Resolve unexpected_cfgs warning

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/build/src/gen/write.rs:333:15
        |
    333 |         #[cfg(feature = "experimental-enum-variants-from-header")]
        |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`, `parallel`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
        = note: `#[warn(unexpected_cfgs)]` on by default

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/build/src/gen/write.rs:393:15
        |
    393 |         #[cfg(feature = "experimental-enum-variants-from-header")]
        |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`, `parallel`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/build/src/gen/write.rs:415:15
        |
    415 |         #[cfg(feature = "experimental-enum-variants-from-header")]
        |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`, `parallel`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/build/src/syntax/attrs.rs:147:21
        |
    147 |             && cfg!(feature = "experimental-enum-variants-from-header")
        |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`, `parallel`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/build/src/syntax/discriminant.rs:182:11
        |
    182 |     #[cfg(feature = "experimental-enum-variants-from-header")]
        |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`, `parallel`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/build/src/syntax/tokens.rs:296:19
        |
    296 |             #[cfg(feature = "experimental-enum-variants-from-header")]
        |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`, `parallel`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
      --> gen/build/src/syntax/types.rs:95:31
       |
    95 |                         #[cfg(feature = "experimental-enum-variants-from-header")]
       |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       |
       = note: expected values for `feature` are: `experimental-async-fn`, `parallel`
       = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
       = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/build/src/syntax/mod.rs:146:11
        |
    146 |     #[cfg(feature = "experimental-enum-variants-from-header")]
        |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`, `parallel`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/lib/src/gen/write.rs:333:15
        |
    333 |         #[cfg(feature = "experimental-enum-variants-from-header")]
        |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the condition
        |
        = note: no expected values for `feature`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
        = note: `#[warn(unexpected_cfgs)]` on by default

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/lib/src/gen/write.rs:393:15
        |
    393 |         #[cfg(feature = "experimental-enum-variants-from-header")]
        |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the condition
        |
        = note: no expected values for `feature`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/lib/src/gen/write.rs:415:15
        |
    415 |         #[cfg(feature = "experimental-enum-variants-from-header")]
        |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the condition
        |
        = note: no expected values for `feature`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/lib/src/syntax/attrs.rs:147:21
        |
    147 |             && cfg!(feature = "experimental-enum-variants-from-header")
        |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the condition
        |
        = note: no expected values for `feature`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/lib/src/syntax/discriminant.rs:182:11
        |
    182 |     #[cfg(feature = "experimental-enum-variants-from-header")]
        |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the condition
        |
        = note: no expected values for `feature`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-async-fn`
       --> gen/lib/src/syntax/parse.rs:565:52
        |
    565 |     if foreign_fn.sig.asyncness.is_some() && !cfg!(feature = "experimental-async-fn") {
        |                                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the condition
        |
        = note: no expected values for `feature`
        = help: consider adding `experimental-async-fn` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/lib/src/syntax/tokens.rs:296:19
        |
    296 |             #[cfg(feature = "experimental-enum-variants-from-header")]
        |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the condition
        |
        = note: no expected values for `feature`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
      --> gen/lib/src/syntax/types.rs:95:31
       |
    95 |                         #[cfg(feature = "experimental-enum-variants-from-header")]
       |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the condition
       |
       = note: no expected values for `feature`
       = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
       = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/lib/src/syntax/mod.rs:146:11
        |
    146 |     #[cfg(feature = "experimental-enum-variants-from-header")]
        |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the condition
        |
        = note: no expected values for `feature`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/cmd/src/gen/write.rs:333:15
        |
    333 |         #[cfg(feature = "experimental-enum-variants-from-header")]
        |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
        = note: `#[warn(unexpected_cfgs)]` on by default

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/cmd/src/gen/write.rs:393:15
        |
    393 |         #[cfg(feature = "experimental-enum-variants-from-header")]
        |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/cmd/src/gen/write.rs:415:15
        |
    415 |         #[cfg(feature = "experimental-enum-variants-from-header")]
        |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/cmd/src/syntax/attrs.rs:147:21
        |
    147 |             && cfg!(feature = "experimental-enum-variants-from-header")
        |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/cmd/src/syntax/discriminant.rs:182:11
        |
    182 |     #[cfg(feature = "experimental-enum-variants-from-header")]
        |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/cmd/src/syntax/tokens.rs:296:19
        |
    296 |             #[cfg(feature = "experimental-enum-variants-from-header")]
        |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
      --> gen/cmd/src/syntax/types.rs:95:31
       |
    95 |                         #[cfg(feature = "experimental-enum-variants-from-header")]
       |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       |
       = note: expected values for `feature` are: `experimental-async-fn`
       = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
       = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition value: `experimental-enum-variants-from-header`
       --> gen/cmd/src/syntax/mod.rs:146:11
        |
    146 |     #[cfg(feature = "experimental-enum-variants-from-header")]
        |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: expected values for `feature` are: `experimental-async-fn`
        = help: consider adding `experimental-enum-variants-from-header` as a feature in `Cargo.toml`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `trybuild`
     --> tests/ffi/build.rs:4:13
      |
    4 |     if cfg!(trybuild) {
      |             ^^^^^^^^
      |
      = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `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`, `windows`
      = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(trybuild)");` to the top of the `build.rs`
      = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
      = note: `#[warn(unexpected_cfgs)]` on by default

