be prescriptive about topic names; require lower_snake

backend/controller/pubsub/testdata/go/publisher/publisher.go

@@ -38,12 +38,12 @@ func PublishOne(ctx context.Context) error {
 }
 
 //ftl:export
-var Topic2 = ftl.Topic[PubSubEvent]("topic2")
+var Topic2 = ftl.Topic[PubSubEvent]("topic_2")
 
 //ftl:verb
 func PublishOneToTopic2(ctx context.Context) error {
 	logger := ftl.LoggerFromContext(ctx)
 	t := time.Now()
-	logger.Infof("Publishing to topic2 %v", t)
+	logger.Infof("Publishing to topic_2 %v", t)
 	return Topic2.Publish(ctx, PubSubEvent{Time: t})
 }

go-runtime/compile/testdata/pubsub/pubsub.go

@@ -32,11 +32,11 @@ var Payins = ftl.Topic[PayinEvent]("payins")
 
 var _ = ftl.Subscription(PublicBroadcast, "broadcastSubscription")
 
-// publicBroadcast is a topic that broadcasts payin events to the public.
+// public_broadcast is a topic that broadcasts payin events to the public.
 // out of order with subscription registration to test ordering doesn't matter.
 //
 //ftl:export
-var PublicBroadcast = ftl.Topic[PayinEvent]("publicBroadcast")
+var PublicBroadcast = ftl.Topic[PayinEvent]("public_broadcast")
 
 //ftl:verb export
 func Broadcast(ctx context.Context) error {

go-runtime/schema/subscription/analyzer.go

@@ -3,9 +3,9 @@ package subscription
 import (
 	"go/ast"
 	"go/types"
-	"strings"
 
 	"github.com/TBD54566975/ftl/backend/schema"
+	"github.com/TBD54566975/ftl/backend/schema/strcase"
 	"github.com/TBD54566975/ftl/go-runtime/schema/common"
 	"github.com/TBD54566975/golang-tools/go/analysis"
 	"github.com/TBD54566975/golang-tools/go/analysis/passes/inspect"
@@ -91,10 +91,7 @@ func extractSubscription(pass *analysis.Pass, obj types.Object, node *ast.CallEx
 			common.Errorf(pass, node, "subscription registration must have a topic")
 			return optional.None[*schema.Subscription]()
 		}
-		name := strings.ToLower(string(varName[0]))
-		if len(varName) > 1 {
-			name += varName[1:]
-		}
+		name := strcase.ToLowerSnake(varName)
 		topicRef = &schema.Ref{
 			Module: moduleIdent.Name,
 			Name:   name,

go-runtime/schema/topic/analyzer.go

@@ -62,6 +62,11 @@ func extractTopic(pass *analysis.Pass, node *ast.GenDecl, callExpr *ast.CallExpr
 	}
 
 	topicName := common.ExtractStringLiteralArg(pass, callExpr, 0)
+	expTopicName := strcase.ToLowerSnake(topicName)
+	if topicName != expTopicName {
+		common.Errorf(pass, node, "unsupported topic name %q, did you mean to use %q?", topicName, expTopicName)
+		return optional.None[*schema.Topic]()
+	}
 
 	if len(node.Specs) > 0 {
 		if t, ok := node.Specs[0].(*ast.ValueSpec); ok {

lsp/markdown/completion/pubSubTopic.md

@@ -1,7 +1,7 @@
 Declare a PubSub topic.
 
 ```go
-var invoicesTopic = ftl.Topic[Invoice]("invoices")
+var Invoices = ftl.Topic[Invoice]("invoices")
 ```
 
 See https://tbd54566975.github.io/ftl/docs/reference/pubsub/