Reduce system calls by activating the `did_you_mean` gem.

Activating the gem puts the gem on the load path, where simply requiring
the file will search every gem that's installed until it can find a gem
that contains the `did_you_mean` file.

Calling RubyGems' `require` will search each installed gem until it can
find one that contains the file it should require.  This means that the
more gems you have installed, the longer it can take to require that
gem.

To see this in action, lets compare the number of `stat` calls for a
"bare require" vs the number of `stat` calls for a require that follows
a gem activation by using these two programs:

```
[aaron@TC rubygems (master)]$ cat req_dym.rb
begin
  require 'did_you_mean'
rescue LoadError
end
[aaron@TC rubygems (master)]$ cat gem_dym.rb
begin
  gem 'did_you_mean'
  require 'did_you_mean'
rescue Gem::LoadError, LoadError
end
```

The first program just requires the `did_you_mean` gem, where the second
one activates the gem, then requires it.  We can count the number of
`stat` calls using `dtrace`:

```
[aaron@TC rubygems (master)]$ ruby -v
ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin15]
[aaron@TC rubygems (master)]$ sudo dtrace -q -n 'syscall::stat*:entry { printf("%s\n", copyinstr(arg0)); }' -c`rbenv which ruby`" --disable-did_you_mean req_dym.rb" | wc -l

dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
     283
[aaron@TC rubygems (master)]$ sudo dtrace -q -n 'syscall::stat*:entry { printf("%s\n", copyinstr(arg0)); }' -c`rbenv which ruby`" --disable-did_you_mean gem_dym.rb" | wc -l

dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
      13
```

The "bare require" version does over 10x the number of stat calls
compared to the "gem, then require" version.  Of course the number for
the first one depends on the number of gems you have installed that sort
before the `did_you_mean` gem.

Lets also look at trunk Ruby:

```
[aaron@TC rubygems (master)]$ ruby -v
ruby 2.4.0dev (2016-02-25 trunk 53940) [x86_64-darwin15]
[aaron@TC rubygems (master)]$ sudo dtrace -q -n 'syscall::stat*:entry { printf("%s\n", copyinstr(arg0)); }' -c`rbenv which ruby`" --disable-did_you_mean req_dym.rb" | wc -l

dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
    2325
[aaron@TC rubygems (master)]$ sudo dtrace -q -n 'syscall::stat*:entry { printf("%s\n", copyinstr(arg0)); }' -c`rbenv which ruby`" --disable-did_you_mean gem_dym.rb" | wc -l

dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
dtrace: error on enabled probe ID 3 (ID 826: syscall::stat64:entry): invalid user access in action #1 at DIF offset 24
     685
```

This change will reduce the number of `stat` calls on trunk Ruby too,
but since this installation doesn't have the `did_you_mean` gem,
RubyGems is still reading every gem spec file so that it can raise a
`Gem::LoadError` exception with a nice error message.  If we can modify
RubyGems a little, it may be possible to drop the number of stat calls
even on a Ruby installation that doesn't have the `did_you_mean` gem.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53941 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,11 @@
+Fri Feb 26 08:11:58 2016  Aaron Patterson <tenderlove@ruby-lang.org>
+
+	* gem_prelude.rb: Reduce system calls by activating the `did_you_mean`
+	  gem before requiring the gem.  Activating the gem puts the gem on
+	  the load path, where simply requiring the file will search every gem
+	  that's installed until it can find a gem that contains the
+	  `did_you_mean` file.
+
 Thu Feb 25 19:04:13 2016  Martin Duerst  <duerst@it.aoyama.ac.jp>
 
 	* enc/unicode/case-folding.rb: Adding possibility for debugging output

gem_prelude.rb

@@ -1,7 +1,8 @@
 if defined?(Gem)
   require 'rubygems.rb'
   begin
+    gem 'did_you_mean'
     require 'did_you_mean'
-  rescue LoadError
+  rescue Gem::LoadError, LoadError
   end if defined?(DidYouMean)
 end