Comment by comex

Technically it’s only the standard library that lacks custom allocator support. Box and Vec and all the rest are “just” types defined in the standard library, and if you define your own, you can use any allocator you want and still take advantage of the borrow checker and other language features. This has been possible since 1.0.

…Well, except for certain magic features that the standard library types get that you can’t (yet) replicate in a custom type. This is an annoying wart, but they’re relatively minor.

But yeah, custom allocator support for standard library containers is planned.