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.


  • 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.


~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:

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


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.

It may be possible that unforeseen interactions will appear between the struct nature of Box<T> and the built-in nature of ~T when merged.