It seems to me like the compiler is pretty good at detecting when a value has an unknown size, and thus needs to be heap allocated, so why do we need the Box type? It seems to me like it could just heap allocate(wrap in a box internally) a type whenever it detects it doesn't implement Sized, and may…

Automatic heap allocations are against the goals of Rust - for a systems language, that's pretty much unacceptable, warnings or not. Box also represents unique ownership over heap allocated storage, as a type, so it serves that role. How would you represent that without a type? Is there a more sp…

Box is not the only, or even the most common, way to pass a pointer to a dynamically sized type. For example, types like str and [T] are usually passed as borrowed references like &str and &[T], or through specialized owned types like String and Vec<T>. Trait objects can be passed as &Trait or Rc<T…

[image] alvitawa: maybe give a compile warning that the value is heap-allocated What are you expecting programmers to do with this warning, just ignore it? Warnings for standard, correct code are not helpful.

How would you represent that without a type? It wouldn't need to be specifically represented, you would just write: struct A { arr: [T] } Instead of: struct A { arr: Box<[T]> } Same for function signatures, and all usage of the value would just remain the same, since Box already imple…

What are you expecting programmers to do with this warning, just ignore it? Warnings for standard, correct code are not helpful. When applicable, it could be ignored or suppressed, or the code could be changed to be generic. Most of the time, using Box is a convenience, and code that utilizes thi…

The example you have is for a slice, which is something specific. How would you put a scalar value on the heap? [image] alvitawa: No, I was just wondering, because sometimes boxing values can get a bit tiresome. Hopefully you're not boxing all that much in Rust :slight_smile:.

The example you have is for a slice, which is something specific. How would you put a scalar value on the heap? Why would you want to put a scalar value on the heap? If something can go on the stack, it should, in general. If, for some reason, you need to heap-allocate an f32 when it could also …

[image] alvitawa: Why would you want to put a scalar value on the heap? If something can go on the stack, it should, in general. If, for some reason, you need to heap-allocate an f32 when it could also be stack-allocated, you can use unsafe functions like the ones the implementation of Vec uses…

There's another use of Box that we've not mentioned yet, which is to pass/receive raw pointers via FFI. You can get a raw pointer to Rust-allocated memory from a Box, pass that pointer via FFI, and then take ownership back when FFI returns a raw pointer back (i.e. put it back into a Box). This is …

[image] alvitawa: as long as I’m aware the heap allocation occurs, whether it’s explicitly written in the code or communicated through a warning doesn’t matter to me. This, I think, is probably going to be where you will find the most resistance to your point of view. There is very good reaso…

Why doesn't rust just heap allocate automatically?

vitalyd August 28, 2017, 10:16am 7

The example you have is for a slice, which is something specific. How would you put a scalar value on the heap?

Hopefully you're not boxing all that much in Rust .

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Why doesn't rust just heap allocate automatically?

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