Improving "Empty 'do' block" error message

Motivation

With GHC-9.2 NondecreasingIndentation will no longer be on by default. This means that users will run into the Empty 'do' block error much more often. It would be nice if this error message gave users hints on how to fix it.

Context

There's broadly two ways you can get this error message. Either (a) you can write something with an explicit empty block:

a = do {}

or (b) you can write something like:

{-# LANGUAGE NoNondecreasingIndentation #-} -- implicit with GHC 9.2
b = do
  print "doing things with X"
  withX $ \x -> do
  doStuffWithX x

where the layout rules lead to an empty block.

Real world example: https://github.com/TomMD/cipher-aes128/blob/0c3311cf9fc696c2bc22a890b409e5bf37541913/Setup.hs#L36

Proposal

When this error is thrown from a situation like (b), we should tell users:

• that the error can be avoided by indenting the lines of the inner do block
• that the error can be avoided by enabling NondecreasingIndentation