configs/configupgrade: Upgrade rules for the "terraform" block type

This includes the backend configuration, so we need to load the requested
backend and migrate the included configuration per that schema.

configs/configupgrade/upgrade_body.go

@@ -158,3 +158,27 @@ func schemaDefaultBodyRules(filename string, schema *configschema.Block, an *ana
 
 	return ret
 }
+
+// schemaNoInterpBodyRules constructs standard body content rules for the given
+// schema. Each call is guaranteed to produce a distinct object so that
+// callers can safely mutate the result in order to impose custom rules
+// in addition to or instead of those created by default, for situations
+// where schema-based and predefined items mix in a single body.
+func schemaNoInterpBodyRules(filename string, schema *configschema.Block, an *analysis) bodyContentRules {
+	ret := make(bodyContentRules)
+	if schema == nil {
+		// Shouldn't happen in any real case, but often crops up in tests
+		// where the mock schemas tend to be incomplete.
+		return ret
+	}
+
+	for name, attrS := range schema.Attributes {
+		ret[name] = noInterpAttributeRule(filename, attrS.Type, an)
+	}
+	for name, blockS := range schema.BlockTypes {
+		nestedRules := schemaDefaultBodyRules(filename, &blockS.Block, an)
+		ret[name] = nestedBlockRule(filename, nestedRules, an)
+	}
+
+	return ret
+}

configs/configupgrade/upgrade_native.go

@@ -18,6 +18,7 @@ import (
 	"github.com/zclconf/go-cty/cty"
 
 	"github.com/hashicorp/terraform/addrs"
+	backendinit "github.com/hashicorp/terraform/backend/init"
 	"github.com/hashicorp/terraform/tfdiags"
 )
 
@@ -117,6 +118,19 @@ func (u *Upgrader) upgradeNativeSyntaxFile(filename string, src []byte, an *anal
 			moreDiags := u.upgradeNativeSyntaxProvider(filename, &buf, pType, item, an, adhocComments)
 			diags = diags.Append(moreDiags)
 
+		case "terraform":
+			if len(labels) != 0 {
+				diags = diags.Append(&hcl2.Diagnostic{
+					Severity: hcl2.DiagError,
+					Summary:  fmt.Sprintf("Invalid %s block", blockType),
+					Detail:   fmt.Sprintf("A %s block must not have any labels.", blockType),
+					Subject:  &declRange,
+				})
+				continue
+			}
+			moreDiags := u.upgradeNativeSyntaxTerraformBlock(filename, &buf, item, an, adhocComments)
+			diags = diags.Append(moreDiags)
+
 		case "variable":
 			if len(labels) != 1 {
 				diags = diags.Append(&hcl2.Diagnostic{
@@ -319,6 +333,63 @@ func (u *Upgrader) upgradeNativeSyntaxProvider(filename string, buf *bytes.Buffe
 	return diags
 }
 
+func (u *Upgrader) upgradeNativeSyntaxTerraformBlock(filename string, buf *bytes.Buffer, item *hcl1ast.ObjectItem, an *analysis, adhocComments *commentQueue) tfdiags.Diagnostics {
+	var diags tfdiags.Diagnostics
+
+	body := item.Val.(*hcl1ast.ObjectType)
+
+	rules := bodyContentRules{
+		"required_version": noInterpAttributeRule(filename, cty.String, an),
+		"backend": func(buf *bytes.Buffer, blockAddr string, item *hcl1ast.ObjectItem) tfdiags.Diagnostics {
+			var diags tfdiags.Diagnostics
+
+			declRange := hcl1PosRange(filename, item.Keys[0].Pos())
+			if len(item.Keys) != 2 {
+				diags = diags.Append(&hcl2.Diagnostic{
+					Severity: hcl2.DiagError,
+					Summary:  `Invalid backend block`,
+					Detail:   `A backend block must have one label: the backend type name.`,
+					Subject:  &declRange,
+				})
+				return diags
+			}
+
+			typeName := item.Keys[1].Token.Value().(string)
+			beFn := backendinit.Backend(typeName)
+			if beFn == nil {
+				diags = diags.Append(&hcl2.Diagnostic{
+					Severity: hcl2.DiagError,
+					Summary:  "Unsupported backend type",
+					Detail:   fmt.Sprintf("Terraform does not support a backend type named %q.", typeName),
+					Subject:  &declRange,
+				})
+				return diags
+			}
+			be := beFn()
+			schema := be.ConfigSchema()
+			rules := schemaNoInterpBodyRules(filename, schema, an)
+
+			body := item.Val.(*hcl1ast.ObjectType)
+
+			printComments(buf, item.LeadComment)
+			printBlockOpen(buf, "backend", []string{typeName}, item.LineComment)
+			bodyDiags := u.upgradeBlockBody(filename, fmt.Sprintf("terraform.backend.%s", typeName), buf, body.List.Items, rules, adhocComments)
+			diags = diags.Append(bodyDiags)
+			buf.WriteString("}\n")
+
+			return diags
+		},
+	}
+
+	printComments(buf, item.LeadComment)
+	printBlockOpen(buf, "terraform", nil, item.LineComment)
+	bodyDiags := u.upgradeBlockBody(filename, "terraform", buf, body.List.Items, rules, adhocComments)
+	diags = diags.Append(bodyDiags)
+	buf.WriteString("}\n\n")
+
+	return diags
+}
+
 func (u *Upgrader) upgradeBlockBody(filename string, blockAddr string, buf *bytes.Buffer, args []*hcl1ast.ObjectItem, rules bodyContentRules, adhocComments *commentQueue) tfdiags.Diagnostics {
 	var diags tfdiags.Diagnostics
 

configs/configupgrade/upgrade_test.go

@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"testing"
 
+	backendinit "github.com/hashicorp/terraform/backend/init"
 	"github.com/hashicorp/terraform/configs/configschema"
 	"github.com/hashicorp/terraform/providers"
 	"github.com/hashicorp/terraform/terraform"
@@ -188,3 +189,8 @@ var testProviders = map[string]providers.Factory{
 		return p, nil
 	}),
 }
+
+func init() {
+	// Initialize the backends
+	backendinit.Init(nil)
+}