    warning: unexpected `cfg` condition name: `deny_warnings`
      --> tests/ffi/build.rs:13:37
       |
    13 |     build.warnings_into_errors(cfg!(deny_warnings));
       |                                     ^^^^^^^^^^^^^
       |
       = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(deny_warnings)");` to the top of the `build.rs`
       = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `deny_warnings`
      --> build.rs:14:36
       |
    14 |         .warnings_into_errors(cfg!(deny_warnings))
       |                                    ^^^^^^^^^^^^^
       |
       = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `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`, `windows`
       = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(deny_warnings)");` to the top of the `build.rs`
       = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
       = note: `#[warn(unexpected_cfgs)]` on by default

    warning: unexpected `cfg` condition name: `doc_cfg`
       --> src/lib.rs:368:13
        |
    368 | #![cfg_attr(doc_cfg, feature(doc_cfg))]
        |             ^^^^^^^
        |
        = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `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`, `windows`
        = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(doc_cfg)");` to the top of the `build.rs`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
        = note: `#[warn(unexpected_cfgs)]` on by default

    warning: unexpected `cfg` condition name: `built_with_cargo`
       --> src/lib.rs:409:7
        |
    409 | #[cfg(built_with_cargo)]
        |       ^^^^^^^^^^^^^^^^
        |
        = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(built_with_cargo)");` to the top of the `build.rs`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `cxx_experimental_no_alloc`
       --> src/lib.rs:433:34
        |
    433 | #[cfg(not(any(feature = "alloc", cxx_experimental_no_alloc)))]
        |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(cxx_experimental_no_alloc)");` to the top of the `build.rs`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `compile_error_if_alloc`
       --> src/lib.rs:438:11
        |
    438 | #[cfg(all(compile_error_if_alloc, feature = "alloc"))]
        |           ^^^^^^^^^^^^^^^^^^^^^^
        |
        = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(compile_error_if_alloc)");` to the top of the `build.rs`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `compile_error_if_std`
       --> src/lib.rs:443:11
        |
    443 | #[cfg(all(compile_error_if_std, feature = "std"))]
        |           ^^^^^^^^^^^^^^^^^^^^
        |
        = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(compile_error_if_std)");` to the top of the `build.rs`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `doc_cfg`
       --> src/lib.rs:480:12
        |
    480 | #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
        |            ^^^^^^^
        |
        = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(doc_cfg)");` to the top of the `build.rs`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `no_core_ffi_c_char`
      --> src/c_char.rs:11:11
       |
    11 | #[cfg(not(no_core_ffi_c_char))]
       |           ^^^^^^^^^^^^^^^^^^
       |
       = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(no_core_ffi_c_char)");` to the top of the `build.rs`
       = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `no_core_ffi_c_char`
      --> src/c_char.rs:16:7
       |
    16 | #[cfg(no_core_ffi_c_char)]
       |       ^^^^^^^^^^^^^^^^^^
       |
       = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(no_core_ffi_c_char)");` to the top of the `build.rs`
       = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `doc_cfg`
     --> src/exception.rs:7:12
      |
    7 | #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
      |            ^^^^^^^
      |
      = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(doc_cfg)");` to the top of the `build.rs`
      = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `doc_cfg`
      --> src/exception.rs:20:12
       |
    20 | #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
       |            ^^^^^^^
       |
       = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(doc_cfg)");` to the top of the `build.rs`
       = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `doc_cfg`
       --> src/extern_type.rs:220:16
        |
    220 |     #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
        |                ^^^^^^^
        |
        = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(doc_cfg)");` to the top of the `build.rs`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `doc_cfg`
       --> src/cxx_string.rs:151:16
        |
    151 |     #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
        |                ^^^^^^^
        |
        = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(doc_cfg)");` to the top of the `build.rs`
        = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

    warning: unexpected `cfg` condition name: `skip_ui_tests`
     --> tests/compiletest.rs:3:12
      |
    3 | #[cfg_attr(skip_ui_tests, ignore)]
      |            ^^^^^^^^^^^^^
      |
      = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `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`, `windows`
      = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(skip_ui_tests)");` to the top of the `build.rs`
      = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
      = note: `#[warn(unexpected_cfgs)]` on by default

build.rs

@@ -1,3 +1,6 @@
+#![allow(unknown_lints)]
+#![allow(unexpected_cfgs)]
+
 use std::env;
 use std::path::{Path, PathBuf};
 use std::process::Command;
@@ -24,6 +27,16 @@ fn main() {
     }
 
     if let Some(rustc) = rustc_version() {
+        if rustc.minor >= 80 {
+            println!("cargo:rustc-check-cfg=cfg(built_with_cargo)");
+            println!("cargo:rustc-check-cfg=cfg(compile_error_if_alloc)");
+            println!("cargo:rustc-check-cfg=cfg(compile_error_if_std)");
+            println!("cargo:rustc-check-cfg=cfg(cxx_experimental_no_alloc)");
+            println!("cargo:rustc-check-cfg=cfg(doc_cfg)");
+            println!("cargo:rustc-check-cfg=cfg(no_core_ffi_c_char)");
+            println!("cargo:rustc-check-cfg=cfg(skip_ui_tests)");
+        }
+
         if rustc.minor < 63 {
             println!("cargo:warning=The cxx crate requires a rustc version 1.63.0 or newer.");
             println!(

tests/ffi/build.rs

@@ -1,3 +1,6 @@
+#![allow(unknown_lints)]
+#![allow(unexpected_cfgs)]
+
 use cxx_build::CFG;
 
 fn main() {

tools/cargo/build.rs

@@ -48,6 +48,9 @@ through crates.io.
 ";
 
 fn main() {
+    println!("cargo:rustc-check-cfg=cfg(feature, values(\"experimental-async-fn\"))");
+    println!("cargo:rustc-check-cfg=cfg(feature, values(\"experimental-enum-variants-from-header\"))");
+
     if Path::new("src/syntax/mod.rs").exists() {
         return;
     }