo<'a>()
where
    AsOwned: ArchiveWith<'a>,
{
    let _ = unsafe { &*std::ptr::dangling::<<AsOwned as ArchiveWith<'a>>::Archived>() };
}

fn main() {
    foo();
}

Error output:

error[E0080]: the type `<AsOwned as ArchiveWith<'_>>::Archived` has an unknown layout
    --> /Users/timch/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/mem/mod.rs:1251:26
     |
1251 | ...ze = intrinsics::align_of::<Self>();
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `<<AsOwned as ArchiveWith<'_>>::Archived as std::mem::SizedTypeProperties>::ALIGN` failed here

For more information about this error, try `rustc --explain E0080`.