fix providers source format and add special case for terraform builtin provider

Author: yanndegat

tests/bcr/tf/BUILD.bazel

@@ -6,6 +6,7 @@ tf_providers_versions(
     providers = {
         "random" : "hashicorp/random:3.3.2",
         "null"   : "hashicorp/null:3.1.1",
+        "terraform"   : "terraform.io/builtin/terraform",
     },
 )
 

tests/bcr/tf/my-tflint-config.hcl

@@ -57,5 +57,9 @@ rule "terraform_unused_declarations" {
 }
 
 rule "terraform_unused_required_providers" {
-  enabled = true
+# tf lint support for provider functions is implemented but not released
+# by the time of this patch (tflint v0.9.1). Once released, please enable
+# cf https://github.com/terraform-linters/tflint-ruleset-terraform/pull/214
+# enabled = true
+  enabled = false
 }

tests/bcr/tf/root-modules/root-mod-a/BUILD.bazel

@@ -4,9 +4,11 @@ tf_module(
     name = "root-mod-a",
     providers = [
         "random",
+        "terraform",
     ],
     deps = [
         "//tf/modules/mod-a",
     ],
     providers_versions = "//tf:versions",
+    tflint_config = "//tf:tflint-custom-config",
 )

tests/bcr/tf/root-modules/root-mod-a/main.tf

@@ -7,3 +7,9 @@ module "amodule" {
 
   a_string = random_string.a_random_string.result
 }
+
+module "a2ndmodule" {
+  source = "../../modules/mod-a"
+
+  a_string = provider::terraform::decode_tfvars("a_string = \"hello\"").a_string
+}

tests/bcr/tf/root-modules/root-mod-a/versions.tf.json

@@ -1 +1 @@
-{"terraform":[{"required_providers":[{"random":{"source":"hashicorp/random","version":"3.3.2"}}],"required_version":">= 1.9, <= 1.10"}]}
+{"terraform":[{"required_providers":[{"random":{"source":"hashicorp/random","version":"3.3.2"},"terraform":{"source":"terraform.io/builtin/terraform"}}],"required_version":">= 1.9, <= 1.10"}]}

tf/rules/tf-providers-versions.bzl

@@ -5,13 +5,20 @@ def _impl(ctx):
     providers = {}
     for k in ctx.attr.providers:
         v = ctx.attr.providers[k]
+        if v == "terraform.io/builtin/terraform":
+            # special case of builtin provider
+            providers[k] = {
+                "source": v,
+            }
+            continue
+
         elems = v.split(":")
         if len(elems) != 2:
-            fail("provider version format must be org/provider:version, was: %s".format(v))
+            fail("provider version format must be [hostname/]org/provider:version, was: %s".format(v))
 
         provider_elems = elems[0].split("/")
-        if len(provider_elems) != 2:
-            fail("provider version format must be org/provider:version, was: %s".format(v))
+        if len(provider_elems) < 2 or len(provider_elems) > 3:
+            fail("provider version format must be [hostname/]org/provider:version, was: %s".format(v))
 
         providers[k] = {
             "source": elems[0], "version": elems[1],