Add ultimate solution for practice example.

practice/unreferenced_types_in_rust/src/bin/8_solution_ultimate.rs

@@ -0,0 +1,28 @@
+fn main() {}
+
+trait Trait1 {
+    type Item;
+    type T1: core::iter::Iterator<Item = Self::Item>;
+    fn f1() -> Self::T1;
+}
+
+struct Once1 {}
+
+impl Trait1 for Once1 {
+    type Item = i32;
+    type T1 = core::iter::Once<Self::Item>;
+    fn f1() -> Self::T1 {
+        core::iter::once(13_i32)
+    }
+}
+
+struct Map1 {}
+
+impl Trait1 for Map1 {
+    type Item = i32;
+    type T1 = core::iter::Map<core::iter::Once<Self::Item>, &'static dyn Fn(i32) -> i32>;
+    fn f1() -> Self::T1 {
+        // Just put the existential type behind the &dyn and you're ready to go =)
+        core::iter::once(13_i32).map(&(|e| e + 10))
+    }
+}