Given that Enumerable#inject can take either a symbol or a block as the method to be used in the iteration, as explained in an answer to this question, is there any reason to use the & in conjunction with Symbol#to_proc within Enumerable#inject? The following pairs return the same result:
[1, 2, 3, 4, 5].inject(:+)
[1, 2, 3, 4, 5].inject(&:+)
[:a, :b, :c].inject({a: {b: {c: 1}}}, :fetch)
[:a, :b, :c].inject({a: {b: {c: 1}}}, &:fetch)
Is there any use case where using a symbol and using a block (created by &) have different results? Are there any cases where one can be used and not the other?
If you have to support old versions of Ruby (1.8.6 or older) and you're using a library that defines Symbol#to_proc for those versions (like active support), the version using & will work and the other will not.
Other than that the only differences between the two versions are that the version using a symbol is faster and that the version using & will be affected if you redefine Symbol#to_proc - though I can't think of a case where that'd be useful. So no, if you don't need to support Ruby 1.8.6, there's no reason to not use the symbol form.