Shim LazyLock when before 1.70

common/std-shims/Cargo.toml

@@ -7,7 +7,7 @@ repository = "https://github.com/serai-dex/serai/tree/develop/common/std-shims"
 authors = ["Luke Parker <lukeparker5132@gmail.com>"]
 keywords = ["nostd", "no_std", "alloc", "io"]
 edition = "2021"
-rust-version = "1.70"
+rust-version = "1.64"
 
 [package.metadata.docs.rs]
 all-features = true

common/std-shims/src/sync.rs

@@ -25,41 +25,101 @@ mod mutex_shim {
 }
 pub use mutex_shim::{ShimMutex as Mutex, MutexGuard};
 
-#[cfg(feature = "std")]
-pub use std::sync::OnceLock;
 #[cfg(not(feature = "std"))]
 pub use spin::Once as OnceLock;
+#[cfg(feature = "std")]
+mod std_oncelock {
+  #[rustversion::before(1.70)]
+  mod before_1_70_oncelock {
+    use core::cell::Cell;
+    use std::sync::RwLock;
 
-#[rustversion::before(1.80)]
-mod before_1_80_lazylock {
-  use core::ops::Deref;
-  use super::{Mutex, OnceLock};
+    /// Shim for `std::sync::OnceLock`.
+    pub struct OnceLock<T> {
+      value: Cell<*mut T>,
+      init: RwLock<bool>,
+    }
+    // We use the `RwLock` (which is `Sync`) to control access to the `!Sync` `RefCell`
+    unsafe impl<T: Sync> Sync for OnceLock<T> {}
 
-  /// Shim for `std::sync::LazyLock`.
-  pub struct LazyLock<T, F = fn() -> T> {
-    f: Mutex<Option<F>>,
-    once: OnceLock<T>,
-  }
-  impl<T, F: FnOnce() -> T> LazyLock<T, F> {
-    /// Shim for `std::sync::LazyLock::new`.
-    pub const fn new(f: F) -> Self {
-      Self { f: Mutex::new(Some(f)), once: OnceLock::new() }
+    impl<T> OnceLock<T> {
+      /// Shim for `std::sync::OnceLock::new`.
+      pub const fn new() -> Self {
+        Self { value: Cell::new(core::ptr::null_mut()), init: RwLock::new(false) }
+      }
+      /// Shim for `std::sync::OnceLock::get_or_init`.
+      pub fn get_or_init<F>(&'a self, f: F) -> &'a T
+      where
+        F: FnOnce() -> T,
+      {
+        let initialized = *self.init.read().unwrap();
+        if !initialized {
+          // Obtain an exclusive reference
+          let mut initialized = self.init.write().unwrap();
+          // If this still isn't initialized (by someone who first obtained an exlusive reference)
+          if !*initialized {
+            // Set the value and mark it initialized
+            self.value.set(Box::into_raw(Box::new(f())));
+            *initialized = true;
+          }
+        }
+        // SAFETY: We always initialize the value before this and it's only written to once
+        unsafe { &*self.value.get() }
+      }
     }
-    /// Shim for `std::sync::LazyLock::get_or_init`.
-    pub fn get(&self) -> &T {
-      // Since this initializer will only be called once, the value in the Mutex will be `Some`
-      self.once.get_or_init(|| (self.f.lock().take().unwrap())())
-    }
-  }
-  impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> {
-    type Target = T;
-    fn deref(&self) -> &T {
-      self.get()
+
+    // SAFETY: `OnceLock` doesn't implement `Clone` so this doesn't risk dropping the `Box`
+    // multiple times
+    impl<T> Drop for OnceLock<T> {
+      fn drop(&mut self) {
+        unsafe { drop(Box::from_raw(self.value.get())) }
+      }
     }
   }
+  #[rustversion::before(1.70)]
+  pub use before_1_70_oncelock::OnceLock;
+  #[rustversion::since(1.70)]
+  pub use std::sync::OnceLock;
 }
-#[rustversion::before(1.80)]
-pub use before_1_80_lazylock::LazyLock;
+#[cfg(feature = "std")]
+pub use std_oncelock::OnceLock;
 
-#[rustversion::since(1.80)]
-pub use std::sync::LazyLock;
+#[cfg(not(feature = "std"))]
+pub use spin::Lazy as LazyLock;
+#[cfg(feature = "std")]
+mod std_lazylock {
+  #[rustversion::before(1.80)]
+  mod before_1_80_lazylock {
+    use core::ops::Deref;
+    use crate::sync::{Mutex, OnceLock};
+
+    /// Shim for `std::sync::LazyLock`.
+    pub struct LazyLock<T, F = fn() -> T> {
+      f: Mutex<Option<F>>,
+      once: OnceLock<T>,
+    }
+    impl<T, F: FnOnce() -> T> LazyLock<T, F> {
+      /// Shim for `std::sync::LazyLock::new`.
+      pub const fn new(f: F) -> Self {
+        Self { f: Mutex::new(Some(f)), once: OnceLock::new() }
+      }
+      /// Shim for `std::sync::LazyLock::get`.
+      pub fn get(&self) -> &T {
+        // Since this initializer will only be called once, the value in the Mutex will be `Some`
+        self.once.get_or_init(|| (self.f.lock().take().unwrap())())
+      }
+    }
+    impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> {
+      type Target = T;
+      fn deref(&self) -> &T {
+        self.get()
+      }
+    }
+  }
+  #[rustversion::before(1.80)]
+  pub use before_1_80_lazylock::LazyLock;
+  #[rustversion::since(1.80)]
+  pub use std::sync::LazyLock;
+}
+#[cfg(feature = "std")]
+pub use std_lazylock::LazyLock;