Add `vec_in!` macro

[Rua]

Currently there is no way to construct a Vec with a non-default allocator, using an array literal. vec! only works with the default allocator.

[Amanieu]

It's not clear whether vectors with custom allocators will be sufficiently widely used to justify adding another macro to the prelude. This should probably be re-visited once custom allocator support has been stabilized.

[bergkvist]

Is there a reason something like this couldn't work?

let v: Vec<u32, MyAllocator> = vec![1, 2, 3, 4];

[Shadlock0133]

@bergkvist it's missing the allocator parameter, e.g. vec_in![in MyAllocator; 1, 2, 3, 4];

[bergkvist]

Yeah, I just realized that something like this:

let v: Vec<u32, MyAllocator> = vec![1, 2, 3, 4];

could only work if the Allocator trait didn't require &self in its methods.

Which could generally only be true for allocators that outsource memory management to the operating system, or who otherwise behave like singeltons/manage memory globally.

For example a typical arena/bump allocator (where you can make multiple independent instances of the same allocator implementation), would not be possible without &self.

[bergkvist]

I guess the closest thing without a vec_in![in MyAllocator; 1, 2, 3, 4] macro currently is something like this:

let v = <[_]>::into_vec(Box::new_in([1, 2, 3, 4], MyAllocator));

[gagbo]

Hello,

(I’ve been lurking a bit about the topic, just want to weigh in as I recently made a small library that wanted custom allocated Vec.)

Would it be possible then to add extra rules in the vec![] macro like checking for a ;:

let v = vec![in my_arena; 1, 2, 3, 4];

When I want to create vector in custom allocators using allocator_api2, it gets very verbose and I find the macro helps a lot for readability.