Box is parameterized by an allocator: https://doc.rust-lang.org/stable/std/boxed/struct.Box.html

Rust doesn’t let you utilize this fact on stable yet, which is what you’re thinking of, but the implementation in this case already exists.

If by "STL" you mean the standard library, it's not true that _all_ containers accept allocators. At least valarray and filesystem::path do not accept allocators.