Add HOMEBREW_BOOTSNAP to optionally use Bootsnap

> Bootsnap is a library that plugs into Ruby, with optional support
> for ActiveSupport and YAML, to optimize and cache expensive
> computations.
https://github.com/Shopify/bootsnap

For our case that translates to "repeated calls to `brew` have
reductions in the time spend `require`ing speeding up the process
boot time".

For example:

```
$ hyperfine --warmup=2 "unset HOMEBREW_BOOTSNAP; brew info wget" "export HOMEBREW_BOOTSNAP=1; brew info wget"
Benchmark #1: unset HOMEBREW_BOOTSNAP; brew info wget
  Time (mean ± σ):      2.417 s ±  0.032 s    [User: 659.0 ms, System: 855.5 ms]
  Range (min … max):    2.382 s …  2.464 s    10 runs

Benchmark #2: export HOMEBREW_BOOTSNAP=1; brew info wget
  Time (mean ± σ):      1.862 s ±  0.064 s    [User: 425.3 ms, System: 566.8 ms]
  Range (min … max):    1.736 s …  1.952 s    10 runs

Summary
  'export HOMEBREW_BOOTSNAP=1; brew info wget' ran
    1.30 ± 0.05 times faster than 'unset HOMEBREW_BOOTSNAP; brew info wget'
```

.gitignore

@@ -85,6 +85,7 @@
 
 # Ignore dependencies we don't wish to vendor
 **/vendor/bundle/ruby/*/gems/ast-*/
+**/vendor/bundle/ruby/*/gems/bootsnap-*/
 **/vendor/bundle/ruby/*/gems/bundler-*/
 **/vendor/bundle/ruby/*/gems/byebug-*/
 **/vendor/bundle/ruby/*/gems/coderay-*/
@@ -105,6 +106,7 @@
 **/vendor/bundle/ruby/*/gems/mime-types-*/
 **/vendor/bundle/ruby/*/gems/mini_portile2-*/
 **/vendor/bundle/ruby/*/gems/minitest-*/
+**/vendor/bundle/ruby/*/gems/msgpack-*/
 **/vendor/bundle/ruby/*/gems/mustache-*/
 **/vendor/bundle/ruby/*/gems/net-http-digest_auth-*/
 **/vendor/bundle/ruby/*/gems/net-http-persistent-*/

Library/.rubocop.yml

@@ -370,6 +370,7 @@ Sorbet/FalseSigil:
     - "/**/{Formula,Casks}/*.rb"
     - "**/{Formula,Casks}/*.rb"
     - "Homebrew/test/**/Casks/**/*.rb"
+    - "Homebrew/bootsnap.rb"
 
 Sorbet/StrictSigil:
   Enabled: true

Library/Homebrew/Gemfile

@@ -3,6 +3,7 @@
 source "https://rubygems.org"
 
 # installed gems (should all be require: false)
+gem "bootsnap", require: false if ENV["HOMEBREW_BOOTSNAP"]
 gem "byebug", require: false
 gem "codecov", require: false
 gem "nokogiri", require: false

Library/Homebrew/bootsnap.rb

@@ -0,0 +1,20 @@
+# typed: ignore
+# frozen_string_literal: true
+
+# TODO: make this `typed: true` when HOMEBREW_BOOTSNAP is enabled by
+# default and/or we vendor bootsnap and the RBI file.
+
+raise "Needs HOMEBREW_BOOTSNAP!" unless ENV["HOMEBREW_BOOTSNAP"]
+
+require "rubygems"
+require "bootsnap"
+
+Bootsnap.setup(
+  cache_dir:            "#{ENV["HOMEBREW_TEMP"]}/homebrew-bootsnap",
+  development_mode:     false, # TODO: use ENV["HOMEBREW_DEVELOPER"]?,
+  load_path_cache:      true,
+  autoload_paths_cache: true,
+  disable_trace:        true,
+  compile_cache_iseq:   true,
+  compile_cache_yaml:   true,
+)

Library/Homebrew/global.rb

@@ -1,6 +1,8 @@
 # typed: false
 # frozen_string_literal: true
 
+require_relative "load_path"
+
 require "English"
 require "json"
 require "json/add/exception"
@@ -14,8 +16,6 @@ require "rbconfig"
 RUBY_PATH = Pathname.new(RbConfig.ruby).freeze
 RUBY_BIN = RUBY_PATH.dirname.freeze
 
-require_relative "load_path"
-
 require "rubygems"
 # Only require "core_ext" here to ensure we're only requiring the minimum of
 # what we need.

Library/Homebrew/load_path.rb

@@ -9,5 +9,9 @@ $LOAD_PATH.push HOMEBREW_LIBRARY_PATH.to_s
 
 require "vendor/bundle/bundler/setup"
 
-$LOAD_PATH.select! { |d| Pathname(d).directory? }
-$LOAD_PATH.uniq!
+if ENV["HOMEBREW_BOOTSNAP"]
+  require "bootsnap"
+else
+  $LOAD_PATH.select! { |d| Pathname(d).directory? }
+  $LOAD_PATH.uniq!
+end