Haskell's GHC partially does it. LLVM can do it in principle, if your frontend gives enough information. Some JVMs can partially do some of it.
The above is about the optimiser figuring out whether to box or unbox by itself.
If you are willing to give the compiler a hand: Rust can do it just fine and it's the default when you define data structures. If you need boxing, you need to explicitly ask for it, eg via https://doc.rust-lang.org/std/boxed/struct.Box.html
Haskell's GHC partially does it. LLVM can do it in principle, if your frontend gives enough information. Some JVMs can partially do some of it.
The above is about the optimiser figuring out whether to box or unbox by itself.
If you are willing to give the compiler a hand: Rust can do it just fine and it's the default when you define data structures. If you need boxing, you need to explicitly ask for it, eg via https://doc.rust-lang.org/std/boxed/struct.Box.html