rust-analyzer doesn't like `zerocopy::KnownLayout` on DSTs

[allisonkarlitskaya]

rust-analyzer version: rust-analyzer 1.83.0 via rust-analyzer-1.83.0-1.fc41.x86_64
rustc version: rustc 1.83.0 (90b35a623 2024-11-26) (Fedora 1.83.0-1.fc41) via rust-1.83.0-1.fc41.x86_64
editor or extension: nvim from Fedora 41: neovim-0.10.2-1.fc41.x86_64
relevant settings:

LSP configs active in this buffer (bufnr: 1) ~
- Language client log: ~/.local/state/nvim/lsp.log
- Detected filetype: `rust`
- 1 client(s) attached to this buffer
- Client: `rust_analyzer` (id: 1, bufnr: [1])
  root directory:    ~/src/reproducer/
  filetypes:         rust
  cmd:               /usr/local/bin/rust-analyzer
  version:           `rust-analyzer 1.85.0-nightly (d4025ee 2024-12-12)`
  executable:        true
  autostart:         true

code snippet to reproduce:

// add your code here
use zerocopy::KnownLayout;

#[derive(KnownLayout)]
#[repr(C)]
struct Flexible {
    header: u32,
    body: [u8],
}

This gets you

1. casting `*mut _` as `*mut __ZerocopyKnownLayoutMaybeUninit` is invalid: vtable kinds may not match [E0606]
2. casting `*mut __ZerocopyKnownLayoutMaybeUninit` as `*mut _` is invalid: vtable kinds may not match [E0606]

but with cargo build it's fine.

This is caused by Flexible being a DST (dynamically sized type). Dropping the variable-sized body: [u8] field makes the problem go away.

I also tried with nightly (rust-analyzer 1.85.0-nightly (d4025ee 2024-12-12)) without improvement.

[sudosilico]

If you don't get any complaints elsewhere from rust-analyzer (only from rustc) when you remove the KnownLayout derive, then a potential workaround is to tell rust-analyzer to ignore the KnownLayout macro:

"rust-analyzer.procMacro.ignored": {
  "zerocopy_derive": ["KnownLayout"]
},

[joshlf]

For help in debugging this issue, here's the cargo expand output of the provided code snippet using zerocopy 0.8.15:

Code

Source:

use zerocopy::KnownLayout;

#[derive(KnownLayout)]
#[repr(C)]
struct Flexible {
    header: u32,
    body: [u8],
}

Output of cargo expand:

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
use zerocopy::KnownLayout;
#[repr(C)]
struct Flexible {
    header: u32,
    body: [u8],
}
const _: () = {
    #[allow(deprecated)]
    #[automatically_derived]
    unsafe impl ::zerocopy::KnownLayout for Flexible
    where
        [u8]: ::zerocopy::KnownLayout,
    {
        fn only_derive_is_allowed_to_implement_this_trait() {}
        type PointerMetadata = <[u8] 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(DstLayout::for_type::<u32>(), repr_packed)
                .extend(<[u8] 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] 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]>::pointer_to_metadata(ptr as *mut _)
        }
    }
    #[allow(non_camel_case_types)]
    struct __Zerocopy_Field_header;
    #[allow(non_camel_case_types)]
    struct __Zerocopy_Field_body;
    unsafe impl ::zerocopy::util::macro_util::Field<__Zerocopy_Field_header>
    for Flexible {
        type Type = u32;
    }
    unsafe impl ::zerocopy::util::macro_util::Field<__Zerocopy_Field_body> for Flexible {
        type Type = [u8];
    }
    #[repr(C)]
    #[doc(hidden)]
    #[allow(private_bounds)]
    struct __ZerocopyKnownLayoutMaybeUninit(
        ::zerocopy::util::macro_util::core_reexport::mem::MaybeUninit<
            <Flexible as ::zerocopy::util::macro_util::Field<
                __Zerocopy_Field_header,
            >>::Type,
        >,
        <<Flexible as ::zerocopy::util::macro_util::Field<
            __Zerocopy_Field_body,
        >>::Type as ::zerocopy::KnownLayout>::MaybeUninit,
    )
    where
        <Flexible as ::zerocopy::util::macro_util::Field<
            __Zerocopy_Field_body,
        >>::Type: ::zerocopy::KnownLayout;
    unsafe impl ::zerocopy::KnownLayout for __ZerocopyKnownLayoutMaybeUninit
    where
        <Flexible as ::zerocopy::util::macro_util::Field<
            __Zerocopy_Field_body,
        >>::Type: ::zerocopy::KnownLayout,
    {
        #[allow(clippy::missing_inline_in_public_items)]
        fn only_derive_is_allowed_to_implement_this_trait() {}
        type PointerMetadata = <Flexible as ::zerocopy::KnownLayout>::PointerMetadata;
        type MaybeUninit = Self;
        const LAYOUT: ::zerocopy::DstLayout = <Flexible 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 = <<<Flexible as ::zerocopy::util::macro_util::Field<
                __Zerocopy_Field_body,
            >>::Type 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 {
            <<<Flexible as ::zerocopy::util::macro_util::Field<
                __Zerocopy_Field_body,
            >>::Type as ::zerocopy::KnownLayout>::MaybeUninit>::pointer_to_metadata(
                ptr as *mut _,
            )
        }
    }
};

[ShoyuVanilla]

I've been planning to resolve this issue, but couldn't make time for it 😅 I'll work on this today

[ShoyuVanilla]

It seems that r-a is failing to infer the projection types in the definition of __ZerocopyKnownLayoutMaybeUninit.
Replacing them with direct, actual type removes those false positive diagnostics.
r-a fails to infer projection types in some places, and as we are migrating its trait solver from chalk to rustc's next-gen solver, this might be mitigated in future.
I'll add a code that skips this diagnostics if we are failing to infer the projection types.
edit) It looks like that I've missed resolving the projection type 😅 I'll fix this soon.

Sorry for the inconveniences and delayed fix

[cloneable]

There's still a problem with KnownLayout when the DST is wrapped:

use zerocopy::KnownLayout;

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

#[derive(KnownLayout)]
#[repr(C)]
pub struct Struct {
    pub dst: Dst,
}

cannot cast thin pointer `*mut __ZerocopyKnownLayoutMaybeUninit` to fat pointer `*mut _` rust-analyzer[E0607]

rust-analyzer 1.85.1 (4eb16125 2025-03-15)

No problem with rustc.

[ShoyuVanilla]

There's still a problem with KnownLayout when the DST is wrapped:

I've addressed a new issue for this and a new PR fixing it