Update to stabilized const_fn_trait_bound

Author: taiki-e

crossbeam-epoch/Cargo.toml

@@ -27,22 +27,26 @@ std = ["alloc", "crossbeam-utils/std", "lazy_static"]
 # NOTE: Disabling both `std` *and* `alloc` features is not supported yet.
 alloc = []
 
+# These features are no longer used.
+# TODO: remove in the next major version.
 # Enable to use of unstable functionality.
 # This is disabled by default and requires recent nightly compiler.
 #
 # NOTE: This feature is outside of the normal semver guarantees and minor or
 # patch versions of crossbeam may make breaking changes to them at any time.
-nightly = ["crossbeam-utils/nightly", "const_fn"]
+nightly = ["crossbeam-utils/nightly"]
 
 # Enable the use of loom for concurrency testing.
 #
 # NOTE: This feature is outside of the normal semver guarantees and minor or
 # patch versions of crossbeam may make breaking changes to them at any time.
 loom = ["loom-crate", "crossbeam-utils/loom"]
 
+[build-dependencies]
+autocfg = "1"
+
 [dependencies]
 cfg-if = "1"
-const_fn = { version = "0.4.4", optional = true }
 memoffset = "0.6"
 
 # Enable the use of loom for concurrency testing.
@@ -67,3 +71,4 @@ default-features = false
 
 [dev-dependencies]
 rand = "0.8"
+rustversion = "1"

crossbeam-epoch/build.rs

@@ -29,6 +29,18 @@ fn main() {
         }
     };
 
+    let cfg = match autocfg::AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to determine rustc version: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
+            return;
+        }
+    };
+
     // Note that this is `no_*`, not `has_*`. This allows treating
     // `cfg(target_has_atomic = "ptr")` as true when the build script doesn't
     // run. This is needed for compatibility with non-cargo build systems that
@@ -37,5 +49,10 @@ fn main() {
         println!("cargo:rustc-cfg=crossbeam_no_atomic_cas");
     }
 
+    if cfg.probe_rustc_version(1, 61) {
+        // TODO: invert cfg once Rust 1.61 became stable.
+        println!("cargo:rustc-cfg=crossbeam_const_fn_trait_bound");
+    }
+
     println!("cargo:rerun-if-changed=no_atomic.rs");
 }

crossbeam-epoch/src/atomic.rs

@@ -342,8 +342,16 @@ impl<T: ?Sized + Pointable> Atomic<T> {
     ///
     /// let a = Atomic::<i32>::null();
     /// ```
-    ///
-    #[cfg_attr(all(feature = "nightly", not(crossbeam_loom)), const_fn::const_fn)]
+    #[cfg(all(crossbeam_const_fn_trait_bound, not(crossbeam_loom)))]
+    pub const fn null() -> Atomic<T> {
+        Self {
+            data: AtomicUsize::new(0),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns a new null atomic pointer.
+    #[cfg(not(all(crossbeam_const_fn_trait_bound, not(crossbeam_loom))))]
     pub fn null() -> Atomic<T> {
         Self {
             data: AtomicUsize::new(0),
@@ -1594,7 +1602,7 @@ mod tests {
         Shared::<i64>::null().with_tag(7);
     }
 
-    #[cfg(feature = "nightly")]
+    #[rustversion::since(1.61)]
     #[test]
     fn const_atomic_null() {
         use super::Atomic;

crossbeam-epoch/src/lib.rs

@@ -62,7 +62,6 @@
     unreachable_pub
 )]
 #![cfg_attr(not(feature = "std"), no_std)]
-#![cfg_attr(feature = "nightly", feature(const_fn_trait_bound))]
 
 #[cfg(crossbeam_loom)]
 extern crate loom_crate as loom;