一通り学習して「さあ組むぞ！」「せっかくだから最新版にしちゃえ！」と軽い気持ちでアップデートしたら「bundle install」でドスンと絶望したので、備忘録として残します。

実行ログ

$ bundle install
The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x86-mingw32, x86-mswin32, x64-mingw32, java. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x86-mswin32 x64-mingw32 java`.
Fetching gem metadata from https://rubygems.org/............
Using rake 12.3.3
Using concurrent-ruby 1.1.5
Using i18n 1.6.0
Using minitest 5.11.3
Using thread_safe 0.3.6
Using tzinfo 1.2.5
Using activesupport 5.2.3
Using builder 3.2.3
Using erubi 1.8.0
Using mini_portile2 2.4.0
Using nokogiri 1.10.4
Using rails-dom-testing 2.0.3
Using crass 1.0.4
Using loofah 2.2.3
Using rails-html-sanitizer 1.2.0
Using actionview 5.2.3
Using rack 2.0.7
Using rack-test 1.1.0
Using actionpack 5.2.3
Using nio4r 2.5.1
Using websocket-extensions 0.1.4
Using websocket-driver 0.7.1
Using actioncable 5.2.3
Using globalid 0.4.2
Using activejob 5.2.3
Using mini_mime 1.0.2
Using mail 2.7.1
Using actionmailer 5.2.3
Using activemodel 5.2.3
Using arel 9.0.0
Using activerecord 5.2.3
Using mimemagic 0.3.3
Using marcel 0.3.3
Using activestorage 5.2.3
Using public_suffix 4.0.1
Using addressable 2.7.0
Using io-like 0.3.0
Using archive-zip 0.12.0
Using bindex 0.8.1
Using msgpack 1.3.1
Using bootsnap 1.4.5
Using bundler 1.17.2
Using byebug 11.0.1
Using regexp_parser 1.6.0
Using xpath 3.2.0
Using capybara 3.29.0
Using childprocess 2.0.0
Using chromedriver-helper 2.1.1
Using coffee-script-source 1.12.2
Using execjs 2.7.0
Using coffee-script 2.4.1
Using method_source 0.9.2
Using thor 0.20.3
Using railties 5.2.3
Using coffee-rails 4.2.2
Using ffi 1.11.1
Using jbuilder 2.9.1
Using rb-fsevent 0.10.3
Using rb-inotify 0.10.0
Using ruby_dep 1.5.0
Using listen 3.1.5
Fetching mysql2 0.5.2
Installing mysql2 0.5.2 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/ーー/.rbenv/versions/2.6.4/lib/ruby/gems/2.6.0/gems/mysql2-0.5.2/ext/mysql2
/Users/ーー/.rbenv/versions/2.6.4/bin/ruby -I /Users/ーー/.rbenv/versions/2.6.4/lib/ruby/2.6.0 -r ./siteconf20190930-59870-zfftb8.rb
extconf.rb --with-ldflags\=-L/usr/local/opt/openssl/lib
checking for rb_absint_size()... yes
checking for rb_absint_singlebit_p()... yes
checking for rb_wait_for_single_fd()... yes
checking for -lmysqlclient... no
-----
mysql client is missing. You may need to 'brew install mysql' or 'port install mysql', and try again.
-----
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/Users/ーー/.rbenv/versions/2.6.4/bin/$(RUBY_BASE_NAME)
        --with-mysql-dir
        --without-mysql-dir
        --with-mysql-include
        --without-mysql-include=${mysql-dir}/include
        --with-mysql-lib
        --without-mysql-lib=${mysql-dir}/lib
        --with-mysql-config
        --without-mysql-config
        --with-mysql-dir
        --without-mysql-dir
        --with-mysql-include
        --without-mysql-include=${mysql-dir}/include
        --with-mysql-lib
        --without-mysql-lib=${mysql-dir}/lib
        --with-mysqlclientlib
        --without-mysqlclientlib

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /Users/ーー/.rbenv/versions/2.6.4/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-17/2.6.0-static/mysql2-0.5.2/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/ーー/.rbenv/versions/2.6.4/lib/ruby/gems/2.6.0/gems/mysql2-0.5.2 for inspection.
Results logged to /Users/ーー/.rbenv/versions/2.6.4/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-17/2.6.0-static/mysql2-0.5.2/gem_make.out

An error occurred while installing mysql2 (0.5.2), and Bundler cannot continue.
Make sure that `gem install mysql2 -v '0.5.2' --source 'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  mysql2

原因っぽそうな箇所を推測

候補1

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

候補2

mysql client is missing. You may need to 'brew install mysql' or 'port install mysql', and try again.

候補3

An error occurred while installing mysql2 (0.5.2), and Bundler cannot continue.
Make sure that `gem install mysql2 -v '0.5.2' --source 'https://rubygems.org/'` succeeds before bundling.

色々とエラー文で検索したりしてみましたが、結果手がかりとなったのは「候補２」のエラー文でした。
どうやらMACにmySQLがインストールできていなかったことが原因だったようです。。

こちらの方の記事を見つけました。
mysql2 が原因でbundle installにてエラーを吐く(Mac OS X) - Qiita

解決方法

1.brew install mysqlを実行。
2.bundle installを実行。
3.止まらないのキモチエエ！

次回予告

開始早々初めて詰まったので、実行ログから原因を探っていく作業が新鮮でした。

別件でまた詰まったら・・・

1.とりあえずエラー文っぽい箇所を探す。（「not」などの否定形や、「faild」などネガティブワードを見つける）
2.とりあえずエラー文で検索してみる。
3.とりあえず色々試してみる。

この３本でお送りしよう。
今後の自分に役立ちそう！

参考サイト

探求中に色々見つけたのでまとめ。

bundle installでmysql2のgemがインストールできなかった時に試したこと - Qiita
mysql2 gemインストール時のトラブルシュート - Qiita