Support unknown variable evaluation

[notnoop]

Expose a callback to handle unknown variables found in hcl syntax. This is useful in situations where the variables aren't known statically upfront.

When a chain of EvalContexts is used, the deepest child context with set UnknownVariable is used.

[apparentlymart]

Hi @notnoop!

This seems reasonable to me in principle. I'd like to suggest a couple small tweaks to it in the hope of making it fit better with the existing features of this module:

• The word "unknown variable" confused me a little at first because HCL (via cty) already has a special idea of a "unknown value", which is a variable that is explicitly defined as not being known yet. I'd prefer to call this new field UndefinedVariable or MissingVariable to avoid creating two meanings of the word "unknown" in this context.

• In HCL, functions that might fail typically return hcl.Diagnostics instead of error, so that the function can return zero or more structured diagnostic messages rather than an opaque error objects. Since this callback function is called during expression evaluation, I'd prefer its signature to return (cty.Value, hcl.Diagnostics) and then return the diagnostics to the TraverseAbs caller.

  The typical pattern for that is for the function to start by declaring a zero-value Diagnostics and then gradually append new diagnostics to it as it goes:

  var diags Diagnostics
  // ...
  if unknownHandler != nil {
        v, moreDiags := unknownHandler(t)
        diags = append(diags, moreDiags...)
        return v, diags
  }
  //
  diags = diags.Append(&Diagnostic{
      Severity: DiagError,
      Summary:  "Unknown variable",
       // etc
  })
  return cty.DynamicVal, diags
  

  (Note that diags can be non-nil even if there are no errors, because it can potentially contain warnings too.)

[notnoop]

Thanks @apparentlymart for the feedback and sorry for taking long to get back. I have updated the PR to use UndefinedVariable.

I have opted not go through the diags accumulation approach. The function TraverseAbs never returns the accumulation of multiple diagnostics. If UndefinedVariables returns some error diagnostics, we probably should return those as is, without additional "did you mean ..." suggestions, showing multiple, maybe conflicting, error messages for the same variable reference will be confusing to users, I suspect.

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