reg-struct-return

The tracking issue for this feature is: https://github.com/rust-lang/rust/issues/116973.

Option -Zreg-struct-return causes the compiler to return small structs in registers instead of on the stack for extern "C"-like functions. It is UNSOUND to link together crates that use different values for this flag. It is only supported on x86.

It is equivalent to Clang's and GCC's -freg-struct-return.