Convert points with components of one type to another

See:
rust-lang/rust#42721 (comment)

And:
https://stackoverflow.com/questions/49710852/what-is-an-auto-trait-in-rust

Author: tuzz

src/geometry/point/mod.rs

@@ -4,3 +4,28 @@ use generic_array::{ArrayLength, GenericArray};
 pub struct Point<T, N: ArrayLength<T>> {
     pub components: GenericArray<T, N>,
 }
+
+impl<T, N: ArrayLength<T>, I, X> From<I> for Point<T, N>
+    where I: IntoIterator<Item=T, IntoIter=X>,
+          X: ExactSizeIterator<Item=T>,
+{
+    fn from(iter: I) -> Self {
+        Self { components: GenericArray::from_exact_iter(iter).unwrap() }
+    }
+}
+
+// NotEq is a marker trait that's auto-implemented on all 2-tuples...
+pub auto trait NotEq { }
+
+// ...except those with two values of the same type...
+impl<T> !NotEq for (T, T) { }
+
+impl<T, U, N: ArrayLength<T> + ArrayLength<U>> From<Point<U, N>> for Point<T, N>
+    where (T, U): NotEq, // ...which lets us add a trait bound that avoids a conflict
+          T: From<U>,    //    with the auto-implemented From<T> trait.
+          U: Copy,
+{
+    fn from(p: Point<U, N>) -> Self {
+        p.components.iter().map(|&a| a.into()).into()
+    }
+}

src/geometry/point2/test.rs

@@ -50,4 +50,12 @@ mod conversions {
         assert_eq!(subject.x(), 1);
         assert_eq!(subject.y(), 2);
     }
+
+    #[test]
+    fn it_can_build_a_point2_from_a_point2_with_different_component_types() {
+        let subject: Point2f = Point2i::new(1, 2).into();
+
+        assert_eq!(subject.x(), 1.0);
+        assert_eq!(subject.y(), 2.0);
+    }
 }

src/geometry/point3/test.rs

@@ -41,3 +41,16 @@ mod default {
         assert_eq!(subject.z(), 0);
     }
 }
+
+mod conversions {
+    use super::*;
+
+    #[test]
+    fn it_can_build_a_point3_from_a_point3_with_different_component_types() {
+        let subject: Point3f = Point3i::new(1, 2, 3).into();
+
+        assert_eq!(subject.x(), 1.0);
+        assert_eq!(subject.y(), 2.0);
+        assert_eq!(subject.z(), 3.0);
+    }
+}

src/main.rs

@@ -1,3 +1,5 @@
+#![feature(optin_builtin_traits)]
+
 mod geometry;
 
 fn main() {