What do you consider a mainstream language?

Rust uses it in their standard library[0]. Haskell uses it[1]. Lisp seems to have originated the concept in the form of box diagrams[2]. F# uses it [3].

I think it might be a functional programming thing.

[0] https://doc.rust-lang.org/std/boxed/struct.Box.html [1] https://downloads.haskell.org/~ghc/6.12.1/docs/html/users_gu... [2] https://www.gnu.org/software/emacs/manual/html_node/elisp/Bo... [3] https://fsharpforfunandprofit.com/posts/cli-types/

While slightly different, the first place I remember hearing the term was Java. It was solving a slightly different problem: primitive types were not objects. You’d have an int, but many data structures require the things you stored to be subclasses of Object. There were a bunch of objects with names like Integer that you could use instead, but in the early days there was a non-trivial performance hit associated with the “boxed” values.

Java isn't mainstream?

Granted, Java has 'references' instead of pointers, but they're close enough that the concept of boxing is basically the same.