feat: Allow predicates to own object and evaluate against borrowed types

It is very useful to be able to dynamically construct an object and
have that object owned by the predicate, yet evaluate against an
unowned type related to the owned one. An obvious example is a String
being owned by the predicate but being compared against &strs.

Therefore, implement Predicate for Eq/OrdPredicate that store an
object that implements Borrow for the predicate type, replacing
existing impls of Predicate<T> for Eq/OrdPredicate<T> and
Eq/OrdPredicate<&T>. This is backwards compatible as there are blanket
implementations of Borrow<T> for T and Borrow<T> for &T.

Note that Borrow imposes more requirements than are actually required
and AsRef would be sufficient. However, AsRef doesn't have a blanket
implementation for T and thus the existing impl of Predicate<T> for
EqPredicate<T> is still required, but results in a conflict since T
may also implement AsRef<T>. Requiring Borrow instead of AsRef is
sufficient for common use cases though.

This addresses assert-rs#20 more completely.

src/ord.rs

@@ -1,4 +1,4 @@
-// Copyright (c) 2018 The predicates-rs Project Developers.
+// Copyright (c) 2018, 2022 The predicates-rs Project Developers.
 //
 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
 // http://www.apache.org/license/LICENSE-2.0> or the MIT license
@@ -35,26 +35,24 @@ impl fmt::Display for EqOps {
 ///
 /// This is created by the `predicate::{eq, ne}` functions.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct EqPredicate<T>
-where
-    T: fmt::Debug + PartialEq,
-{
+pub struct EqPredicate<T> {
     constant: T,
     op: EqOps,
 }
 
-impl<T> Predicate<T> for EqPredicate<T>
+impl<P, T> Predicate<P> for EqPredicate<T>
 where
-    T: fmt::Debug + PartialEq,
+    T: std::borrow::Borrow<P> + fmt::Debug,
+    P: fmt::Debug + PartialEq + ?Sized,
 {
-    fn eval(&self, variable: &T) -> bool {
+    fn eval(&self, variable: &P) -> bool {
         match self.op {
-            EqOps::Equal => variable.eq(&self.constant),
-            EqOps::NotEqual => variable.ne(&self.constant),
+            EqOps::Equal => variable.eq(self.constant.borrow()),
+            EqOps::NotEqual => variable.ne(self.constant.borrow()),
         }
     }
 
-    fn find_case<'a>(&'a self, expected: bool, variable: &T) -> Option<reflection::Case<'a>> {
+    fn find_case<'a>(&'a self, expected: bool, variable: &P) -> Option<reflection::Case<'a>> {
         utils::default_find_case(self, expected, variable).map(|case| {
             case.add_product(reflection::Product::new(
                 "var",
@@ -64,32 +62,11 @@ where
     }
 }
 
-impl<'a, T> Predicate<T> for EqPredicate<&'a T>
-where
-    T: fmt::Debug + PartialEq + ?Sized,
-{
-    fn eval(&self, variable: &T) -> bool {
-        match self.op {
-            EqOps::Equal => variable.eq(self.constant),
-            EqOps::NotEqual => variable.ne(self.constant),
-        }
-    }
-
-    fn find_case<'b>(&'b self, expected: bool, variable: &T) -> Option<reflection::Case<'b>> {
-        utils::default_find_case(self, expected, variable).map(|case| {
-            case.add_product(reflection::Product::new(
-                "var",
-                utils::DebugAdapter::new(variable).to_string(),
-            ))
-        })
-    }
-}
-
-impl<T> reflection::PredicateReflection for EqPredicate<T> where T: fmt::Debug + PartialEq {}
+impl<T> reflection::PredicateReflection for EqPredicate<T> where T: fmt::Debug {}
 
