Remove the check_cfg job

check-cfg was stabilized in 1.80, so there is no longer any need to have
a specific job and environment variable to enable it only under certain
conditions.

(backport <#4312>)
(cherry picked from commit 2065a4a)

Author: tgross35

.github/workflows/ci.yaml

@@ -204,19 +204,6 @@ jobs:
             export PATH=$HOME/.rust_solaris/bin:$PATH
             ./ci/run.sh ${{ matrix.target }}
 
-  check_cfg:
-    name: "Check #[cfg]s"
-    runs-on: ubuntu-24.04
-    env:
-      TOOLCHAIN: nightly
-    timeout-minutes: 10
-    steps:
-      - uses: actions/checkout@v4
-      - name: Setup Rust toolchain
-        run: ./ci/install-rust.sh
-      - name: Build with check-cfg
-        run: LIBC_CHECK_CFG=1 cargo build -Z unstable-options -Z check-cfg
-
   # One job that "summarizes" the success state of this pipeline. This can then be added to branch
   # protection, rather than having to add each job separately.
   success:

build.rs

@@ -44,7 +44,6 @@ fn main() {
     let (rustc_minor_ver, _is_nightly) = rustc_minor_nightly();
     let rustc_dep_of_std = env::var("CARGO_FEATURE_RUSTC_DEP_OF_STD").is_ok();
     let libc_ci = env::var("LIBC_CI").is_ok();
-    let libc_check_cfg = env::var("LIBC_CHECK_CFG").is_ok() || rustc_minor_ver >= 80;
 
     // The ABI of libc used by std is backward compatible with FreeBSD 12.
     // The ABI of libc from crates.io is backward compatible with FreeBSD 11.
@@ -100,12 +99,9 @@ fn main() {
     // Set unconditionally when ctest is not being invoked.
     set_cfg("libc_const_extern_fn");
 
-    // check-cfg is a nightly cargo/rustc feature to warn when unknown cfgs are used across the
-    // codebase. libc can configure it if the appropriate environment variable is passed. Since
-    // rust-lang/rust enforces it, this is useful when using a custom libc fork there.
-    //
-    // https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg
-    if libc_check_cfg {
+    // Since Rust 1.80, configuration that isn't recognized by default needs to be provided to
+    // avoid warnings.
+    if rustc_minor_ver >= 80 {
         for cfg in ALLOWED_CFGS {
             if rustc_minor_ver >= 75 {
                 println!("cargo:rustc-check-cfg=cfg({})", cfg);