Fix set function unification

[alisdair]

Two changes here to fix panics when using the stdlib set functions (setintersect, setunion, etc) where one of the parameters is an empty collection.

Add Equals method to set.Rules

Previously, we checked if two sets had the same rules using a simple equality check. This panics if the sets contain object types, as they cannot be compared using ==.

Adding an Equals method to the Rules interface allows us to delegate to the Type.Equals method where necessary, fixing this problem.

Fix set function crashes with empty sets

If one or more arguments to the stdlib set functions was an empty set of dynamic pseudo type, the functions would panic due to incompatible set rules.

We can special case empty dynamic pseudo type sets to be ignored through type unification, because they are always capable of being converted to any other type.

Downstream crash

This PR fixes this Terraform crash:

$ echo 'setsubtract([{}], [])' | tf console

Error: Error in function call

  on /Users/alisdair/code/terraform/main.tf line 2, in output "x":
   2:   value = setsubtract([{}], [])

Call to function "setsubtract" failed: panic in function implementation:
incompatible set rules: cty.setRules{Type:cty.EmptyObject},
cty.setRules{Type:cty.DynamicPseudoType}

[apparentlymart]

Nice, thanks for digging into this! I have a little inline comment about naming, but functionality-wise this looks great.

[apparentlymart]

Equals is consistent with how a method like this is named elsewhere, but it feels a little confusing in this case because it's alongside Equivalent which is about comparing potential values, rather than the rules object itself.

What do you think about naming this SameRules instead, just to make it a bit clearer that we're talking about comparing the rules object itself? 🤔

[codecov]

Codecov Report

Merging #52 into master will increase coverage by 0.05%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   70.48%   70.53%   +0.05%     
==========================================
  Files          79       79              
  Lines        7138     7151      +13     
==========================================
+ Hits         5031     5044      +13     
  Misses       1664     1664              
  Partials      443      443              

Impacted Files	Coverage Δ
cty/set/set.go	66.66% <50.00%> (ø)
cty/function/stdlib/set.go	69.23% <100.00%> (+3.27%)	⬆️
cty/path_set.go	67.08% <100.00%> (+1.29%)	⬆️
cty/set_internals.go	97.69% <100.00%> (+0.09%)	⬆️

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[apparentlymart]

I did the change to rename Equals to SameRules myself, because I'm about to tag the 1.4.1 release and wanted to make sure this would be in it. Thanks again for looking into this!

[alisdair]

Thanks, that rename makes sense to me!