One change is that test/unit is now no longer part of the standard library, which Rails requires. The workaround is simple, "test-unit" must be added to the Gemfile.

A less easily fixed change is that the following syntax no longer works the same as it did in previous versions of Ruby:

    def foo(bar = bar)

The recommended fix is the following: (I believe)

    def foo(bar = bar())

Rails uses this syntax occasionally. It's simple to change, but the Rails team refuses to merge in the fixes because it's not a security issue.