Ruby: Improve desugaring of for loops

[hvitved]

Previously, we would desugar for loops

for x in xs do
  # body
end

into

xs.each do |synth|
  x = synth
  # body
end

However, when the iteration variable x is used after the loop (as a captured variable inside the loop), the rewrite only becomes correct when x either has a prior assignment, or if we add a nil initialization.

This PR therefor improves the desugaring by adding a conditional initial assignment:

if not defined?(x) then
  x = nil
end

xs.each do |synth|
  x = synth
  # body
end

The rewrite also works when there are multiple iteration variables:

for x, y in xs do
  # body
end

gets desugared into

if not defined?(x) then
  x = nil
end

if not defined?(y) then
  y = nil
end

xs.each do |synth|
  x, y = synth
  # body
end

[aibaars]

This does not match the behaviour mentioned in parse.y, but it seems to have the right effect. See also https://github.com/ruby/ruby/blob/master/parse.y#L3558-L3567. I think we don't need the "if defined" checks; instead we could insert x = nil assignments for each variable that does not exist in the current scope.

[hvitved]

I think we don't need the "if defined" checks; instead we could insert x = nil assignments for each variable that does not exist in the current scope.

I would rather not make the desugaring transformation context sensitive, which is why I did it this way.

[aibaars]

Looks good to me. Let's improve the locations of the synthesized nodes though.