Summary

The tilde (~) operator and type construction do not support allocators and therefore should be removed in favor of the box keyword and a language item for the type.

Motivation

There will be a unique pointer type in the standard library, Box<T,A> where A is an allocator. The ~T type syntax does not allow for custom allocators. Therefore, in order to keep ~T around while still supporting allocators, we would need to make it an alias for Box<T,Heap>. In the spirit of having one way to do things, it seems better to remove ~ entirely as a type notation.
~EXPR and box EXPR are duplicate functionality; the former does not support allocators. Again in the spirit of having one and only one way to do things, I would like to remove ~EXPR.
Some people think ~ is confusing, as it is less self-documenting than Box.
~ can encourage people to blindly add sigils attempting to get their code to compile instead of consulting the library documentation.

Drawbacks

~T may be seen as convenient sugar for a common pattern in some situations.

Detailed design

The ~EXPR production is removed from the language, and all such uses are converted into box.

Add a lang item, box. That lang item will be defined in liballoc (NB: not libmetal/libmini, for bare-metal programming) as follows:

#[lang="box"]
pub struct Box<T,A=Heap>(*T);

All parts of the compiler treat instances of Box<T> identically to the way it treats ~T today.

The destructuring form for Box<T> will be box PAT, as follows:

let box(x) = box(10);
println!("{}", x); // prints 10

Alternatives

The other possible design here is to keep ~T as sugar. The impact of doing this would be that a common pattern would be terser, but I would like to not do this for the reasons stated in “Motivation” above.

Unresolved questions

The allocator design is not yet fully worked out.