configs: Reserve various names for future use

We want the forthcoming v0.12.0 release to be the last significant
breaking change to our main configuration constructs for a long time, but
not everything could be implemented in that release.

As a compromise then, we reserve various names we have some intent of
using in a future release so that such future uses will not be a further
breaking change later.

Some of these names are associated with specific short-term plans, while
others are reserved conservatively for possible later work and may be
"un-reserved" in a later release if we don't end up using them. The ones
that we expect to use in the near future were already being handled, so
we'll continue to decode them at the config layer but also produce an
error so that we don't get weird behavior downstream where the
corresponding features don't work yet.

configs/module_call.go

@@ -68,16 +68,40 @@ func decodeModuleBlock(block *hcl.Block, override bool) (*ModuleCall, hcl.Diagno
 
 	if attr, exists := content.Attributes["count"]; exists {
 		mc.Count = attr.Expr
+
+		// We currently parse this, but don't yet do anything with it.
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Reserved argument name in module block",
+			Detail:   fmt.Sprintf("The name %q is reserved for use in a future version of Terraform.", attr.Name),
+			Subject:  &attr.NameRange,
+		})
 	}
 
 	if attr, exists := content.Attributes["for_each"]; exists {
 		mc.ForEach = attr.Expr
+
+		// We currently parse this, but don't yet do anything with it.
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Reserved argument name in module block",
+			Detail:   fmt.Sprintf("The name %q is reserved for use in a future version of Terraform.", attr.Name),
+			Subject:  &attr.NameRange,
+		})
 	}
 
 	if attr, exists := content.Attributes["depends_on"]; exists {
 		deps, depsDiags := decodeDependsOn(attr)
 		diags = append(diags, depsDiags...)
 		mc.DependsOn = append(mc.DependsOn, deps...)
+
+		// We currently parse this, but don't yet do anything with it.
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Reserved argument name in module block",
+			Detail:   fmt.Sprintf("The name %q is reserved for use in a future version of Terraform.", attr.Name),
+			Subject:  &attr.NameRange,
+		})
 	}
 
 	if attr, exists := content.Attributes["providers"]; exists {
@@ -113,6 +137,16 @@ func decodeModuleBlock(block *hcl.Block, override bool) (*ModuleCall, hcl.Diagno
 		}
 	}
 
+	// Reserved block types (all of them)
+	for _, block := range content.Blocks {
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Reserved block type name in module block",
+			Detail:   fmt.Sprintf("The block type name %q is reserved for use by Terraform in a future version.", block.Type),
+			Subject:  &block.TypeRange,
+		})
+	}
+
 	return mc, diags
 }
 
@@ -145,4 +179,10 @@ var moduleBlockSchema = &hcl.BodySchema{
 			Name: "providers",
 		},
 	},
+	Blocks: []hcl.BlockHeaderSchema{
+		// These are all reserved for future use.
+		{Type: "lifecycle"},
+		{Type: "locals"},
+		{Type: "provider", LabelNames: []string{"type"}},
+	},
 }

configs/module_call_test.go

@@ -9,7 +9,7 @@ import (
 )
 
 func TestLoadModuleCall(t *testing.T) {
-	src, err := ioutil.ReadFile("test-fixtures/valid-files/module-calls.tf")
+	src, err := ioutil.ReadFile("test-fixtures/invalid-files/module-calls.tf")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -19,13 +19,11 @@ func TestLoadModuleCall(t *testing.T) {
 	})
 
 	file, diags := parser.LoadConfigFile("module-calls.tf")
-	if len(diags) != 0 {
-		t.Errorf("Wrong number of diagnostics %d; want 0", len(diags))
-		for _, diag := range diags {
-			t.Logf("- %s", diag)
-		}
-		return
-	}
+	assertExactDiagnostics(t, diags, []string{
+		`module-calls.tf:19,3-8: Reserved argument name in module block; The name "count" is reserved for use in a future version of Terraform.`,
+		`module-calls.tf:20,3-11: Reserved argument name in module block; The name "for_each" is reserved for use in a future version of Terraform.`,
+		`module-calls.tf:22,3-13: Reserved argument name in module block; The name "depends_on" is reserved for use in a future version of Terraform.`,
+	})
 
