frb macro causing compilation error

[GregoryConrad]

Describe the bug

Had a new compilation error pop up in my CI yesterday. Guessing it's due to the new stable Rust release.

Steps to reproduce

Use the frb macro.

Logs

error: unexpected `cfg` condition name: `frb_expand`
  --> packages/mimir/native/src/api.rs:62:1
   |
62 | #[frb(dart_metadata=("freezed"))]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = 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(frb_expand)'] }
   = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(frb_expand)");` 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: `-D unexpected-cfgs` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unexpected_cfgs)]`
   = note: this error originates in the attribute macro `frb` (in Nightly builds, run with -Z macro-backtrace for more info)

error: unexpected `cfg` condition name: `frb_expand`
  --> packages/mimir/native/src/api.rs:70:1
   |
70 | #[frb(dart_metadata=("freezed"))]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = 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(frb_expand)'] }
   = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(frb_expand)");` 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: this error originates in the attribute macro `frb` (in Nightly builds, run with -Z macro-backtrace for more info)

error: unexpected `cfg` condition name: `frb_expand`
   --> packages/mimir/native/src/api.rs:207:5
    |
207 |     #[frb(ignore)]
    |     ^^^^^^^^^^^^^^
    |
    = 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(frb_expand)'] }
    = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(frb_expand)");` 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: this error originates in the attribute macro `frb` (in Nightly builds, run with -Z macro-backtrace for more info)

error: unexpected `cfg` condition name: `frb_expand`
   --> packages/mimir/native/src/api.rs:218:5
    |
218 |     #[frb(ignore)]
    |     ^^^^^^^^^^^^^^
    |
    = 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(frb_expand)'] }
    = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(frb_expand)");` 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: this error originates in the attribute macro `frb` (in Nightly builds, run with -Z macro-backtrace for more info)

[GregoryConrad]

Can be silenced with (in Cargo.toml):

[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(frb_expand)'] }

But is there any fix that can be done on FRB side?

[fzyzcjy]

I quickly glanced it before and it seemed not trivial: #2425 (comment)