Custom lint for Mockall static expectations

[asomers]

Lint explanation

When mocking a static method with Mockall, it's necessary to create a context object ¹. Normally, the context object should live for the entire duration of the test case. But it's easy for users to forget, in which case they'll get confusing errors at runtime. The easiest way to forget is when users use method chaining inappropriately.

Example code

The lint should flag code like this:

MockA::foo_context().expect().returning(|| 99);

And suggest instead writing it like this:

let ctx = MockA::foo_context();
ctx.expect().returning(|| 99);

Notes

Possibly the easiest way to implement this would be to detect when a user uses a *_context() method without a let binding. And since the *_context() methods are autogenerated, Mockall can emit any attributes needed to guide the linter.

Footnotes

1. https://docs.rs/mockall/latest/mockall/#static-methods ↩