 	gotModules := file.ModuleCalls
 	wantModules := []*ModuleCall{

configs/named_values.go

@@ -68,6 +68,16 @@ func decodeVariableBlock(block *hcl.Block, override bool) (*Variable, hcl.Diagno
 			})
 		}
 	}
+	for _, blockS := range moduleBlockSchema.Blocks {
+		if blockS.Type == v.Name {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Invalid variable name",
+				Detail:   fmt.Sprintf("The variable name %q is reserved due to its special meaning inside module blocks.", blockS.Type),
+				Subject:  &block.LabelRanges[0],
+			})
+		}
+	}
 
 	if attr, exists := content.Attributes["description"]; exists {
 		valDiags := gohcl.DecodeExpression(attr.Expr, nil, &v.Description)

configs/parser_test.go

@@ -85,6 +85,36 @@ func assertDiagnosticSummary(t *testing.T, diags hcl.Diagnostics, want string) b
 	return true
 }
 
+func assertExactDiagnostics(t *testing.T, diags hcl.Diagnostics, want []string) bool {
+	t.Helper()
+
+	gotDiags := map[string]bool{}
+	wantDiags := map[string]bool{}
+
+	for _, diag := range diags {
+		gotDiags[diag.Error()] = true
+	}
+	for _, msg := range want {
+		wantDiags[msg] = true
+	}
+
+	bad := false
+	for got := range gotDiags {
+		if _, exists := wantDiags[got]; !exists {
+			t.Errorf("unexpected diagnostic: %s", got)
+			bad = true
+		}
+	}
+	for want := range wantDiags {
+		if _, exists := gotDiags[want]; !exists {
+			t.Errorf("missing expected diagnostic: %s", want)
+			bad = true
+		}
+	}
+
+	return bad
+}
+
 func assertResultDeepEqual(t *testing.T, got, want interface{}) bool {
 	t.Helper()
 	if !reflect.DeepEqual(got, want) {

configs/provider.go

@@ -56,6 +56,28 @@ func decodeProviderBlock(block *hcl.Block) (*Provider, hcl.Diagnostics) {
 		diags = append(diags, versionDiags...)
 	}
 
+	// Reserved attribute names
+	for _, name := range []string{"count", "depends_on", "for_each", "source"} {
+		if attr, exists := content.Attributes[name]; exists {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Reserved argument name in provider block",
+				Detail:   fmt.Sprintf("The provider argument name %q is reserved for use by Terraform in a future version.", name),
+				Subject:  &attr.NameRange,
+			})
+		}
+	}
+
+	// Reserved block types (all of them)
+	for _, block := range content.Blocks {
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Reserved block type name in provider block",
+			Detail:   fmt.Sprintf("The block type name %q is reserved for use by Terraform in a future version.", block.Type),
+			Subject:  &block.TypeRange,
+		})
+	}
+
 	return provider, diags
 }
 
@@ -107,5 +129,16 @@ var providerBlockSchema = &hcl.BodySchema{
 		{
 			Name: "version",
 		},
+
+		// Attribute names reserved for future expansion.
+		{Name: "count"},
+		{Name: "depends_on"},
+		{Name: "for_each"},
+		{Name: "source"},
+	},
+	Blocks: []hcl.BlockHeaderSchema{
+		// _All_ of these are reserved for future expansion.
+		{Type: "lifecycle"},
+		{Type: "locals"},
 	},
 }

