> Collections are not parametrized by an allocator, like in C++ or (future) Rust.

What does he mean by this?

Psuedo-Rust code here, showing off both styles.

Parameterized by an allocator:

  struct Foo {
      // ...
  }

  impl Foo {
      fn new() -> Foo {
          // do something to make a new foo, calling functions via A
      }
  }

contrast with "an allocator is passed in explicitly to every method which actually needs to allocate."

  struct Foo {
      // ...
  }

  impl Foo {
      fn new(a: A) -> Foo {
          // do something to make a new foo, calling functions via A
      }
  }

(EDIT: "new" was a bad choice for me to pick here, see the child comment that uses 'put' instead; I was not trying to suggest that the allocator only parameterizes a constructor, but any call that would need to allocate.)

MrBuddyCasino

Thanks for the example. I wasn't aware this is something that was planned for Rust, seems like a pretty large breaking change. Interesting!

steveklabnik

Nothing is breaking! The change isn't a move to a zig-like style, but instead, that in today's Rust, you have

  struct Foo {
      // ...
  }

and not

  struct Foo {
      // ...
  }

This isn't breaking because there is a default type provided for A.

The handwave is over "Today's Rust", as this machinery has already landed, in a sense, it's just not really possible to use with the standard library because the allocator API isn't fully stable.

Here's an actual example: https://doc.rust-lang.org/stable/std/vec/struct.Vec.html See that "A = Global"?

MrBuddyCasino

I see. I'm interested in the motivation and design goals behind it, is there something that spells those out? The only thing I could find quickly is this [0].

[0] https://internals.rust-lang.org/t/why-bring-your-own-allocat...

steveklabnik

https://rust-lang.github.io/rfcs/1974-global-allocators.html was the original RFC.

My vague understanding is that there's a working group https://github.com/rust-lang/wg-allocators

The further I get from working on Rust day to day, the less I know about these things, so that's all I've got for you.