fix AVR support by using atomic-polyfill

Cargo.toml

@@ -36,6 +36,9 @@ runtime-rng = ["getrandom"]
 # If this is disabled and runtime-rng is unavailable constant keys are used.
 compile-time-rng = ["const-random"]
 
+# in case this is being used on an architecture lacking core::sync::atomic::AtomicUsize and friends
+atomic-polyfill = [ "dep:atomic-polyfill", "once_cell/atomic-polyfill"]
+
 [[bench]]
 name = "ahash"
 path = "tests/bench.rs"
@@ -72,9 +75,10 @@ getrandom = { version = "0.2.3", optional = true }
 const-random = { version = "0.1.12", optional = true }
 serde = { version = "1.0.117", optional = true }
 cfg-if = "1.0"
+atomic-polyfill = { version="1.0.1", optional=true}
 
 [target.'cfg(not(all(target_arch = "arm", target_os = "none")))'.dependencies]
-once_cell = { version = "1.8", default-features = false, features = ["unstable", "alloc"] }
+once_cell = { version = "1.13.1", default-features = false, features = ["unstable", "alloc"] }
 
 [dev-dependencies]
 no-panic = "0.1.10"

src/random_state.rs

@@ -23,8 +23,13 @@ cfg_if::cfg_if! {
     }
 }
 
+#[cfg(feature = "atomic-polyfill")]
+use atomic_polyfill as atomic;
+#[cfg(not(feature = "atomic-polyfill"))]
+use core::sync::atomic;
+
 use alloc::boxed::Box;
-use core::sync::atomic::{AtomicUsize, Ordering};
+use atomic::{AtomicUsize, Ordering};
 use core::any::{Any, TypeId};
 use core::fmt;
 use core::hash::BuildHasher;