Checking basic code formatting with Ruby

You may not have seen an example of it, but Ruby actually ships with a built-in syntax checker that will warn about syntax that is almost universally considered problematic. It can catch issues such as the following:

• Unused variables:

  def a
    b = 1 # b not used
    2
  end

• Unreachable code:

  def a
    return
    2 # not reachable
  end

• Mismatched and possibly misleading indentation:

  if a
    if b
      p 3
  end # misleading, appears to close "if a" instead of "if b"
  end

• Unused literal expressions:

  def a
    1 # unused literal value
    2
  end

• Duplicated keyword arguments and hash keys:

  a(b: 1, b: 2) # duplicate keyword argument
   {c: 3, c: 4} # duplicate hash key

• Using meth *args when meth is a local variable (which is parsed as meth.*(args) instead of meth(*args))
• Using if var = val conditionals, where val is a static value such...