src/generics/phantom/testcase_units.md

# Testcase: unit clarification

A useful method of unit conversions can be examined by implementing `Add`
with a phantom type parameter. The `Add` `trait` is examined below:

```rust,ignore
// This construction would impose: `Self + RHS = Output`
// where RHS defaults to Self if not specified in the implementation.
pub trait Add<RHS = Self> {
    type Output;

    fn add(self, rhs: RHS) -> Self::Output;
}

// `Output` must be `T<U>` so that `T<U> + T<U> = T<U>`.
impl<U> Add for T<U> {
    type Output = T<U>;
    ...
}
```

The whole implementation:

```rust,editable
use std::ops::Add;
use std::marker::PhantomData;

/// Create void enumerations to define unit types.
#[derive(Debug, Clone, Copy)]
enum Inch {}
#[derive(Debug, Clone, Copy)]
enum Mm {}

/// `Length` is a type with phantom type parameter `Unit`,
/// and is not generic over the length type (that is `f64`).
///
/// `f64` already implements the `Clone` and `Copy` traits.
#[derive(Debug, Clone, Copy)]
struct Length<Unit>(f64, PhantomData<Unit>);

/// The `Add` trait defines the behavior of the `+` operator.
impl<Unit> Add for Length<Unit> {
    type Output = Length<Unit>;

    // add() returns a new `Length` struct containing the sum.
    fn add(self, rhs: Length<Unit>) -> Length<Unit> {
        // `+` calls the `Add` implementation for `f64`.
        Length(self.0 + rhs.0, PhantomData)
    }
}

fn main() {
    // Specifies `one_foot` to have phantom type parameter `Inch`.
    let one_foot:  Length<Inch> = Length(12.0, PhantomData);
    // `one_meter` has phantom type parameter `Mm`.
    let one_meter: Length<Mm>   = Length(1000.0, PhantomData);

    // `+` calls the `add()` method we implemented for `Length<Unit>`.
    //
    // Since `Length` implements `Copy`, `add()` does not consume
    // `one_foot` and `one_meter` but copies them into `self` and `rhs`.
    let two_feet = one_foot + one_foot;
    let two_meters = one_meter + one_meter;

    // Addition works.
    println!("one foot + one_foot = {:?} in", two_feet.0);
    println!("one meter + one_meter = {:?} mm", two_meters.0);

    // Nonsensical operations fail as they should:
    // Compile-time Error: type mismatch.
    //let one_feter = one_foot + one_meter;
}
```

### See also:

[Borrowing (`&`)], [Bounds (`X: Y`)], [enum], [impl & self],
[Overloading], [ref], [Traits (`X for Y`)], and [TupleStructs].

[Borrowing (`&`)]: ../../scope/borrow.md
[Bounds (`X: Y`)]: ../../generics/bounds.md
[enum]: ../../custom_types/enum.md
[impl & self]: ../../fn/methods.md
[Overloading]: ../../trait/ops.md
[ref]: ../../scope/borrow/ref.md
[Traits (`X for Y`)]: ../../trait.md
[TupleStructs]: ../../custom_types/structs.md
[std::marker::PhantomData]: https://doc.rust-lang.org/std/marker/struct.PhantomData.html