Regression: "unexpected `cfg` condition name: `coverage_nightly`"

[joshlf]

Originally discovered in google/zerocopy#2117

Using zerocopy 0.8.11, the following code succeeds cargo check on nightly-2024-11-19 but produces a warning on nightly-2024-11-20:

use zerocopy::KnownLayout;

#[derive(KnownLayout)]
#[repr(C)]
pub struct Test(pub [u8; 32]);

Here's the warning:

$ cargo +nightly-2024-11-20 check
    Checking tmp v0.1.0 (.../tmp)
warning: unexpected `cfg` condition name: `coverage_nightly`
 --> src/lib.rs:3:10
  |
3 | #[derive(KnownLayout)]
  |          ^^^^^^^^^^^
  |
  = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
  = help: consider using a Cargo feature instead
  = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
           [lints.rust]
           unexpected_cfgs = { level = "warn", check-cfg = ['cfg(coverage_nightly)'] }
  = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(coverage_nightly)");` to the top of the `build.rs`
  = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
  = note: `#[warn(unexpected_cfgs)]` on by default
  = note: this warning originates in the derive macro `KnownLayout` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: `tmp` (lib) generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.04s

If we remove the #[repr(C)] attribute, then this succeeds without warning on both toolchains.

Here is the output of cargo expand both with and without the #[repr(C)] attribute. You can see that neither include cfg(coverage_nightly).

With #[repr(C)]

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
use zerocopy::KnownLayout;
#[repr(C)]
pub struct Test(pub [u8; 32]);
const _: () = {
    #[allow(deprecated)]
    #[automatically_derived]
    unsafe impl ::zerocopy::KnownLayout for Test
    where
        [u8; 32]: ::zerocopy::KnownLayout,
    {
        fn only_derive_is_allowed_to_implement_this_trait() {}
        type PointerMetadata = <[u8; 32] as ::zerocopy::KnownLayout>::PointerMetadata;
        type MaybeUninit = __ZerocopyKnownLayoutMaybeUninit;
        const LAYOUT: ::zerocopy::DstLayout = {
            use ::zerocopy::util::macro_util::core_reexport::num::NonZeroUsize;
            use ::zerocopy::{DstLayout, KnownLayout};
            let repr_align = ::zerocopy::util::macro_util::core_reexport::option::Option::None;
            let repr_packed = ::zerocopy::util::macro_util::core_reexport::option::Option::None;
            DstLayout::new_zst(repr_align)
                .extend(<[u8; 32] as KnownLayout>::LAYOUT, repr_packed)
                .pad_to_align()
        };
        #[inline(always)]
        fn raw_from_ptr_len(
            bytes: ::zerocopy::util::macro_util::core_reexport::ptr::NonNull<u8>,
            meta: Self::PointerMetadata,
        ) -> ::zerocopy::util::macro_util::core_reexport::ptr::NonNull<Self> {
            use ::zerocopy::KnownLayout;
            let trailing = <[u8; 32] as KnownLayout>::raw_from_ptr_len(bytes, meta);
            let slf = trailing.as_ptr() as *mut Self;
            unsafe {
                ::zerocopy::util::macro_util::core_reexport::ptr::NonNull::new_unchecked(
                    slf,
                )
            }
        }
        #[inline(always)]
        fn pointer_to_metadata(ptr: *mut Self) -> Self::PointerMetadata {
            <[u8; 32]>::pointer_to_metadata(ptr as *mut _)
        }
    }
    #[repr(C)]
    #[doc(hidden)]
    pub struct __ZerocopyKnownLayoutMaybeUninit(
        <[u8; 32] as ::zerocopy::KnownLayout>::MaybeUninit,
    )
    where
        [u8; 32]: ::zerocopy::KnownLayout;
    unsafe impl ::zerocopy::KnownLayout for __ZerocopyKnownLayoutMaybeUninit
    where
        [u8; 32]: ::zerocopy::KnownLayout,
        <[u8; 32] as ::zerocopy::KnownLayout>::MaybeUninit: ::zerocopy::KnownLayout,
    {
        #[allow(clippy::missing_inline_in_public_items)]
        fn only_derive_is_allowed_to_implement_this_trait() {}
        type PointerMetadata = <Test as ::zerocopy::KnownLayout>::PointerMetadata;
        type MaybeUninit = Self;
        const LAYOUT: ::zerocopy::DstLayout = <Test as ::zerocopy::KnownLayout>::LAYOUT;
        #[inline(always)]
        fn raw_from_ptr_len(
            bytes: ::zerocopy::util::macro_util::core_reexport::ptr::NonNull<u8>,
            meta: Self::PointerMetadata,
        ) -> ::zerocopy::util::macro_util::core_reexport::ptr::NonNull<Self> {
            use ::zerocopy::KnownLayout;
            let trailing = <<[u8; 32] as ::zerocopy::KnownLayout>::MaybeUninit as KnownLayout>::raw_from_ptr_len(
                bytes,
                meta,
            );
            let slf = trailing.as_ptr() as *mut Self;
            unsafe {
                ::zerocopy::util::macro_util::core_reexport::ptr::NonNull::new_unchecked(
                    slf,
                )
            }
        }
        #[inline(always)]
        fn pointer_to_metadata(ptr: *mut Self) -> Self::PointerMetadata {
            <<[u8; 32] as ::zerocopy::KnownLayout>::MaybeUninit>::pointer_to_metadata(
                ptr as *mut _,
            )
        }
    }
};

