#[no_mangle] wrapped in unsafe produces strange syntax error

[fidelicura]

Sorry, I couldn't think of a more appropriate name for this issue, and it should be renamed.

Reproducible Example

• Edition: 2021
• Version: 1.85.0

1. create a library using cargo new --lib cool
2. add a abi_stable crate using cargo add abi_stable:

`Cargo.toml`

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

[dependencies]
abi_stable = "0.11.3"

3. paste following code:

#[abi_stable::sabi_extern_fn]
#[no_mangle]
fn something() -> i32 {
    52
}

It compiles successfully.

$ cargo build
    Updating crates.io index
     Locking 51 packages to latest compatible versions
   Compiling proc-macro2 v1.0.94
   Compiling unicode-ident v1.0.18
   Compiling syn v1.0.109
   Compiling cfg-if v1.0.0
   Compiling libc v0.2.170
   Compiling autocfg v1.4.0
   Compiling serde v1.0.218
   Compiling parking_lot_core v0.9.10
   Compiling crossbeam-utils v0.8.21
   Compiling core_extensions v1.5.3
   Compiling smallvec v1.14.0
   Compiling serde_json v1.0.140
   Compiling core_extensions_proc_macros v1.5.3
   Compiling paste v1.0.15
   Compiling tstr_proc_macros v0.2.2
   Compiling scopeguard v1.2.0
   Compiling ryu v1.0.20
   Compiling itoa v1.0.15
   Compiling memchr v2.7.4
   Compiling typed-arena v2.0.2
   Compiling generational-arena v0.2.9
   Compiling libloading v0.7.4
   Compiling const_panic v0.2.12
   Compiling abi_stable_shared v0.11.0
   Compiling lock_api v0.4.12
   Compiling tstr v0.2.4
   Compiling repr_offset v0.2.2
   Compiling crossbeam-channel v0.5.14
   Compiling quote v1.0.39
   Compiling syn v2.0.99
   Compiling parking_lot v0.12.3
   Compiling serde_derive v1.0.218
   Compiling as_derive_utils v0.11.0
   Compiling abi_stable_derive v0.11.3
   Compiling abi_stable v0.11.3
   Compiling cool v0.1.0 (/home/kbezuglyi/cool)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 10.43s

4. try to update to 2024 edition:

`cargo fix --edition --allow-dirty`

   Migrating Cargo.toml from 2021 edition to 2024
    Updating crates.io index
     Locking 51 packages to latest compatible versions
   Compiling proc-macro2 v1.0.94
   Compiling unicode-ident v1.0.18
   Compiling autocfg v1.4.0
   Compiling syn v1.0.109
    Checking cfg-if v1.0.0
   Compiling libc v0.2.170
   Compiling serde v1.0.218
   Compiling core_extensions v1.5.3
   Compiling parking_lot_core v0.9.10
   Compiling crossbeam-utils v0.8.21
   Compiling paste v1.0.15
   Compiling serde_json v1.0.140
    Checking smallvec v1.14.0
   Compiling core_extensions_proc_macros v1.5.3
   Compiling tstr_proc_macros v0.2.2
    Checking scopeguard v1.2.0
    Checking itoa v1.0.15
    Checking ryu v1.0.20
    Checking memchr v2.7.4
   Compiling typed-arena v2.0.2
    Checking libloading v0.7.4
    Checking generational-arena v0.2.9
    Checking const_panic v0.2.12
   Compiling abi_stable_shared v0.11.0
   Compiling lock_api v0.4.12
    Checking tstr v0.2.4
    Checking repr_offset v0.2.2
    Checking crossbeam-channel v0.5.14
   Compiling quote v1.0.39
   Compiling syn v2.0.99
    Checking parking_lot v0.12.3
   Compiling serde_derive v1.0.218
   Compiling as_derive_utils v0.11.0
   Compiling abi_stable_derive v0.11.3
    Checking abi_stable v0.11.3
    Checking cool v0.1.0 (/home/kbezuglyi/cool)
   Migrating src/lib.rs from 2021 edition to 2024
warning: failed to automatically apply fixes suggested by rustc to crate `cool`

after fixes were automatically applied the compiler reported errors within these files:

  * src/lib.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/rust/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error: expected path
 --> src/lib.rs:3:3
  |
3 | #[unsafe(no_mangle)]
  |   ^^^^^^

error: aborting due to 1 previous error

Original diagnostics will follow.

warning: unsafe attribute used without unsafe
 --> src/lib.rs:3:3
  |
3 | #[no_mangle]
  |   ^^^^^^^^^ usage of unsafe attribute
  |
  = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
  = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/unsafe-attributes.html>
  = note: `--force-warn unsafe-attr-outside-unsafe` implied by `--force-warn rust-2024-compatibility`
help: wrap the attribute in `unsafe(...)`
  |
3 | #[unsafe(no_mangle)]
  |   +++++++         +

warning: `cool` (lib) generated 1 warning (run `cargo fix --lib -p cool` to apply 1 suggestion)
warning: failed to automatically apply fixes suggested by rustc to crate `cool`

after fixes were automatically applied the compiler reported errors within these files:

  * src/lib.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/rust/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error: expected path
 --> src/lib.rs:3:3
  |
3 | #[unsafe(no_mangle)]
  |   ^^^^^^

error: aborting due to 1 previous error

Original diagnostics will follow.

warning: `cool` (lib test) generated 1 warning (1 duplicate)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 9.85s

5. manual edit didn't work either:

#[abi_stable::sabi_extern_fn]
#[unsafe(no_mangle)]
fn something() -> i32 {
    52
}

`cargo build`

error: expected path
 --> src/lib.rs:2:3
  |
2 | #[unsafe(no_mangle)]
  |   ^^^^^^

error: could not compile `cool` (lib) due to 1 previous error

Additional Information

`cargo expand` on non-unsafe no-mangle attribute

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
#[no_mangle]
extern "C" fn something() -> i32 {
    #[allow(clippy::redundant_closure_call)]
    {
        {
            let aborter_guard = {
                use ::abi_stable::utils::{AbortBomb, PanicInfo};
                #[allow(dead_code)]
                const BOMB: AbortBomb = AbortBomb {
                    fuse: &PanicInfo {
                        file: "src/lib.rs",
                        line: 1u32,
                    },
                };
                BOMB
            };
            let res = {
                let a = ::abi_stable::marker_type::NotCopyNotClone;
                (move || {
                    { a };
                    { 52 }
                })()
            };
            ::std::mem::forget(aborter_guard);
            res
        }
    }
}

[bjorn3]

Fair chance that abi-stable is at fault here. In any case it looks like the abi-stable crate hasn't been updated in 2 years, so you may want to use another crate instead.

[fidelicura]

Fair chance that abi-stable is at fault here. In any case it looks like the abi-stable crate hasn't been updated in 2 years, so you may want to use another crate instead.

Yes, probably. I think this is because of incorrect logic of handling syn tokens on a new edition. Sadly, but maybe it is really the case I will change my dependency from abi_stable to something else. Thanks.