Crate implementation

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A crate for targeting and accessing actual implementation.

Take an example trait:

trait ScrapeTheInternet {
    fn scrape_the_internet(&self) -> Vec<Website>;

The trait represents some abstract computation. The trait exports a method signature that can be implemented by types. In this case, we can imagine what a true implementation of the trait will do: Actually scrape the internet.

implementation provides the Impl type as an implementation target for traits having the following semantics:

  • The trait has only one actual, true implementation.
  • Other implementations of the trait may exist, but these are interpreted as fake, mocked in some way.

implementation enables a standardized way of writing these actual implementations in a way that allows the actual Self-receiver type to be unknown.


To define the actual, generic implementation of ScrapeTheInternet, we can write the following impl:

impl<T> ScrapeTheInternet for implementation::Impl<T> {
    fn scrape_the_internet(&self) -> Vec<Website> {
        todo!("find all the web pages, etc")

This code implements the trait for Impl, and by doing that we have asserted that it is the actual, true implementation.

The implementation is fully generic, and works for any T.

use implementation::Impl;

struct MyType;

let websites = Impl::new(MyType).scrape_the_internet();

Trait bounds

The advantage of keeping trait implementations generic, is that the self type might live in a downstream crate. Let’s say we need to access a configuration parameter from scrape_the_internet. E.g. the maximum number of pages to scrape:

use implementation::Impl;

trait GetMaxNumberOfPages {
    fn get_max_number_of_pages(&self) -> Option<usize>;

impl<T> ScrapeTheInternet for Impl<T>
    where Impl<T>: GetMaxNumberOfPages
    fn scrape_the_internet(&self) -> Vec<Website> {
        let max_number_of_pages = self.get_max_number_of_pages();
        todo!("find all the web pages, etc")

Now, for this to work, Impl<T> also needs to implement GetMaxNumberOfPages (for the same T that is going to be used).

GetMaxNumberOfPages would likely be implemented for a specific T rather than a generic one, since that T would typically be some configuration holding that number:

struct Config {
    max_number_of_pages: Option<usize>

impl GetMaxNumberOfPages for implementation::Impl<Config> {
    fn get_max_number_of_pages(&self) -> Option<usize> {


This crate is the solution to a trait coherence problem.

Given the trait above, we would like to provide an actual and a mocked implementation. We might know what its actual implementation looks like as an algorithm, but not what type it should be implemented for. There could be several reasons to have a generic Self:

  • The Self type might live in a downstream crate
  • It is actually designed to work generically

If we had used a generic Self type (impl<T> DoSomething for T), the trait would be unable to also have distinct fake implementations, because that would break the coherence rules: A generic (“blanket”) impl and a specialized impl are not allowed to exist at the same time, because that would lead to ambiguity.

To solve that, a concrete type is needed as implementation target. But that type is allowed to be generic internally. It’s just the root level that needs to be a concretely named type.

That type is the Impl type.

When we use this implementation, we can create as many fake implementations as we want.


  • Wrapper type for targeting and accessing actual implementation.