no_std: support platforms that do not have native atomics

Fixes BurntSushi#162, Closes BurntSushi#177

.github/workflows/ci.yml

@@ -155,6 +155,26 @@ jobs:
     - run: cross test --verbose --target ${{ matrix.target }} --all
     - run: cross test --verbose --target ${{ matrix.target }} -p jiff-cli
 
+  # This is meant to test that Jiff's use of `Arc` from `portable-atomic-util`
+  # on targets that don't have `alloc::sync::Arc` in `std` works.
+  #
+  # Normally I'd do this with Cross, but I guess `riscv32imc-unknown-none-elf`
+  # isn't in Cross, and that's the target that the end user wanting this
+  # cares about[1,2].
+  #
+  # [1]: https://github.com/BurntSushi/jiff/issues/162
+  # [2]: https://github.com/BurntSushi/jiff/pull/177
+  riscv32imc-unknown-none-elf:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: dtolnay/rust-toolchain@master
+      with:
+        targets: riscv32imc-unknown-none-elf
+    - uses: actions/checkout@v4
+    - name: Test the build only
+      run: |
+        cargo build --target riscv32imc-unknown-none-elf --features alloc --no-default-features --features portable-atomic/critical-section
+
   # Test the wasm32-wasip1 target via wasmtime.
   wasm32-wasip1:
     runs-on: ubuntu-latest

CHANGELOG.md

@@ -6,6 +6,8 @@ TBD.
 
 Enhancements:
 
+* [#162](https://github.com/BurntSushi/jiff/issues/162):
+Support platforms that do not have atomics in `std`.
 * [#169](https://github.com/BurntSushi/jiff/issues/169):
 Add `TimeZone::to_fixed_offset` for accessing an invariant offset if possible.
 

Cargo.toml

@@ -31,7 +31,7 @@ members = [
 [features]
 default = ["std", "tz-system", "tzdb-bundle-platform", "tzdb-zoneinfo"]
 std = ["alloc", "serde?/std"]
-alloc = ["serde?/alloc"]
+alloc = ["serde?/alloc", "portable-atomic-util/alloc"]
 serde = ["dep:serde"]
 logging = ["dep:log"]
 
@@ -96,6 +96,16 @@ optional = true
 js-sys = { version = "0.3.50", optional = true }
 wasm-bindgen = { version = "0.2.70", optional = true }
 
+# For targets that have no atomics in `std`, we add a dependency on
+# `portable-atomic-util` for its Arc implementation.
+#
+# Note that for this to work, you may need to enable a `portable-atomic`
+# feature like `portable-atomic/unsafe-assume-single-core` or
+# `portable-atomic/critical-section`.
+[target.'cfg(not(target_has_atomic = "ptr"))'.dependencies]
+portable-atomic = { version = "1.10.0", default-features = false }
+portable-atomic-util = { version = "0.2.4", default-features = false }
+
 [dev-dependencies]
 anyhow = "1.0.81"
 chrono = { version = "0.4.38", features = ["serde"] }

src/error.rs

@@ -1,4 +1,6 @@
-use alloc::{boxed::Box, string::String, sync::Arc};
+use alloc::{boxed::Box, string::String};
+
+use crate::sync::Arc;
 
 /// Creates a new ad hoc error with no causal chain.
 ///

src/lib.rs

@@ -743,6 +743,18 @@ pub mod _documentation {
     pub mod changelog {}
 }
 
+/// This module re-exports `Arc`.
+///
+/// That is, it provides some indirection for the case when `alloc::sync::Arc`
+/// is unavailable.
+mod sync {
+    #[cfg(not(target_has_atomic = "ptr"))]
+    pub use portable_atomic_util::Arc;
+
+    #[cfg(target_has_atomic = "ptr")]
+    pub use alloc::sync::Arc;
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/tz/db/mod.rs

@@ -1,6 +1,6 @@
-use alloc::{string::String, sync::Arc, vec::Vec};
+use alloc::{string::String, vec::Vec};
 
-use crate::{error::Error, tz::TimeZone};
+use crate::{error::Error, sync::Arc, tz::TimeZone};
 
 use self::{bundled::BundledZoneInfo, zoneinfo::ZoneInfo};
 

src/tz/mod.rs

@@ -83,14 +83,12 @@ TO get the system's default time zone, use [`TimeZone::system`].
 [`GetDynamicTimeZoneInformation`]: https://learn.microsoft.com/en-us/windows/win32/api/timezoneapi/nf-timezoneapi-getdynamictimezoneinformation
 */
 
-use alloc::{
-    string::{String, ToString},
-    sync::Arc,
-};
+use alloc::string::{String, ToString};
 
 use crate::{
     civil::DateTime,
     error::{err, Error, ErrorContext},
+    sync::Arc,
     util::array_str::ArrayStr,
     Timestamp, Zoned,
 };

src/tz/system/mod.rs

@@ -1,9 +1,10 @@
 use std::{sync::RwLock, time::Duration};
 
-use alloc::{string::ToString, sync::Arc};
+use alloc::string::ToString;
 
 use crate::{
     error::{err, Error, ErrorContext},
+    sync::Arc,
     tz::{posix::PosixTz, TimeZone, TimeZoneDatabase},
     util::cache::Expiration,
 };

src/tz/zic.rs

@@ -105,7 +105,6 @@ use core::{ops::RangeInclusive, str::FromStr};
 use alloc::{
     collections::BTreeMap,
     string::{String, ToString},
-    sync::Arc,
     vec,
     vec::Vec,
 };
@@ -114,6 +113,7 @@ use crate::{
     civil::{Date, DateTime, Time, Weekday},
     error::{err, Error, ErrorContext},
     span::{Span, ToSpan},
+    sync::Arc,
     timestamp::Timestamp,
     tz::{Dst, Offset},
     util::{