Remove #![feature(stdsimd)] used for Miri/ThreadSanitizer

bench/benches/bench.rs

@@ -3,7 +3,7 @@
 #![warn(rust_2018_idioms, single_use_lifetimes, unsafe_op_in_unsafe_fn)]
 #![allow(dead_code, unused_extern_crates)]
 #![allow(clippy::only_used_in_recursion)]
-#![feature(asm_experimental_arch, core_intrinsics, stdsimd)]
+#![feature(asm_experimental_arch, core_intrinsics)]
 
 use std::{
     hint::black_box,

build.rs

@@ -147,6 +147,10 @@ fn main() {
         if target_arch == "arm" && !version.probe(67, 2022, 11, 5) {
             println!("cargo:rustc-cfg=portable_atomic_unstable_isa_attribute");
         }
+        // https://github.com/rust-lang/rust/pull/109359 (includes https://github.com/rust-lang/stdarch/pull/1358) merged in Rust 1.70 (nightly-2023-03-24).
+        if target_arch == "x86_64" && !version.probe(70, 2023, 3, 23) {
+            println!("cargo:rustc-cfg=portable_atomic_unstable_cmpxchg16b_intrinsic");
+        }
 
         // `cfg(sanitize = "..")` is not stabilized.
         let sanitize = env::var("CARGO_CFG_SANITIZE").unwrap_or_default();

src/lib.rs

@@ -320,10 +320,13 @@ RUSTFLAGS="--cfg portable_atomic_no_outline_atomics" cargo ...
     ),
     feature(core_intrinsics)
 )]
-// This feature will be unnecessary once stdarch submodule in rust-lang/rust is
-// updated to include https://github.com/rust-lang/stdarch/pull/1358.
+// This feature is only enabled for old nightly.
 #![cfg_attr(
-    all(target_arch = "x86_64", any(miri, portable_atomic_sanitize_thread)),
+    all(
+        target_arch = "x86_64",
+        portable_atomic_unstable_cmpxchg16b_intrinsic,
+        any(miri, portable_atomic_sanitize_thread),
+    ),
     feature(stdsimd)
 )]
 // docs.rs only