terraform console: enable use of impure functions

[mildwonkey]

Allow impure funcs in walkEval, which is used by terraform console.

I wrote a bit of a unit test for this, but it was so very simplistic that it did not seem worth doing (the test seemed more likely to give a false sense of coverage). I will happily do it anyway, or take other suggestions!

I am also considering what additional documentation might be useful here, if any. I started writing a note about impure functions in the console documentation to warn users about impure functions, but it felt pretty awkward. I'm inclined to skip it for now unless we start getting questions, but I can go back to that if y'all think it's something we should add now.

[codecov]

Codecov Report

Merging #25442 into master will increase coverage by 0.10%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
terraform/evaluate.go	54.36% <100.00%> (+0.44%)	⬆️
dag/marshal.go	54.44% <0.00%> (+1.11%)	⬆️
terraform/node_resource_plan.go	93.44% <0.00%> (+1.63%)	⬆️
terraform/context.go	86.00% <0.00%> (+6.00%)	⬆️
terraform/graph_builder_eval.go	100.00% <0.00%> (+100.00%)	⬆️
terraform/node_provider_eval.go	100.00% <0.00%> (+100.00%)	⬆️

[apparentlymart]

This looks good to me!

The sort of testing I might suggest here is testing of the sort we have in the context_*_test.go files... perhaps we could add a new context_eval_test.go file and have it mimic how terraform console calls ctx.Eval, and then get an evaluation scope out of it the same way as terraform console does and run a table of "this expression in, that result out" tests against it so we can verify that it's able to access the sorts of things we expect console to be able to access: these functions, local values defined in the configuration test fixture, and maybe some other bits and bobs if they are easy to write without blowing up the scope of what you were trying to solve here.

[ghost]

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