core: Invalid for_each argument messaging improvements

[apparentlymart]

Our original messaging here was largely just lifted from the equivalent message for unknown values in "count", and it didn't really include any specific advice on how to update a configuration to make for_each valid, instead focusing only on the workaround of using the -target planning option.

It's tough to pack in a fully-actionable suggestion here since unknown values in for_each keys tends to be a gnarly architectural problem rather than a local quirk -- when data flows between modules it can sometimes be unclear whether it'll end up being used in a context which allows unknown values.

I did my best to summarize the advice we've been giving in community forum though, in the hope that more people will be able to address this for themselves without asking for help, until we're one day able to smooth this out better with a mechanism such as "partial apply".

╷
│ Error: Invalid for_each argument
│ 
│   on for-each-unknown.tf line 6, in resource "null_resource" "other":
│    6:   for_each = toset([null_resource.example.id])
│     ├────────────────
│     │ null_resource.example.id is a string, known only after apply
│ 
│ The "for_each" set includes values derived from resource attributes that
│ cannot be determined until apply, and so Terraform cannot determine the full
│ set of keys that will identify the instances of this resource.
│ 
│ When working with unknown values in for_each, it's better to use a map value
│ where the keys are defined statically in your configuration and where only
│ the values contain apply-time results.
│ 
│ Alternatively, you could use the -target planning option to first apply only
│ the resources that the for_each value depends on, and then apply a second
│ time to fully converge.
╵
╷
│ Error: Invalid for_each argument
│ 
│   on for-each-unknown.tf line 10, in resource "null_resource" "other2":
│   10:   for_each = {
│   11:     (null_resource.example.id) = "baz"
│   12:   }
│     ├────────────────
│     │ null_resource.example.id is a string, known only after apply
│ 
│ The "for_each" map includes keys derived from resource attributes that cannot
│ be determined until apply, and so Terraform cannot determine the full set of
│ keys that will identify the instances of this resource.
│ 
│ When working with unknown values in for_each, it's better to define the map
│ keys statically in your configuration and place apply-time results only in
│ the map values.
│ 
│ Alternatively, you could use the -target planning option to first apply only
│ the resources that the for_each value depends on, and then apply a second
│ time to fully converge.
╵

[github-actions]

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