fix(predicate): Break out boxing from trait

BREAKING CHANGE: `Predicate` trait is changed but most use cases should
not be impacted.

src/boxed.rs

@@ -57,3 +57,46 @@ where
         self.0.eval(variable)
     }
 }
+
+/// `Predicate` extension for boxing a `Predicate`.
+pub trait PredicateBoxExt<Item: ?Sized>
+where
+    Self: Predicate<Item>,
+{
+    /// Returns a `BoxPredicate` wrapper around this `Predicate` type.
+    ///
+    /// Returns a `BoxPredicate` wrapper around this `Predicate type. The
+    /// `BoxPredicate` type has a number of useful properties:
+    ///
+    ///   - It stores the inner predicate as a trait object, so the type of
+    ///     `BoxPredicate` will always be the same even if steps are added or
+    ///     removed from the predicate.
+    ///   - It is a common type, allowing it to be stored in vectors or other
+    ///     collection types.
+    ///   - It implements `Debug` and `Display`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use predicates::prelude::*;
+    ///
+    /// let predicates = vec![
+    ///     predicate::always().boxed(),
+    ///     predicate::never().boxed(),
+    /// ];
+    /// assert_eq!(true, predicates[0].eval(&4));
+    /// assert_eq!(false, predicates[1].eval(&4));
+    /// ```
+    fn boxed(self) -> BoxPredicate<Item>
+    where
+        Self: Sized + Send + Sync + 'static,
+    {
+        BoxPredicate::new(self)
+    }
+}
+
+impl<P, Item> PredicateBoxExt<Item> for P
+where
+    P: Predicate<Item>,
+{
+}

src/core.rs

@@ -6,8 +6,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use boxed::BoxPredicate;
-
 /// Trait for generically evaluating a type against a dynamically created
 /// predicate function.
 ///
@@ -19,35 +17,4 @@ pub trait Predicate<Item: ?Sized> {
     /// Execute this `Predicate` against `variable`, returning the resulting
     /// boolean.
     fn eval(&self, variable: &Item) -> bool;
-
-    /// Returns a `BoxPredicate` wrapper around this `Predicate` type.
-    ///
-    /// Returns a `BoxPredicate` wrapper around this `Predicate type. The
-    /// `BoxPredicate` type has a number of useful properties:
-    ///
-    ///   - It stores the inner predicate as a trait object, so the type of
-    ///     `BoxPredicate` will always be the same even if steps are added or
-    ///     removed from the predicate.
-    ///   - It is a common type, allowing it to be stored in vectors or other
-    ///     collection types.
-    ///   - It implements `Debug` and `Display`.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use predicates::prelude::*;
-    ///
-    /// let predicates = vec![
-    ///     predicate::always().boxed(),
-    ///     predicate::never().boxed(),
-    /// ];
-    /// assert_eq!(true, predicates[0].eval(&4));
-    /// assert_eq!(false, predicates[1].eval(&4));
-    /// ```
-    fn boxed(self) -> BoxPredicate<Item>
-    where
-        Self: Sized + Send + Sync + 'static,
-    {
-        BoxPredicate::new(self)
-    }
 }

src/prelude.rs

@@ -10,6 +10,7 @@
 
 pub use core::Predicate;
 pub use boolean::PredicateBooleanExt;
+pub use boxed::PredicateBoxExt;
 
 /// Predicate factories
 pub mod predicate {