fix memory leak when unpacking at top-level scope

[marius311]

The current macro expands @unpack x = foo() to

var### = foo()
begin
    x = unpack(var###, Val{:x})
end
var###

where var### is a new gensymed name each time, but if you ever do @unpack from the REPL / IJulia, then all the var### are in Main forever, thus leaking memory.

This makes it so we expand to

local x
let var### = foo()
    begin
        x = unpack(var###, Val{:x})
    end
    var###
end

where the local is needed to make sure scoping is always right.

[codecov-commenter]

Codecov Report

Merging #11 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #11   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           28        28           
=========================================
  Hits            28        28           

Impacted Files	Coverage Δ
src/UnPack.jl	100.00% <100.00%> (ø)

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[marius311]

Wait, sorry, still an issue in the top-level scope. Fixing...

[marius311]

Hmmm crap I can't think of any way to solve this in a way that works both in the global and local scope? Any ideas @mauro3?

[marius311]

Ok, got some guidance on discourse and I think its ready for review.

[mauro3]

Looks good. I thought about requiring a test but I think it would be too fiddly and brittle (searching through the variables returned by names(Main, all=true)).

Thanks to @yuyichao for the help on discourse! I didn't know that trick.

PS: for a bit of history, here the original PR mauro3/Parameters.jl#13 adding @unpack.

[marius311]

PS: for a bit of history, here the original PR mauro3/Parameters.jl#13 adding @unpack.

Nice. I hope one day we can look back at this whole repo too as a historical anecdote because your proposal here will have been accepted :)