How to pass &:read as argument to File.open as indicated by Rubocop

I just created a file named "t.txt" that contains "Hello, World\n". We can read that as follows.

File.open('t.txt', 'r', &:read)
  #=> "Hello, World\n"

Incidentally, as the default of the second argument is 'r', it suffices to write:

File.open('t.txt', &:read)

Here's another example:

"This is A Test".gsub('i', &:upcase)
  #=> "ThIs Is A Test" 

In other words, include the proc (e.g., &:read) as the last argument.

File.open(file_name, 'r', &:read)

Rubocop wants you to use the 'symbol to proc' feature in Ruby instead of defining a complete block. This is purely stylistic, and doesn't affect the code execution. You can find it in the Rubocop style guide.