Rename "Const" trait to "Freeze"

[bblum]

The word const currently means three things:

• &const as a mutability qualifier, supertype of "mut" and "" (immut). Directly analogous to C/C++'s meaning.
• const x: ty = val; for constant globals. Same as the meaning in C/C++/Java/etc.
• T: Const as a trait, which indicates "no embedded mutability". Corresponds to nothing in any other language, as far as I know.

I propose we change the third one, and leave the other two as they are. Const doesn't even mean the data is constant (it can be borrowed as mutable). In working on the regions paper, I figured this is always used, conceptually, to indicate a "freezable" data structure -- one which becomes deeply constant only when borrowed from immutably. Let's call it that.

related #3542 #3529

[nikomatsakis]

Copying a comment I accidentally wrote in #3542:

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!

[pcwalton]

&any int?

[bblum]

"any what?"

not to reject any, but it might also conflict with the any type which I believe is in the far-future plans.

[pnkfelix]

• I believe &const is going away in the future
• const x: ty = val has been replaced by static x: ty = val

so I don't think we need to get rid of Const as a trait at this point. Closing bug.

[brson]

I am still interested in renaming both Const and Owned

[thestinger]

+1 for renaming Const to Freeze

The word Const doesn't really get across the meaning, which is that something can be statically frozen by the compiler (either it inherits mutability, or it's immutable).

[toddaaro]

Visiting for triage. Whatever happens, this should be settled by milestone "maturity number 2 - backwards compatible".

[thestinger]

This landed as part of #7451.