Add portable-atomic Feature

Cargo.toml

@@ -12,7 +12,7 @@ rustdoc-args = ["--cfg", "docsrs"]
 all-features = true
 
 [dependencies]
-serde = { version = "1.0", optional = true, default-features = false }
+serde = { version = "1.0", optional = true, default-features = false, features = ["alloc"] }
 borsh = { version = "1.4.0", optional = true, default-features = false }
 arbitrary = { version = "1.3", optional = true }
 
@@ -24,3 +24,13 @@ serde = { version = "1.0", features = ["derive"] }
 [features]
 default = ["std"]
 std = ["serde?/std", "borsh?/std"]
+portable-atomic = [
+  "dep:portable-atomic",
+  "dep:portable-atomic-util",
+]
+
+[target.'cfg(not(target_has_atomic = "ptr"))'.dependencies]
+portable-atomic = { version = "1", default-features = false, optional = true }
+portable-atomic-util = { version = "0.2.4", features = [
+  "alloc",
+], optional = true }

src/borsh.rs

@@ -2,7 +2,7 @@ use crate::{Repr, SmolStr, INLINE_CAP};
 use alloc::string::{String, ToString};
 use borsh::io::{Error, ErrorKind, Read, Write};
 use borsh::{BorshDeserialize, BorshSerialize};
-use core::intrinsics::transmute;
+use core::mem::transmute;
 
 impl BorshSerialize for SmolStr {
     fn serialize<W: Write>(&self, writer: &mut W) -> borsh::io::Result<()> {

src/lib.rs

@@ -3,7 +3,7 @@
 
 extern crate alloc;
 
-use alloc::{borrow::Cow, boxed::Box, string::String, sync::Arc};
+use alloc::{borrow::Cow, boxed::Box, string::String};
 use core::{
     borrow::Borrow,
     cmp::{self, Ordering},
@@ -12,6 +12,18 @@ use core::{
     str::FromStr,
 };
 
+#[cfg(target_has_atomic = "ptr")]
+use alloc::sync::Arc;
+
+#[cfg(target_has_atomic = "ptr")]
+type HeapPtr<T> = Arc<T>;
+
+#[cfg(all(not(target_has_atomic = "ptr"), feature = "portable-atomic"))]
+type HeapPtr<T> = portable_atomic_util::Arc<T>;
+
+#[cfg(all(not(target_has_atomic = "ptr"), not(feature = "portable-atomic")))]
+type HeapPtr<T> = alloc::boxed::Box<T>;
+
 /// A `SmolStr` is a string type that has the following properties:
 ///
 /// * `size_of::<SmolStr>() == 24` (therefor `== size_of::<String>()` on 64 bit platforms)
@@ -377,6 +389,7 @@ impl From<Box<str>> for SmolStr {
     }
 }
 
+#[cfg(target_has_atomic = "ptr")]
 impl From<Arc<str>> for SmolStr {
     #[inline]
     fn from(s: Arc<str>) -> SmolStr {
@@ -392,6 +405,7 @@ impl<'a> From<Cow<'a, str>> for SmolStr {
     }
 }
 
+#[cfg(target_has_atomic = "ptr")]
 impl From<SmolStr> for Arc<str> {
     #[inline(always)]
     fn from(text: SmolStr) -> Self {
@@ -483,7 +497,7 @@ enum Repr {
         buf: [u8; INLINE_CAP],
     },
     Static(&'static str),
-    Heap(Arc<str>),
+    Heap(HeapPtr<str>),
 }
 
 impl Repr {
@@ -524,7 +538,7 @@ impl Repr {
     }
 
     fn new(text: &str) -> Self {
-        Self::new_on_stack(text).unwrap_or_else(|| Repr::Heap(Arc::from(text)))
+        Self::new_on_stack(text).unwrap_or_else(|| Repr::Heap(HeapPtr::from(text)))
     }
 
     #[inline(always)]
@@ -562,7 +576,8 @@ impl Repr {
 
     fn ptr_eq(&self, other: &Self) -> bool {
         match (self, other) {
-            (Self::Heap(l0), Self::Heap(r0)) => Arc::ptr_eq(l0, r0),
+            #[cfg(any(target_has_atomic = "ptr", feature = "portable-atomic"))]
+            (Self::Heap(l0), Self::Heap(r0)) => HeapPtr::ptr_eq(l0, r0),
             (Self::Static(l0), Self::Static(r0)) => core::ptr::eq(l0, r0),
             (
                 Self::Inline {