fix: generated external modules don't import from metadata refs (#1110)

when we generate an external module, we stub the verb bodies. we
shouldn't add imports which are only dependencies of the verb expression
body because this import may be unused once the body is stubbed, causing
build errors.

backend/schema/visit.go

@@ -11,3 +11,19 @@ func Visit(n Node, visit func(n Node, next func() error) error) error {
 		return nil
 	})
 }
+
+// VisitExcludingMetadataChildren visits all nodes in the schema except the children of metadata nodes.
+// This is used when generating external modules to avoid adding imports only referenced in the bodies of
+// stubbed verbs.
+func VisitExcludingMetadataChildren(n Node, visit func(n Node, next func() error) error) error {
+	return visit(n, func() error {
+		if _, ok := n.(Metadata); !ok {
+			for _, child := range n.schemaChildren() {
+				if err := VisitExcludingMetadataChildren(child, visit); err != nil {
+					return err
+				}
+			}
+		}
+		return nil
+	})
+}

buildengine/build_go_test.go

@@ -126,3 +126,53 @@ func Nothing(context.Context) error {
 		assertGeneratedModule("go/modules/other/external_module.go", expected),
 	})
 }
+
+func TestMetadataImportsExcluded(t *testing.T) {
+	sch := &schema.Schema{
+		Modules: []*schema.Module{
+			schema.Builtins(),
+			{Name: "test", Decls: []schema.Decl{
+				&schema.Data{Name: "Req"},
+				&schema.Data{Name: "Resp"},
+				&schema.Verb{
+					Name:     "call",
+					Request:  &schema.Ref{Name: "Req"},
+					Response: &schema.Ref{Name: "Resp"},
+					Metadata: []schema.Metadata{
+						&schema.MetadataCalls{Calls: []*schema.Ref{{Name: "verb", Module: "other"}}},
+					},
+				},
+			}},
+		},
+	}
+	expected := `// Code generated by FTL. DO NOT EDIT.
+
+//ftl:module test
+package test
+
+import (
+  "context"
+)
+
+var _ = context.Background
+
+type Req struct {
+}
+
+type Resp struct {
+}
+
+//ftl:verb
+func Call(context.Context, Req) (Resp, error) {
+  panic("Verb stubs should not be called directly, instead use github.com/TBD54566975/ftl/runtime-go/ftl.Call()")
+}
+`
+	bctx := buildContext{
+		moduleDir: "testdata/modules/another",
+		buildDir:  "_ftl",
+		sch:       sch,
+	}
+	testBuild(t, bctx, []assertion{
+		assertGeneratedModule("go/modules/test/external_module.go", expected),
+	})
+}

buildengine/build_kotlin.go

@@ -157,7 +157,7 @@ var scaffoldFuncs = scaffolder.FuncMap{
 	},
 	"imports": func(m *schema.Module) []string {
 		imports := sets.NewSet[string]()
-		_ = schema.Visit(m, func(n schema.Node, next func() error) error {
+		_ = schema.VisitExcludingMetadataChildren(m, func(n schema.Node, next func() error) error {
 			switch n.(type) {
 			case *schema.Verb:
 				imports.Append("xyz.block.ftl.Context", "xyz.block.ftl.Ignore", "xyz.block.ftl.Verb")

go-runtime/compile/build.go

@@ -190,7 +190,7 @@ var scaffoldFuncs = scaffolder.FuncMap{
 	},
 	"imports": func(m *schema.Module) map[string]string {
 		imports := map[string]string{}
-		_ = schema.Visit(m, func(n schema.Node, next func() error) error {
+		_ = schema.VisitExcludingMetadataChildren(m, func(n schema.Node, next func() error) error {
 			switch n := n.(type) {
 			case *schema.Ref:
 				if n.Module == "" || n.Module == m.Name {