configs/provider_test.go

@@ -0,0 +1,26 @@
+package configs
+
+import (
+	"io/ioutil"
+	"testing"
+)
+
+func TestProviderReservedNames(t *testing.T) {
+	src, err := ioutil.ReadFile("test-fixtures/invalid-files/provider-reserved.tf")
+	if err != nil {
+		t.Fatal(err)
+	}
+	parser := testParser(map[string]string{
+		"config.tf": string(src),
+	})
+	_, diags := parser.LoadConfigFile("config.tf")
+
+	assertExactDiagnostics(t, diags, []string{
+		`config.tf:10,3-8: Reserved argument name in provider block; The provider argument name "count" is reserved for use by Terraform in a future version.`,
+		`config.tf:11,3-13: Reserved argument name in provider block; The provider argument name "depends_on" is reserved for use by Terraform in a future version.`,
+		`config.tf:12,3-11: Reserved argument name in provider block; The provider argument name "for_each" is reserved for use by Terraform in a future version.`,
+		`config.tf:14,3-12: Reserved block type name in provider block; The block type name "lifecycle" is reserved for use by Terraform in a future version.`,
+		`config.tf:15,3-9: Reserved block type name in provider block; The block type name "locals" is reserved for use by Terraform in a future version.`,
+		`config.tf:13,3-9: Reserved argument name in provider block; The provider argument name "source" is reserved for use by Terraform in a future version.`,
+	})
+}

configs/provisioner.go

@@ -93,8 +93,14 @@ func decodeProvisionerBlock(block *hcl.Block) (*Provisioner, hcl.Diagnostics) {
 			}
 
 		default:
-			// Should never happen because there are no other block types
-			// declared in our schema.
+			// Any other block types are ones we've reserved for future use,
+			// so they get a generic message.
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Reserved block type name in provisioner block",
+				Detail:   fmt.Sprintf("The block type name %q is reserved for use by Terraform in a future version.", block.Type),
+				Subject:  &block.TypeRange,
+			})
 		}
 	}
 
@@ -134,16 +140,11 @@ const (
 
 var provisionerBlockSchema = &hcl.BodySchema{
 	Attributes: []hcl.AttributeSchema{
-		{
-			Name: "when",
-		},
-		{
-			Name: "on_failure",
-		},
+		{Name: "when"},
+		{Name: "on_failure"},
 	},
 	Blocks: []hcl.BlockHeaderSchema{
-		{
-			Type: "connection",
-		},
+		{Type: "connection"},
+		{Type: "lifecycle"}, // reserved for future use
 	},
 }

configs/resource.go

@@ -110,7 +110,14 @@ func decodeResourceBlock(block *hcl.Block) (*Resource, hcl.Diagnostics) {
 	}
 
 	if attr, exists := content.Attributes["for_each"]; exists {
-		r.Count = attr.Expr
+		r.ForEach = attr.Expr
+		// We currently parse this, but don't yet do anything with it.
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Reserved argument name in resource block",
+			Detail:   fmt.Sprintf("The name %q is reserved for use in a future version of Terraform.", attr.Name),
+			Subject:  &attr.NameRange,
+		})
 	}
 
 	if attr, exists := content.Attributes["provider"]; exists {
@@ -244,9 +251,14 @@ func decodeResourceBlock(block *hcl.Block) (*Resource, hcl.Diagnostics) {
 			}
 
 		default:
-			// Should never happen, because the above cases should always be
-			// exhaustive for all the types specified in our schema.
-			continue
+			// Any other block types are ones we've reserved for future use,
+			// so they get a generic message.
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Reserved block type name in resource block",
+				Detail:   fmt.Sprintf("The block type name %q is reserved for use by Terraform in a future version.", block.Type),
+				Subject:  &block.TypeRange,
+			})
 		}
 	}
 
