Example showing problem with Bounds2 using Point2::Sub trait.

.gitignore

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+/target
+**/*.rs.bk

Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "nestedsubtraits"
+version = "0.1.0"
+

Cargo.toml

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+[package]
+name = "nestedsubtraits"
+version = "0.1.0"
+authors = ["Bill Thiede <git@xinu.tv>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

src/lib.rs

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+use std::fmt;
+use std::ops::Mul;
+use std::ops::Sub;
+
+type Float = f32;
+
+pub trait Number
+where
+    Self: std::marker::Sized + Copy + fmt::Display + Mul + Sub,
+{
+    // Real version has stuff here
+}
+
+impl Number for Float {}
+impl Number for isize {}
+
+/// Generic type for any 2D point.
+#[derive(Copy, Clone, Debug, Default, PartialEq)]
+pub struct Point2<T>
+where
+    T: Number,
+{
+    pub x: T,
+    pub y: T,
+}
+
+impl<T> From<[T; 2]> for Point2<T>
+where
+    T: Number,
+{
+    fn from(xy: [T; 2]) -> Self {
+        Point2 { x: xy[0], y: xy[1] }
+    }
+}
+
+/// 2D point type with `Float` members.
+pub type Point2f = Point2<Float>;
+/// 2D point type with `isize` members.
+pub type Point2i = Point2<isize>;
+
+impl<T> Sub for Point2<T>
+where
+    T: Number + Sub<Output = T>,
+{
+    type Output = Self;
+
+    /// Implement `-` for Point2<T>
+    ///
+    /// # Examples
+    /// ```
+    /// use nestedsubtraits::Point2i;
+    ///
+    /// let p1: Point2i = [2, 3].into();
+    /// let p2: Point2i = [4, 5].into();
+    /// assert_eq!(p2 - p1, [2, 2].into());
+    ///
+    /// use nestedsubtraits::Point2f;
+    ///
+    /// let p1: Point2f = [2., 3.].into();
+    /// let p2: Point2f = [4., 5.].into();
+    /// assert_eq!(p2 - p1, [2., 2.].into());
+    /// ```
+    fn sub(self, rhs: Self) -> Self::Output {
+        Point2 {
+            x: self.x - rhs.x,
+            y: self.y - rhs.y,
+        }
+    }
+}
+
+/// Generic type for 2D bounding boxes.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Bounds2<T>
+where
+    T: Number,
+{
+    pub p_min: Point2<T>,
+    pub p_max: Point2<T>,
+}
+
+impl<T> Bounds2<T>
+where
+    T: Number,
+{
+    pub fn area(&self) -> T {
+        // TODO(glenng) make this compile.
+        /*
+        let d = self.p_max - self.p_min;
+        d.x * d.y
+         */
+        unimplemented!();
+    }
+}