test_stubs

Generate todo!() stubs in testing mode for trait methods without default implementations.

Overview

This library provides a proc macro attribute test_stubs which can be attached to traits: for each method in the trait without a default implementation, two variants will be created, one for #[cfg(not(test))] and one for #[cfg(test)]. The latter will have a stubbed method body containing just todo!(), allowing tests to implement the trait without having to manually implement each method.

Roughly speaking, given the following Rust source file:

#[test_stubs]
trait T {
  fn f(&self) { ... }
  fn g(&self);
}

will produce:

trait T {
  fn f(&self) { ... }

  #[cfg(not(test))]
  fn g(&self);

  #[cfg(test)]
  fn g(&self) { todo!() }
}

Note: f was copied over unchanged, but two copies of g were generated, one with and one without a default implementation.

Limitations

There are limitation to what test_stubs can do.

For example, Rust's type inference isn't always happy with just todo!(). This code will not compile:

trait T {
  #[cfg(test)]
  fn f() -> impl Iterator<...> { todo!() }
}

There is no generic solution to this. test_stubs knows about some common types and will generate code for them. For a type such as the above it will generate:

trait T {
  #[cfg(test)]
  fn f() -> impl Iterator<...> { todo!() as std::iter::Empty<_> }
}

When test_stubs has no specific knowledge about a type, it will simply generate todo!() and hope.

If a trait method takes self (rather than &self), test_stubs will add a where Self: Sized constraint to the #[cfg(test)] method.