Subtleties of upgrading Ruby 1.9 to 2.0

27 October 2014
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There are six notable incompatibilities from Ruby 1.9.3 to 2.0.0:

UTF-8 by default: The default encoding for ruby scripts is now UTF-8 [#6679]. Some people report that it affects existing programs, such as some benchmark programs becoming very slow [ruby-dev:46547].
Iconv was removed, which had already been deprecated when M17N was introduced in ruby 1.9. Use String#encode, etc. instead.
There is ABI breakage [ruby-core:48984]. We think that normal users can/should just reinstall extension libraries. You should be aware: DO NOT COPY .so OR .bundle FILES FROM 1.9.
#lines, #chars, #codepoints, #bytes now returns an Array instead of an Enumerator [#6670]. This change allows you to avoid the common idiom “lines.to_a”. Use #each_line, etc. to get an Enumerator.
Object#inspect no longer delegates to #to_s. It always returns a string like #<ClassName:0x…> [#2152]
Objects no longer respond_to? protected methods by default

This last one bit me. Protected methods treated like private for #respond_to? unless true is passed as a second argument.

e.g.

f.respond_to?(:bar)         #=> false
f.respond_to?(:bar, true)   #=> true

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