chore: Do not show "Available on non-loom only." doc label (#7977)

* chore: Do not show "Available on non-loom only." doc label

Closes #7976

Uses the unstable #[doc(cfg()]. https://github.com/rust-lang/rust/issues/43781

* Use build.rs to detect nightly builds

* Use `docsrs` instead of `nightly`. Drop build.rs

* Use doc(auto_cfg(hide(config_name)))

* Use same nightly on CirrusCI (FreeBSD) as on Github Actions CI

.cirrus.yml

@@ -4,7 +4,7 @@ freebsd_instance:
   image_family: freebsd-14-3
 env:
   RUST_STABLE: stable
-  RUST_NIGHTLY: nightly-2025-01-25
+  RUST_NIGHTLY: nightly-2025-10-12
   RUSTFLAGS: -D warnings
   # This excludes unstable features like io_uring, which require '--cfg tokio_unstable'.
   TOKIO_STABLE_FEATURES: full,test-util

tokio/src/lib.rs

@@ -15,6 +15,9 @@
     no_crate_inject,
     attr(deny(warnings, rust_2018_idioms), allow(dead_code, unused_variables))
 ))]
+// loom is an internal implementation detail.
+// Do not show "Available on non-loom only" label
+#![cfg_attr(docsrs, doc(auto_cfg(hide(loom))))]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![cfg_attr(docsrs, allow(unused_attributes))]
 #![cfg_attr(loom, allow(dead_code, unreachable_pub))]