configs/configupgrade: detect invalid resource names and print a TODO (#20856)

* configs/configupgrade: detect invalid resource names and print a TODO
message

In terraform 0.11 and prior it was possible to start a resource name
with a number. This is no longer valid, as the resource name would would
be ambiguous with number values in HCL expressions.

Fixes #19919

* Update configs/configupgrade/test-fixtures/valid/invalid-resource-name/want/resource.tf

Co-Authored-By: mildwonkey <mildwonkey@users.noreply.github.com>

configs/configupgrade/test-fixtures/valid/invalid-resource-name/input/resource.tf

@@ -0,0 +1,2 @@
+resource "test_instance" "1_invalid_resource_name" {
+}

configs/configupgrade/test-fixtures/valid/invalid-resource-name/want/resource.tf

@@ -0,0 +1,8 @@
+# TF-UPGRADE-TODO: In Terraform v0.11 and earlier, it was possible to begin a
+# resource name with a number, but it is no longer possible in Terraform v0.12.
+#
+# Rename the resource and run `terraform state mv` to apply the rename in the
+# state. Detailed information on the `state mv` command can be found in the
+# documentation online: https://www.terraform.io/docs/commands/state/mv.html
+resource "test_instance" "1_invalid_resource_name" {
+}

configs/configupgrade/test-fixtures/valid/invalid-resource-name/want/versions.tf

@@ -0,0 +1,3 @@
+terraform {
+  required_version = ">= 0.12"
+}

configs/configupgrade/test-fixtures/valid/module-source/input/main.tf

@@ -1,3 +1,7 @@
+// Test note: the configupgrade tool will ignore this possibly-relative module
+// source because it does not find a local directory "foo". The example where
+// the configupgrade tool makes a recommendation about relative module sources
+// is is in relative-module-source.
 module "foo" {
   source = "foo"
 }

configs/configupgrade/test-fixtures/valid/module-source/want/main.tf

@@ -1,3 +1,7 @@
+// Test note: the configupgrade tool will ignore this possibly-relative module
+// source because it does not find a local directory "foo". The example where
+// the configupgrade tool makes a recommendation about relative module sources
+// is is in relative-module-source.
 module "foo" {
   source = "foo"
 }

configs/configupgrade/upgrade_native.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"log"
+	"regexp"
 	"sort"
 	"strings"
 
@@ -178,6 +179,9 @@ func (u *Upgrader) upgradeNativeSyntaxFile(filename string, src []byte, an *anal
 			}
 
 			printComments(&buf, item.LeadComment)
+			if invalidLabel(labels[0]) {
+				printLabelTodo(&buf, labels[0])
+			}
 			printBlockOpen(&buf, blockType, labels, item.LineComment)
 
 			rules := bodyContentRules{
@@ -335,6 +339,9 @@ func (u *Upgrader) upgradeNativeSyntaxResource(filename string, buf *bytes.Buffe
 	}
 
 	printComments(buf, item.LeadComment)
+	if invalidLabel(labels[1]) {
+		printLabelTodo(buf, labels[1])
+	}
 	printBlockOpen(buf, blockType, labels, item.LineComment)
 	bodyDiags := upgradeBlockBody(filename, addr.String(), buf, body.List.Items, body.Rbrace, rules, adhocComments)
 	diags = diags.Append(bodyDiags)
@@ -767,3 +774,23 @@ func schemaHasSettableArguments(schema *configschema.Block) bool {
 	}
 	return false
 }
+
+func invalidLabel(name string) bool {
+	matched, err := regexp.Match(`[0-9]`, []byte{name[0]})
+	if err == nil {
+		return matched
+	}
+	// This isn't likely, but if there's an error here we'll just ignore it and
+	// move on.
+	return false
+}
+
+func printLabelTodo(buf *bytes.Buffer, label string) {
+	buf.WriteString("# TF-UPGRADE-TODO: In Terraform v0.11 and earlier, it was possible to begin a\n" +
+		"# resource name with a number, but it is no longer possible in Terraform v0.12.\n" +
+		"#\n" +
+		"# Rename the resource and run `terraform state mv` to apply the rename in the\n" +
+		"# state. Detailed information on the `state move` command can be found in the\n" +
+		"# documentation online: https://www.terraform.io/docs/commands/state/mv.html\n",
+	)
+}