Derive GenericTypeVisitable for RegionConstraint _correctly_

[ada4a]

The derive added in #160164 was incorrect -- it resulted in an overflow during trait solving. This is because #[derive(GenericTypeVisitable)] automatically adds a : GenericTypeVisitable bound to every field of a type -- in this case, Box<[RegionConstraint<I>]>: GenericTypeVisitable<V>.

To fix this, I added a #[generic_type_visitable(bounds(..))] attribute to the derive macro, which allows overriding the added bounds.

I also made the derive macro no longer a no-op in rustc, so that errors like this can be caught on r-l/r CI in the future.

cc @ChayimFriedman2

[rustbot]

r? @JonathanBrouwer

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[ChayimFriedman2]

It doesn't need to be unsafe. If the bounds are wrong it won't compile (at least in r-a).

[ada4a]

What about a case like this?

#[derive(GenericTypeVisitable)]
struct Foo {
    #[generic_type_visitable(unsafe(bounds()))]
    contains_self: (Box<Self>, Bar),
}

struct Bar;

AFAICT bounds are incorrectly missing Bar: GenericTypeVisitable, which leads to unsoundness.

[ChayimFriedman2]

No, because then calling GenericTypeVisitable::visit(&self.contains_self) will fail compilation.

[ada4a]

The newly added complexity of the derive macro makes me wonder if it deserves some ui tests now.. Not sure where they would go though

[ChayimFriedman2]

Note that only making the derive not-no-op won't make forgetting to derive it a CI failure, since rustc does not use it for anything.

[ada4a]

Yes, that will be covered by a future PR. I thought I'd get this one merged now, so that the fixed derive already gets into the source code -- otherwise the list of ra-ap crate versions I need to catch up on will only continue to rise over time 😅

[ada4a]

Apparently, a backtick in an ignore description breaks everything... How silly