Hashify / transform_values is unnecessarily being called for nested Dry::Structs

[rpeng]

dry-struct/lib/dry/struct/hashify.rb

Line 12 in 9fcdbcc

value.to_h.transform_values { self[_1] }

If a value is a Dry::Struct, Hashify.[] is already the default implementation of its to_h method. Doing it again in transform_values will be double calling the hashify method, leading to inefficient serialization.

A simple fix here is to just remove the transform_values in the is_a?(Struct) branch - can expect a huge speed up for nested structs.

Found this on 1.6.0 but I believe the same issue is on the main branch from inspecting the implementation

Example:

class Foo < Dry::Struct
  attribute :value, Types::Coercible::String
end

class Bar < Dry::Struct
  attribute :foo, Foo
end

class Baz < Dry::Struct
  attribute :bar, Bar
end

baz = Baz.new(
  bar: {
    foo: {
      value: "hello"
    }
  }
)

baz.to_h # <- "hello" is hashified 3 times!

patched it with some logging output

>> baz.to_h
hashify: #<Bar:0x000000034f66cc70>
hashify: #<Foo:0x000000034f66cd10>
hashify: hello
hashify: hello
hashify: {:value=>"hello"}
hashify: hello

[rpeng]

we found this thru doing a benchmark + rbprof and found that Dry::Struct::Hashify#[] was taking a substantial amount of self + cpu time