Recipe

Implement a foreign trait for a foreign type (via a newtype)

Use when: rustc reports E0117 because you want a standard library (or other crate's) trait on a type you didn't define

Goal: the behaviour you wanted, on a local wrapper type the orphan rule allows

Taught in Coherence and the orphan rule

1. 1

   Declare a newtype around the foreign type: struct Readings(Vec<f64>);.

   Example

   The failing program has impl fmt::Display for Vec<f64>. Add one line above it: struct Readings(Vec<f64>);. The impl hasn't moved yet, so rustc still reports E0117 at this point.

   Why: the wrapper is defined in your crate, so it's local, and a foreign trait may be implemented for a local type.

2. 2

   Move the impl to the wrapper, impl fmt::Display for Readings, and reach the inner value as self.0.

   Example

   The header becomes impl fmt::Display for Readings. Inside, self is now a Readings, so self.len() becomes self.0.len(): write!(f, "{} readings", self.0.len()). The E0117 is gone.

   Why: the body is the same logic as before; only the type it's attached to has changed.

3. 3

   Where the program made a Vec<f64>, make a Readings(vec![...]) instead, and pass that to the code that uses the trait.

   Example

   let temps = vec![18.5, 19.0, 21.25]; becomes let temps = Readings(vec![18.5, 19.0, 21.25]);. The line println!("{temps}") stays as it was, and now prints 3 readings.

   Why: the trait belongs to Readings, not to Vec<f64>, so it only applies to values of the wrapper type.

4. 4

   If callers need a few of the inner type's methods, add small methods to the wrapper (fn len(&self) -> usize { self.0.len() }), or use .0.

   Example

   A caller wants the first reading. temps.0[0] gives 18.5. If callers often want the count, add impl Readings { fn len(&self) -> usize { self.0.len() } }, and temps.len() returns 3. Methods you don't add, like push, stay out of reach unless someone goes through .0.

   Why: the wrapper is a real type, so you choose which operations it offers.