Commits on Jun 3, 2024

1. lang/funcs: Allow some more expression types in templatestring …

   The templatestring function has some special constraints on its first
   argument that are included to add some intentional friction for those who
   are new to Terraform, want to do some simple template rendering, but have
   only found the templatestring function so far.
   
   We know from previous experience with the hashicorp/template provider that
   this sort of functionality tends to attract those who haven't yet learned
   that the Terraform language has built-in support for string templates
   (without calling any function), who would then get confused by the need
   for an extra level of escaping to render a template string only indirectly
   through this function.
   
   However, this rule is not intended to be onerous and require writing the
   rest of the containing module in an unnatural way to work around it, so
   here we loosen the rule to allow some additional forms:
    - An index expression whose collection operand meets these rules.
    - A relative traversal whose source operand meets these rules.
   
   In particular this makes it possible to write an expression like:
       data.example.example[each.key].result
   ...which is a relative traversal from an index from a scope traversal,
   and is a very reasonable thing to write if you've retrieved multiple
   templates using a data resource that uses for_each.
   
   This also treats splat expressions in the same way as index expressions
   at the static check stage, but that's only to allow us to reach the
   dynamic type check that will ultimately report that a string is required,
   because the result of a splat expression is a tuple. The type-related
   error message is (subjectively) more helpful/relevant than the
   syntax-related one for this case.
   
   Finally, this includes some revisions to the documentation for this
   function to correct some editing errors from the first pass and to slightly
   loosen the language about what's allowed. It's still a little vague about
   what exactly is allowed, but I'm doubtful that a precise definition in
   terms of HCL's expression types would be very enlightening for a typical
   reader anyway. We can tweak the specificity of the language here if we
   start to see repeated questions about what is and is not valid.

   

   apparentlymart committed Jun 3, 2024
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