Rename Send trait to Owned

[brson]

The concepts around the core language are much clearer now than when we added the kind system. At the time I was disappointed about including the notion of 'sendability' in the language, while all the actual sending was going on in libraries. Now we have a much better term for sendable things, 'owned', and owning a thing is useful for more than just sending.

If we want we can keep a Send trait that derives from Owned to use in the comms mods. This would require tagging types as Send, which may or may not be desirable. Regardless, Send should be demoted.

Owned 2012.

[brson]

Also, per @nikomatsakis, the current Owned trait shouldn't exist, represents something other than 'owned' as we usually think of it.

[bblum]

I am somewhat confused by this. As I understand it, today, send means no & pointers and no @ pointers. owned means no & pointers but @ pointers are OK. send precisely describes what's legal to e.g. send over pipes, and owned precisely describes what's legal to store in TLS.

With this change, would we forbid storing @ pointers in TLS (restrictive)? Would we allow storing & pointers in TLS (unsound)? I must be missing something.

[brson]

The current Owned trait is misnamed.

[bblum]

oh, it will just be renamed also?

[brson]

@nikomatsakis believes that further refinements to borrowed pointers will make the current Owned trait unnecessary, and that we can rename it to Static or something until.

[nikomatsakis]

I strongly agree with not having const have so many overloaded meanings. To be honest, I think const-the-mutability-qualifier is the odd man out here, and I would like to change it to something else, but I don't know what (read? too valuable to use as a keyword. mut?? looks funny, can't pronounce it. rd or ro? Eh.). I think there is another issue to this effect. I know @jruderman has raised the issue before (and rightly so).

That said, I think Freeze may not be the right name for the trait we currently call Const. Here is my reasoning: a freezable data structure is one which is based purely on owned data and with no explicit mutable qualifiers. This is currently described by Send Const---the Const captures the lack of mutability qualifiers and the Send captures the "relies purely on owned data". So it seems to me that Freeze, if it exists, should be a "subtrait" of Send and Const.

Now, it is not clear to me whether a trait that means "immutable" is necessary on its own. @pcwalton and I have talked about some ideas for removing pure functions and replacing them with closures with a "Const" bound, so it'd be important then. The best name I can come up for to replace Const is Imm (for Immutable, which seems a touch long), even though it is true that an instance of an Imm type, when placed in a mutable local variable (or other mutable context), is in fact mutable. But somehow NoMut doesn't seem to have the same ring!

[bblum]

@nikomatsakis I expect you meant to reply to #3949?

in any case, I wonder: is there any place we use Const where Send is also not used? I'm alluding to the way that Send (no @-pointers) already implies Owned (no &-pointers).

It seems to me we should either (a) make const/send/owned such that each one implies the next (which means Freeze would indeed mean Const Send Owned), or (b) change Send to not imply no &-pointers (whether or not there would ever be a use case for it?) (such that ARCs would require Const Send Owned, not just Const Send).

If there is a use case for Const-without-Send, I favour (b); otherwise, I'm not sure which one I favour.

[nikomatsakis]

@bblum yes, yes I did. :) Regarding uses of Const where Send is not used, that's precisely what I was talking about in the final paragraph---places we might use immutable on its own.

[bblum]

ah, I must have skipped that paragraph. How would the const bound replace purity? (I assume that means something different than the limitation in #3569.)

Immut might be good instead of Imm or Immutable. I might even favour it just as much as Freeze.

[brson]

Implemented Owned -> Durable, Send -> Owned