How to pass a struct with a string field to a native library, with pre-initialized string field of that struct, via FFI?

Question

I have ruby code which passes a struct to a native Rust library, via FFI gem.

A struct contains a string field. I need to be able to specify string on Ruby side.

    class MyStruct < FFI::Struct
      layout :s1, :string,
             :field2, :uint32,
             # other fields

I've tried this:

     def initialize(ruby_str)
       self[:s1] = ruby_str
       # [.......]

And have gotten this:

`[]=': Cannot set :string fields (ArgumentError)

And this blows segmentation fault:

def initialize(ruby_str)
  p1 = FFI::MemoryPointer.from_string(ruby_str)
  self[:content] = p1
  self[:len] = ruby_str.length

I've read about this issue, but I've not found a clear solution, nor example. I need to be able to pass a string field at least from Ruby (one way, that is).

And, additionally, also convert a string received from a native Rust library (not NULL terminated, but with a length specified in a separate field) into a Ruby string.

How can it be done then?