Shim std::sync::LazyLock for Rust < 1.80

Allows downgrading some crypto crates' MSRV to 1.79 as well.

common/std-shims/src/sync.rs

@@ -26,6 +26,40 @@ mod mutex_shim {
 pub use mutex_shim::{ShimMutex as Mutex, MutexGuard};
 
 #[cfg(feature = "std")]
-pub use std::sync::{OnceLock, LazyLock};
+pub use std::sync::OnceLock;
 #[cfg(not(feature = "std"))]
-pub use spin::{Once as OnceLock, Lazy as LazyLock};
+pub use spin::Once as OnceLock;
+
+#[rustversion::before(1.80)]
+mod before_1_80_lazylock {
+  use core::ops::Deref;
+  use super::{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_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()
+    }
+  }
+}
+#[rustversion::before(1.80)]
+pub use before_1_80_lazylock::LazyLock;
+
+#[rustversion::since(1.80)]
+pub use std::sync::LazyLock;