Allow casting to mutable trait objects.

[jdm]

No description provided.

[jdm]

r? @pcwalton

[nikomatsakis]

@jdm -- I don't think this is quite the right way to handle the representation etc in the compiler

[jdm]

I appreciate the feedback, but would you like to be a bit more specific? It's a bit hard to act on your comment. If this is covered by #5762, that's fine, but I would like to know what the next step is.

[nikomatsakis]

Yes, sorry! That was brief. Let me expand. First off, I apologize, when I first read your patch I was somewhat mislead because you used the letters mt to refer to the mutability, but mt is usually the pair of a mutability and a type. Including an mt in ty_trait wouldn't make much sense. It would probably be better to just use m or mutbl if the value is just a mutability.

Anyhow, adding a mutability to ty_trait doesn't strike me as ideal, since it doesn't apply e.g. to ~Trait, but it's also not a bad thing. We have other cases in our type representation that are "more general" than they need to be. I tend to think we should refactor those, and perhaps we can eventually update this along the lines of #5762, but in the meantime its ok.

I haven't had time to read over the patch in great detail to tell if you're covering all the necessary checks yet though, but it's a good idea to get the basic syntax etc in place.

[jdm]

Getting the syntax in Rust is blocking my work on Servo. I still can't tell whether you're just asking for some renaming for clarity, or whether you're going to need to find some time to read through it more deeply before giving more directed feedback.

[brson]

@nikomatsakis Any further feedback on this?

[nikomatsakis]

@jdm @brson sorry, in short, my feeling is that the patch is basically ok as is, particularly if this is a blocking issue. I think we'll wind up refactoring it some down the line but that goes for just about every patch.

[nikomatsakis]

(though I would certainly prefer not using the mnemonic mt to refer to just a mutability, since that is typically a ty::mt.)

[jdm]

I'll rebase and rename the mts.

[jdm]

Rebased and renamed, and much more pleasant to read after Niko's trait changes! r? @nikomatsakis