offsetof expression returns the byte offset of an instance variable in a struct or class type. It accepts the type as first argument and the instance variable name prefixed by an
@ as second argument:
This is a low-level primitive and only useful if a C API needs to directly interface with the data layout of a Crystal type.
struct Foo @x = 0_i64 @y = 34_i32 end offsetof(Foo, @y) # => 8