 impl<T> fmt::Display for EqPredicate<T>
 where
-    T: fmt::Debug + PartialEq,
+    T: fmt::Debug,
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let palette = crate::Palette::current();
@@ -120,6 +97,10 @@ where
 /// let predicate_fn = predicate::eq("Hello");
 /// assert_eq!(true, predicate_fn.eval("Hello"));
 /// assert_eq!(false, predicate_fn.eval("Goodbye"));
+///
+/// let predicate_fn = predicate::eq(String::from("Hello"));
+/// assert_eq!(true, predicate_fn.eval("Hello"));
+/// assert_eq!(false, predicate_fn.eval("Goodbye"));
 /// ```
 pub fn eq<T>(constant: T) -> EqPredicate<T>
 where
@@ -178,28 +159,26 @@ impl fmt::Display for OrdOps {
 ///
 /// This is created by the `predicate::{gt, ge, lt, le}` functions.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct OrdPredicate<T>
-where
-    T: fmt::Debug + PartialOrd,
-{
+pub struct OrdPredicate<T> {
     constant: T,
     op: OrdOps,
 }
 
-impl<T> Predicate<T> for OrdPredicate<T>
+impl<P, T> Predicate<P> for OrdPredicate<T>
 where
-    T: fmt::Debug + PartialOrd,
+    T: std::borrow::Borrow<P> + fmt::Debug,
+    P: fmt::Debug + PartialOrd + ?Sized,
 {
-    fn eval(&self, variable: &T) -> bool {
+    fn eval(&self, variable: &P) -> bool {
         match self.op {
-            OrdOps::LessThan => variable.lt(&self.constant),
-            OrdOps::LessThanOrEqual => variable.le(&self.constant),
-            OrdOps::GreaterThanOrEqual => variable.ge(&self.constant),
-            OrdOps::GreaterThan => variable.gt(&self.constant),
+            OrdOps::LessThan => variable.lt(self.constant.borrow()),
+            OrdOps::LessThanOrEqual => variable.le(self.constant.borrow()),
+            OrdOps::GreaterThanOrEqual => variable.ge(self.constant.borrow()),
+            OrdOps::GreaterThan => variable.gt(self.constant.borrow()),
         }
     }
 
-    fn find_case<'a>(&'a self, expected: bool, variable: &T) -> Option<reflection::Case<'a>> {
+    fn find_case<'a>(&'a self, expected: bool, variable: &P) -> Option<reflection::Case<'a>> {
         utils::default_find_case(self, expected, variable).map(|case| {
             case.add_product(reflection::Product::new(
                 "var",
@@ -209,34 +188,11 @@ where
     }
 }
 
-impl<'a, T> Predicate<T> for OrdPredicate<&'a T>
-where
-    T: fmt::Debug + PartialOrd + ?Sized,
-{
-    fn eval(&self, variable: &T) -> bool {
-        match self.op {
-            OrdOps::LessThan => variable.lt(self.constant),
-            OrdOps::LessThanOrEqual => variable.le(self.constant),
-            OrdOps::GreaterThanOrEqual => variable.ge(self.constant),
-            OrdOps::GreaterThan => variable.gt(self.constant),
-        }
-    }
-
-    fn find_case<'b>(&'b self, expected: bool, variable: &T) -> Option<reflection::Case<'b>> {
-        utils::default_find_case(self, expected, variable).map(|case| {
-            case.add_product(reflection::Product::new(
-                "var",
-                utils::DebugAdapter::new(variable).to_string(),
-            ))
-        })
-    }
-}
-
-impl<T> reflection::PredicateReflection for OrdPredicate<T> where T: fmt::Debug + PartialOrd {}
+impl<T> reflection::PredicateReflection for OrdPredicate<T> where T: fmt::Debug {}
 
 impl<T> fmt::Display for OrdPredicate<T>
 where
-    T: fmt::Debug + PartialOrd,
+    T: fmt::Debug,
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let palette = crate::Palette::current();
@@ -267,6 +223,10 @@ where
 /// let predicate_fn = predicate::lt("b");
 /// assert_eq!(true, predicate_fn.eval("a"));
 /// assert_eq!(false, predicate_fn.eval("c"));
+///
+/// let predicate_fn = predicate::lt(String::from("b"));
+/// assert_eq!(true, predicate_fn.eval("a"));
+/// assert_eq!(false, predicate_fn.eval("c"));
 /// ```
 pub fn lt<T>(constant: T) -> OrdPredicate<T>
 where