@@ -287,7 +299,14 @@ func decodeDataBlock(block *hcl.Block) (*Resource, hcl.Diagnostics) {
 	}
 
 	if attr, exists := content.Attributes["for_each"]; exists {
-		r.Count = attr.Expr
+		r.ForEach = attr.Expr
+		// We currently parse this, but don't yet do anything with it.
+		diags = append(diags, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Reserved argument name in module block",
+			Detail:   fmt.Sprintf("The name %q is reserved for use in a future version of Terraform.", attr.Name),
+			Subject:  &attr.NameRange,
+		})
 	}
 
 	if attr, exists := content.Attributes["provider"]; exists {
@@ -303,17 +322,23 @@ func decodeDataBlock(block *hcl.Block) (*Resource, hcl.Diagnostics) {
 	}
 
 	for _, block := range content.Blocks {
-		// Our schema only allows for "lifecycle" blocks, so we can assume
-		// that this is all we will see here. We don't have any lifecycle
-		// attributes for data resources currently, so we'll just produce
-		// an error.
+		// All of the block types we accept are just reserved for future use, but some get a specialized error message.
+		switch block.Type {
+		case "lifecycle":
 			diags = append(diags, &hcl.Diagnostic{
 				Severity: hcl.DiagError,
 				Summary:  "Unsupported lifecycle block",
 				Detail:   "Data resources do not have lifecycle settings, so a lifecycle block is not allowed.",
 				Subject:  &block.DefRange,
 			})
-		break
+		default:
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Reserved block type name in data block",
+				Detail:   fmt.Sprintf("The block type name %q is reserved for use by Terraform in a future version.", block.Type),
+				Subject:  &block.TypeRange,
+			})
+		}
 	}
 
 	return r, diags
@@ -431,25 +456,18 @@ var commonResourceAttributes = []hcl.AttributeSchema{
 var resourceBlockSchema = &hcl.BodySchema{
 	Attributes: commonResourceAttributes,
 	Blocks: []hcl.BlockHeaderSchema{
-		{
-			Type: "lifecycle",
-		},
-		{
-			Type: "connection",
-		},
-		{
-			Type:       "provisioner",
-			LabelNames: []string{"type"},
-		},
+		{Type: "locals"}, // reserved for future use
+		{Type: "lifecycle"},
+		{Type: "connection"},
+		{Type: "provisioner", LabelNames: []string{"type"}},
 	},
 }
 
 var dataBlockSchema = &hcl.BodySchema{
 	Attributes: commonResourceAttributes,
 	Blocks: []hcl.BlockHeaderSchema{
-		{
-			Type: "lifecycle",
-		},
+		{Type: "lifecycle"}, // reserved for future use
+		{Type: "locals"},    // reserved for future use
 	},
 }
 

configs/test-fixtures/invalid-files/data-reserved-for_each.tf

@@ -0,0 +1,3 @@
+data "test" "foo" {
+  for_each = ["a"]
+}

configs/test-fixtures/invalid-files/data-reserved-lifecycle.tf

@@ -0,0 +1,3 @@
+data "test" "foo" {
+  lifecycle {}
+}

configs/test-fixtures/invalid-files/data-reserved-locals.tf

@@ -0,0 +1,3 @@
+data "test" "foo" {
+  locals {}
+}

configs/test-fixtures/invalid-files/provider-reserved.tf

@@ -0,0 +1,16 @@
+provider "test" {
+  # These are okay
+  alias   = "foo"
+  version = "1.0.0"
+
+  # Provider-specific arguments are also okay
+  arbitrary = true
+
+  # These are all reserved and should generate errors.
+  count      = 3
+  depends_on = ["foo.bar"]
+  for_each   = ["a", "b"]
+  source     = "foo.example.com/baz/bar"
+  lifecycle {}
+  locals {}
+}

configs/test-fixtures/invalid-files/resource-reserved-for_each.tf

@@ -0,0 +1,3 @@
+resource "test" "foo" {
+  for_each = ["a"]
+}

configs/test-fixtures/invalid-files/resource-reserved-locals.tf

@@ -0,0 +1,3 @@
+resource "test" "foo" {
+  locals {}
+}