feat: implement From<Fn() -> T> for Signal<T> (#1801)

leptos_reactive/src/lib.rs

@@ -3,6 +3,8 @@
 #![cfg_attr(feature = "nightly", feature(fn_traits))]
 #![cfg_attr(feature = "nightly", feature(unboxed_closures))]
 #![cfg_attr(feature = "nightly", feature(type_name_of_val))]
+#![cfg_attr(feature = "nightly", feature(auto_traits))]
+#![cfg_attr(feature = "nightly", feature(negative_impls))]
 
 //! The reactive system for the [Leptos](https://docs.rs/leptos/latest/leptos/) Web framework.
 //!

leptos_reactive/src/signal_wrappers_read.rs

@@ -428,25 +428,6 @@ impl<T> From<Memo<T>> for Signal<T> {
     }
 }
 
-impl<T: Clone> From<T> for Signal<T> {
-    #[track_caller]
-    fn from(value: T) -> Self {
-        Self {
-            inner: SignalTypes::DerivedSignal(store_value(Box::new(
-                move || value.clone(),
-            ))),
-            #[cfg(any(debug_assertions, feature = "ssr"))]
-            defined_at: std::panic::Location::caller(),
-        }
-    }
-}
-
-impl From<&str> for Signal<String> {
-    fn from(value: &str) -> Self {
-        Self::from(value.to_string())
-    }
-}
-
 enum SignalTypes<T>
 where
     T: 'static,
@@ -854,6 +835,36 @@ impl From<&str> for MaybeSignal<String> {
     }
 }
 
+#[cfg(feature = "nightly")]
+mod from_fn_for_signals {
+    use super::{MaybeSignal, Memo, ReadSignal, RwSignal, Signal};
+    auto trait NotSignalMarker {}
+
+    impl<T> !NotSignalMarker for Signal<T> {}
+    impl<T> !NotSignalMarker for ReadSignal<T> {}
+    impl<T> !NotSignalMarker for Memo<T> {}
+    impl<T> !NotSignalMarker for RwSignal<T> {}
+    impl<T> !NotSignalMarker for MaybeSignal<T> {}
+
+    impl<F, T> From<F> for Signal<T>
+    where
+        F: Fn() -> T + NotSignalMarker + 'static,
+    {
+        fn from(value: F) -> Self {
+            Signal::derive(value)
+        }
+    }
+}
+#[cfg(not(feature = "nightly"))]
+impl<F, T> From<F> for Signal<T>
+where
+    F: Fn() -> T + 'static,
+{
+    fn from(value: F) -> Self {
+        Signal::derive(value)
+    }
+}
+
 /// A wrapping type for an optional component prop, which can either be a signal or a
 /// non-reactive value, and which may or may not have a value. In other words, this is
 /// an `Option<MaybeSignal<Option<T>>>` that automatically flattens its getters.