Enhancement of `unreachable_code_linter` to Detect Unreachable Code in Loops

[MEO265]

The unreachable_code_linter should also mark code inside if, else, for, and while loops as unreachable, especially when a return or stop function is called.

The following code currently generates only one annotation:

x <- function() { 
  if(TRUE) { 
    stop('This is a test') 
    5+5 
  } 
  return(5L)
  4+4 
} 

And this one generates none:

x <- function() { 
  if(TRUE) { 
    stop('This is a test') 
    5+5 
  } 
  4+4 
} 

[MichaelChirico]

I think this is a duplicate of #1428, though I don't think if(TRUE) constitutes unreachable code. I think it's more relevant for unnecessary nesting (which we have a linter for, though not in {lintr}) yet IIRC

oh, I misunderstood. yes, I agree it's unreachable.

[MEO265]

In loops, attention should also be paid to next and break.

@MichaelChirico I'm not sure if that's a part of what you might have meant in your TODO as well.

lintr/tests/testthat/test-unreachable_code_linter.R

Lines 175 to 180 in c8f0074

	# TODO(michaelchirico): the logic could be extended to terminal if statements
	# or control flows (for/while). There shouldn't really be such a thing as
	# a terminal for/while (owing to ExplicitReturnLinter forcing these to
	# be followed by return(invisible()) or similar), but could be included to
	# catch comments for completeness / robustness as a standalone function.
	# Terminal if statements are a bit messy, but would have some payoff.