我相信FFI::Struct#[]=会处理这个问题。我没有在源代码中检查它，但我在代码中添加了一些检查，看起来是这样的。

require 'ffi'

class SimpleStruct < FFI::Struct
  layout :value, :pointer
end

class Test1
  def self.foo(s)
    puts "foo is called"
    result = FFI::MemoryPointer.from_string(s)
    ObjectSpace.define_finalizer(result, proc { puts "foo result is garbage collected" })
    result
  end

  def self.bar(s)
    puts "bar is called"
    simple_struct = SimpleStruct.new
    ObjectSpace.define_finalizer(s, proc { puts "bar result is garbage collected" })
    value = FFI::MemoryPointer.from_string(s)
    ObjectSpace.define_finalizer(s, proc { puts "bar result[:value] is garbage collected" })
    simple_struct[:value] = value

    simple_struct
  end

  def self.baz(s)
    puts "baz is called"
    simple_struct = {}
    ObjectSpace.define_finalizer(s, proc { puts "baz result is garbage collected" })
    value = FFI::MemoryPointer.from_string(s)
    ObjectSpace.define_finalizer(s, proc { puts "baz result[:value] is garbage collected" })
    simple_struct[:value] = value

    simple_struct
  end
end

class Test2
  def self.testing
    puts "testing is started"
    a = Test1.foo('foo')
    b = Test1.bar('bar')
    c = Test1.baz('baz')

    puts a.read_string, b[:value].read_string, c[:value].read_string
    puts "testing is finished"
  end
end

GC.stress

Test2.testing

# testing is started
# foo is called
# bar is called
# baz is called
# foo
# bar
# baz
# testing is finished
# baz result is garbage collected
# baz result[:value] is garbage collected
# bar result is garbage collected
# bar result[:value] is garbage collected
# foo result is garbage collected

指针只有在结构体被垃圾回收时才被垃圾回收，就像散列一样。