Add a "nothreads" feature to drop Send+Sync requirements from OwnedKV.

When using slog in a single-threading context, the Send+Sync
requirement means that we cannot use non-threadsafe objects
as values or as part of a lazy value callback. This feature
flag provides support for this scenario in a backward compatible
way.

Author: rasky

Cargo.toml

@@ -29,6 +29,7 @@ debug-assertions = false
 nested-values = ["erased-serde"]
 dynamic-keys = []
 std = []
+nothreads = []
 default = ["std"]
 
 max_level_off   = []

src/lib.rs

@@ -3032,29 +3032,28 @@ where
     }
 }
 
-#[cfg(feature = "std")]
+#[cfg(feature = "nothreads")]
 /// Thread-local safety bound for `KV`
 ///
 /// This type is used to enforce `KV`s stored in `Logger`s are thread-safe.
+pub trait SendSyncRefUnwindSafeKV: KV {}
+
+#[cfg(feature = "nothreads")]
+impl<T> SendSyncRefUnwindSafeKV for T where T: KV + ?Sized {}
+
+#[cfg(all(not(feature = "nothreads"), feature = "std"))]
+/// This type is used to enforce `KV`s stored in `Logger`s are thread-safe.
 pub trait SendSyncRefUnwindSafeKV: KV + Send + Sync + RefUnwindSafe {}
 
-#[cfg(feature = "std")]
-impl<T> SendSyncRefUnwindSafeKV for T
-where
-    T: KV + Send + Sync + RefUnwindSafe + ?Sized,
-{
-}
+#[cfg(all(not(feature = "nothreads"), feature = "std"))]
+impl<T> SendSyncRefUnwindSafeKV for T where T: KV + Send + Sync + RefUnwindSafe + ?Sized {}
 
-#[cfg(not(feature = "std"))]
+#[cfg(all(not(feature = "nothreads"), not(feature = "std")))]
 /// This type is used to enforce `KV`s stored in `Logger`s are thread-safe.
 pub trait SendSyncRefUnwindSafeKV: KV + Send + Sync {}
 
-#[cfg(not(feature = "std"))]
-impl<T> SendSyncRefUnwindSafeKV for T
-where
-    T: KV + Send + Sync + ?Sized,
-{
-}
+#[cfg(all(not(feature = "nothreads"), not(feature = "std")))]
+impl<T> SendSyncRefUnwindSafeKV for T where T: KV + Send + Sync + ?Sized {}
 
 /// Single pair `Key` and `Value`
 pub struct SingleKV<V>(pub Key, pub V)