add comments

src/lib.rs

@@ -1,33 +1,44 @@
+use std::marker::PhantomData;
+
+// a fancy trait
 pub trait CoolTrait {}
 
-impl<'a, D: CoolTrait + ?Sized> CoolTrait for &'a D {}
-
-pub fn foo(parent: Parent<'_>) {
-    requires_parent_fulfill_cool_trait(parent);
-}
-
-pub fn requires_parent_fulfill_cool_trait(_: impl CoolTrait) {}
+// implement that trait for all reference to someone implementing the trait
+impl<'a, D: CoolTrait> CoolTrait for &'a D {}
 
+// a type with a lifetime that contains two other types
 pub enum Parent<'a> {
-    A(&'a A<'a>),
-    B(&'a B<'a>),
-}
-
-impl<'a> CoolTrait for Parent<'a>
-where
-    &'a A<'a>: CoolTrait,
-    &'a B<'a>: CoolTrait,
-{
+    A(Box<A<'a>>),
+    B(Box<B<'a>>),
+    // We need a lifetime or else everything is fine
+    Boo(PhantomData<&'a ()>)
 }
 
+// those two types that in turn contain the parent type recursively
 pub struct A<'a> {
     parent: Parent<'a>,
 }
 
-impl<'a> CoolTrait for A<'a> where Parent<'a>: CoolTrait {}
-
 pub struct B<'a> {
     parent: Parent<'a>,
 }
 
+// implement CoolTrait only when the two types themselves implement it
+impl<'a> CoolTrait for Parent<'a>
+where
+    A<'a>: CoolTrait,
+    B<'a>: CoolTrait,
+{
+}
+
+// implement CoolTrait for the two types only when the Parent also implements it
+// oh no! a cycle!
+impl<'a> CoolTrait for A<'a> where Parent<'a>: CoolTrait {}
 impl<'a> CoolTrait for B<'a> where Parent<'a>: CoolTrait {}
+
+// now test whether CoolTrait is implemented for
+pub fn foo(parent: Parent<'_>) {
+    requires_parent_fulfill_cool_trait(parent);
+}
+
+pub fn requires_parent_fulfill_cool_trait(_: impl CoolTrait) {}