Introduce coerce_container_to_any

[Ralith]

This PR introduces a lint which detects when &Box<dyn Any> is coerced to &dyn Any, which is usually a mistake where the intent was to directly pass the dyn Any inside the box without coercion.

cc @llogiq

Remaining work:

• Documentation
• Generalize from Box to any implementer of Deref<Target=dyn Any>

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changelog: add [coerce_container_to_any] lint

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[matthiasbeyer]

r? llogiq

[llogiq]

This looks good for a start. Let's add the documentation and replace the "default lint description", and then I can start the Final Comment Period for this lint.

[Ralith]

Thanks to guidance from @y21 on Zulip, I've got this as smart as I wanted. Seeking feedback particularly on lint name and description wording. I may iterate on the diagnostic a little more.

[Ralith]

Diagnostic is now pretty good. Not sure if there's a better way to format the expression type, e.g. so it shows up as Box rather than std::boxed::Box.

[Ralith]

Future work: maybe this should also fire on coercing &Box<dyn Foo> to &dyn Any when Foo: Any?

[Ralith]

Thanks to @fu5ha for expanded docs!

[sanbox-irl]

This looks wonderful! I left some bikeshedding comments on the description and what not.

For the name, what about coerce_container_to_any? I think using any_ref to describe &Box<dyn Any> is surprising

[llogiq]

Not sure if there's a better way to format the expression type, e.g. so it shows up as Box rather than std::boxed::Box.

That would be path trimming, which we currently don't have. I'm on that, but you don't need to worry about it right now.

[Ralith]

Applied all feedback. Thanks for the reviews!

[Ralith]

@llogiq friendly ping

[llogiq]

I have one final suggestion regarding the lint message and will start the FCP soon.

[llogiq]

The final comment period is on.

[Ralith]

Fixed a stray ` in the example.

[llogiq]

Thank you!