wit/bindgen: create Go packages on-demand

Author: ydnar

wit/bindgen/generator.go

@@ -227,11 +227,6 @@ func (g *generator) define(dir wit.Direction, v wit.Node) (defined bool) {
 func (g *generator) defineWorlds() error {
 	g.opts.logger.Infof("Generating Go for %d world(s)\n", len(g.res.Worlds))
 	for _, w := range g.res.Worlds {
-		// Define a Go package for every world, regardless of use
-		_, err := g.newPackage(w, nil, "")
-		if err != nil {
-			return err
-		}
 		if w == g.world || g.world == nil {
 			err := g.defineWorld(w)
 			if err != nil {
@@ -2289,9 +2284,17 @@ func (g *generator) cgoFileFor(owner wit.TypeOwner) *gen.File {
 
 func (g *generator) packageFor(owner wit.TypeOwner) *gen.Package {
 	pkg := g.witPackages[owner]
+	if pkg != nil {
+		return pkg
+	}
+	var err error
+	switch owner := (owner).(type) {
+	case *wit.World:
+		pkg, err = g.newPackage(owner, nil, "")
+	}
 	if pkg == nil {
-		panic(fmt.Sprintf("BUG: nil package for wit.TypeOwner %s (%T: %p)",
-			owner.WITPackage().Name.String(), owner, owner))
+		panic(fmt.Sprintf("BUG: cannot create Go package for wit.TypeOwner %s (%T: %p): %v",
+			owner.WITPackage().Name.String(), owner, owner, err))
 	}
 	return pkg
 }