[ruby-core:100589] [Ruby master Feature#17287] Faster Pathname FileUtils methods

Issue #17287 has been reported by schneems (Richard Schneeman).

----------------------------------------
Feature #17287:  Faster Pathname FileUtils methods
https://bugs.ruby-lang.org/issues/17287

* Author: schneems (Richard Schneeman)
* Status: Open
* Priority: Normal
----------------------------------------
I have a patch that I would like to merge into Pathname for increased perfo=
rmance. I understand that akr maintains pathname and may not be on GitHub. =
Here is a link to my patch:

https://github.com/ruby/ruby/pull/3693

Here is the diff:

```
$ git diff master
diff --git a/ext/pathname/lib/pathname.rb b/ext/pathname/lib/pathname.rb
index e6fb90277d..c3af24837f 100644
--- a/ext/pathname/lib/pathname.rb
+++ b/ext/pathname/lib/pathname.rb
@@ -575,12 +575,13 @@ def find(ignore_error: true) # :yield: pathname


 class Pathname    # * FileUtils *
+  autoload(:FileUtils, 'fileutils')
+
   # Creates a full path, including any intermediate directories that don't=
 yet
   # exist.
   #
   # See FileUtils.mkpath and FileUtils.mkdir_p
   def mkpath
-    require 'fileutils'
     FileUtils.mkpath(@path)
     nil
   end
@@ -591,7 +592,6 @@ def mkpath
   def rmtree
     # The name "rmtree" is borrowed from File::Path of Perl.
     # File::Path provides "mkpath" and "rmtree".
-    require 'fileutils'
     FileUtils.rm_r(@path)
     nil
   end
```

## Description

Currently when calling any of the "FileUtils" methods on pathname `require`=
 is called every time even though that library might already be loaded. Thi=
s is slow.

We can speed it up by either checking first if the constant is already defi=
ned, or by using autoload.

Using defined speeds up the action by about 300x and using autoload is abou=
t twice as fast as that (600x faster than current require method).

I'm proposing we use autoload:

```ruby
require 'benchmark/ips'

Benchmark.ips do |x|
  autoload(:FileUtils, "fileutils")
  x.report("require") { require 'fileutils' }
  x.report("defined") { require 'fileutils' unless defined?(FileUtils) }
  x.report("autoload") { FileUtils }

  x.compare!
end

# Warming up --------------------------------------
#              require     3.624k i/100ms
#              defined     1.465M i/100ms
#             autoload     2.320M i/100ms
# Calculating -------------------------------------
#              require     36.282k (=B1 2.4%) i/s -    184.824k in   5.0971=
53s
#              defined     14.539M (=B1 2.0%) i/s -     73.260M in   5.0411=
61s
#             autoload     23.100M (=B1 1.9%) i/s -    115.993M in   5.0232=
71s

# Comparison:
#             autoload: 23099779.2 i/s
#              defined: 14538544.9 i/s - 1.59x  (=B1 0.00) slower
#              require:    36282.3 i/s - 636.67x  (=B1 0.00) slower
```

Because this autoload is scoped to Pathname it will not change the behavior=
 of existing programs that are not expecting FileUtils to be loaded yet:

```
ruby -rpathname -e "class Pathname; autoload(:FileUtils, 'fileutils'); end;=
 puts FileUtils.exist?('foo')"
Traceback (most recent call last):
-e:1:in `<main>': uninitialized constant FileUtils (NameError)
```





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