Add ?Sized bound to SourceCode to allow casting &str` into it.

[Diegovsky]

I'm trying to cast a borrowed &str to a dyn SourceCode but I'm receiving the error:
error[E0277]: the size for values of type str cannot be known at compilation time
and
help: the trait `Sized` is not implemented for `str`
note: required for the cast from `&str` to `&dyn SourceCode`

Which leads me to Source Code's definition, where it is said to be implemented by str and &str (as expected), but it is not marked as ?Sized which I suspect is the reason why this cast can't be made.

[zkat]

I think this is happening because you’re trying to turn &str into &dyn SourceCode… that is, str (which is unsized, and does NOT implement SourceCode), into dyn SourceCode

try casting &&str to &dyn SourceCode

[Diegovsky]

Well, it is implemented for str.

I couldn't do it with &&str because I store an owned String and I'm trying to provide a SourceCode for Diagnostic::source_code.

[JustusFluegel]

actually afaik that is currently a restriction of the rust compiler, you can't actually use CoerceUnsized for !Sized objects. I don't know why the error message is so confusing, that could certainly use some improvement. See the section here, it mentiones T: Sized as a requirement for the unsizing coercion.

https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html

[Diegovsky]

Well, in any case, it seems like this is a rust bug. That trait is clearly implemented for str, which is !Sized.

It is a bit of a paradox that you can implement a trait for an unsized type but can't actually use it.

[JustusFluegel]

well you can use it, you just can't perform the unsizing coercion on !Sized types. It's actually pretty common, for example in rand the recommended way to write a function that takes a rng is fn foo<R: Rng + ?Sized>(rng: &mut R) (which is great since then you can, for example, also pass a &mut dyn RngCore to it since RngCore is implemented for dyn RngCore which itsself is unsized), still behind a reference but since the methods in the trait take references that works just fine. There was some good reason why CoerceUnsized doesn't work (i.e converting from &T to &dyn Trait where T: !Sized + Trait) for unsized types, but I would have to look up why again ( I think it was something with having to store the layout (size, alignment, etc.) in the vtable for the drop impl and that beeing difficult cuz of layouts and supertraits and stuff, not quite sure ) - definetely something that should be fixable in the compiler but I think just a bunch of work no one has gotten around to yet.

[Diegovsky]

Well, that certainly went over my head :)

In any case, I gathered from your explanation that making SourceCode: ?Sized could allow for CoerceUnsized to work, is that correct?