Compare Function bodies by pointer equality

We're changing the `body` field to contain an
`Rc<dyn FunctionBodyObject>`, which does not implement `PartialEq`.
To compare two `Function` objects for equality, we will compare
the `body` fields using pointer equality with `Rc::ptr_eq`.

Author: magicant

yash-env/CHANGELOG.md

@@ -20,6 +20,11 @@ A _private dependency_ is used internally and not visible to downstream users.
   variable name. This allows modules to check variable names without directly
   depending on the `yash-syntax` crate.
 
+### Changed
+
+- `impl PartialEq for function::Function`: Now compares the function bodies
+  using pointer equality instead of deep equality.
+
 ### Removed
 
 - `parser::is_name`: This re-export of `yash_syntax::parser::lex::is_name` has

yash-env/src/function.rs

@@ -64,7 +64,7 @@ impl<T: FunctionBody + ?Sized> FunctionBodyObject for T {
 }
 
 /// Definition of a function.
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Clone, Debug, Eq)]
 pub struct Function {
     /// String that identifies the function.
     pub name: String,
@@ -125,6 +125,19 @@ impl Function {
     }
 }
 
+/// Compares two functions for equality.
+///
+/// Two functions are considered equal if all their members are equal.
+/// This includes comparing the `body` members by pointer equality.
+impl PartialEq for Function {
+    fn eq(&self, other: &Self) -> bool {
+        self.name == other.name
+            && Rc::ptr_eq(&self.body, &other.body)
+            && self.origin == other.origin
+            && self.read_only_location == other.read_only_location
+    }
+}
+
 /// Wrapper of [`Function`] for inserting into a hash set.
 ///
 /// A `HashEntry` wraps a `Function` in `Rc` so that the `Function` object can