Without #[repr(C)]

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
use zerocopy::KnownLayout;
pub struct Test(pub [u8; 32]);
#[allow(deprecated)]
#[automatically_derived]
unsafe impl ::zerocopy::KnownLayout for Test
where
    Self: ::zerocopy::util::macro_util::core_reexport::marker::Sized,
{
    fn only_derive_is_allowed_to_implement_this_trait() {}
    type PointerMetadata = ();
    type MaybeUninit = ::zerocopy::util::macro_util::core_reexport::mem::MaybeUninit<
        Self,
    >;
    const LAYOUT: ::zerocopy::DstLayout = ::zerocopy::DstLayout::for_type::<Self>();
    #[inline(always)]
    fn raw_from_ptr_len(
        bytes: ::zerocopy::util::macro_util::core_reexport::ptr::NonNull<u8>,
        _meta: (),
    ) -> ::zerocopy::util::macro_util::core_reexport::ptr::NonNull<Self> {
        bytes.cast::<Self>()
    }
    #[inline(always)]
    fn pointer_to_metadata(_ptr: *mut Self) -> () {}
}

Note that, while zerocopy 0.8.11 doesn't emit coverage_nightly from its custom derives, it does use coverage_nightly in the library itself. You can see the source code for zerocopy 0.8.11 here.

[jswrenn]

The PR introducing the regression in this rollup is #132577

searched nightlies: from nightly-2024-11-18 to nightly-2024-11-20
regressed nightly: nightly-2024-11-20
searched commit range: 03ee484...ee612c4
regressed commit: 7899368

bisected with cargo-bisect-rustc v0.6.9

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

cargo bisect-rustc --start=2024-11-18 --end=2024-11-20 

[lqd]

cc @Urgau

[Urgau]

I don't think the output of cargo expand is going to help here, as it shows the output post-expension, at which point #[cfg] attributes don't survive.

You could try looking at the expanded with the cfg activated, RUSTFLAGS="--cfg=coverage_nightly", and see if there is any #[coverage] attribute left.

[Urgau]

Looking at the 0.8.11 source code I can see a coverage_nightly cfg in the zerocopy-derive code.

https://github.com/google/zerocopy/blob/7431cbd5546722ff7f435827dd04bfd99395deaa/zerocopy-derive/src/lib.rs#L304

[joshlf]

Ah you're right - I totally forgot about cfgs getting stripped by cargo expand. I've confirmed that this is indeed a zerocopy bug, and put up a fix: google/zerocopy#2123