This is a kind of stupid example, but imagine you have to implement some external trait (e.g. in order to work with some dependency) with the following shape:

trait Foo {
    fn foo(&self, i: usize) -> &Bar;
}

Which is not too unreasonable, for example it's very similar to the stdlib's Index. In order to implement this trait you must return a reference, you can't return e.g. a Cow or an Arc. The fact that it takes a parameter means there might not even be one single value it has to return, so you can't even cache that inside of self with e.g. LazyLock.

Of course I'm not saying I would try to reach for an escape hatch if I had to do something like this. I would first try to see if this is an intrinsic problem in my code, or if maybe I can change the crate I'm working with to be more permissible. That is, I would try to look for proper solutions first, though Cow might not always work for that.

[–] BB_C@programming.dev 3 points 2 weeks ago* (last edited 2 weeks ago) (1 children)

&Bar is a reference to something. That something is either a part of self, or a part of the static context. There is no other context because there is no runtime/GC. So there is no logical not-nonsensical scenario where this would be both a valid and a limiting situation in Rust. And this is why your surface analogy to Index is invalid.

If the return value may depend on something other than self or the static context, and still need to be reference-like, then the trait definition is wrong. It should either return a Cow, or go for the obvious generalization of returning impl AsRef<Bar> values. With that generalization, references, Cows, and more can be returned.

There is also the possibility that the trait definition is right, and you (the implementer) are trying to break a (probably) deliberate constraint (e.g. the return value in Index being tied to &self).

I would wager a guess that what you call an escape hatchet is considered a very bad C# style anyway (or will/should be). Just like how mutable statics are considered very bad in Rust 😉

[–] taladar@sh.itjust.works 1 points 2 weeks ago

That kind of "escape hatch" also makes reasoning about code a lot harder merely because you have to consider that someone used it somewhere. You literally don't want "escape hatches" from safety guarantees all over your language.