Avoid calling to_s at end of ERB

[jhawthorn]

With the recent changes to OutputBuffer, calling to_s is extremely expensive if the buffer later is concatenated to (if the buffer never changes it should be relatively inexpensive, as Ruby will share memory).

This tries to avoid calling to_s where possible, and to explicitly return nil when a buffer is passed to template.render

# frozen_string_literal: true

require 'rails'
require 'action_view'
require 'benchmark/ips'

class BenchView < ActionView::Base
  def compiled_method_container
    self.class
  end

  def self.compiled_method_container
    self
  end
end

view = BenchView.new(
  ActionView::LookupContext,
  {},
  nil,
)
locals = { message: "There aren’t any open alerts.!" }
erb_template = <<~ERB
    Hello! #{"a"*1000}
ERB
template = ActionView::Template.new(
  erb_template,
  "hello template",
  ActionView::Template::Handlers::ERB.new,
  virtual_path: "hello",
  locals: locals.keys,
)

def render_many(view, locals, template)
  buffer = ActionView::OutputBuffer.new
  1000.times do
    template.render(view, locals, buffer)
  end
end

Benchmark.ips do |x|
  x.report("render_many") { render_many(view, locals, template) }
end

Before

$ be ruby benchmark_output_buffer.rb
Warming up --------------------------------------
         render_many     1.000  i/100ms
Calculating -------------------------------------
         render_many     13.724  (± 7.3%) i/s -     69.000  in   5.071279s

After

$ be ruby benchmark_output_buffer.rb
Warming up --------------------------------------
         render_many    89.000  i/100ms
Calculating -------------------------------------
         render_many    929.182  (± 2.4%) i/s -      4.717k in   5.080056s

[casperisfine]

🤦 I definitely didn't think of shared string while writing this.

I'm definitely good with returning the buffer itself, rendering partials directly into the parent buffer is one of the things I'm trying to achieve with these refactors, so 👍 .

Until we get there though, we might need to make sure OutputBuffer can be concatenated into another buffer without having to dup the internal string.

[casperisfine]

But I'm good with merging this as is 👍

[jhawthorn]

Until we get there though, we might need to make sure OutputBuffer can be concatenated into another buffer without having to dup the internal string.

I think that should be fine as template.render will still call to_s if a parent buffer isn't provided