`Borrow` and `Inout` types for safe, first-class references

[Alejandro (Alejandro Alonso)]

@ksluder

Last month, @John_McCall was still considering whether Inout<T> was needed:

@John_McCall

I suppose the counter-argument is that we wouldn't need exclusive use at all if we just special-cased spans and references the right way; maybe I need to think about it more.

No, I think you misunderstand John’s post here. John is referring to not needing the exclusive ownership access if we just special case MutableSpan (and Inout).

@ksluder

As someone trying to come up to speed with exclusive ownership, the fact that the language would admit something like inout Inout<Borrow<T>> makes me anxious. It seems to add so many dimensions to the ownership space that I will never feel confident that I have chosen the right mix of modifiers and wrapper types.

I agree this is confusing. The nice thing about the reborrowing approach discussed in the exclusive ownership proposal that you mentioned is that the most natural spelling is just always consuming Inout<T> (which was also suggested in Difficulty using MutableSpan ) (a great article on what that really is in Rust is here: haibane_tenshi's blog - Obscure Rust: reborrowing is a half-baked feature note &mut u32 is just Inout<UInt32>). Personally speaking, if we had reborrowing support for Inout and MutableSpan, passing these as consuming would become commonplace and could potentially (big if) become the default specifier if you didn’t supply one. E.g.

func foo(with x: Inout<Int>) { // consuming Inout<Int>
  x.value += 1 // ok
}