fix: lowercase request and response fields should produce an error

go-runtime/compile/schema.go

@@ -10,6 +10,7 @@ import (
 	"reflect"
 	"strings"
 	"sync"
+	"unicode"
 
 	"golang.org/x/tools/go/ast/astutil"
 	"golang.org/x/tools/go/packages"
@@ -169,12 +170,16 @@ func visitFile(pctx *parseContext, node *ast.File) error {
 	return nil
 }
 
-func isType[T types.Type](t types.Type) bool {
-	if _, ok := t.(*types.Named); ok {
-		t = t.Underlying()
+func typeCheck[T types.Type](original types.Object) (*T, bool) {
+	t := original.Type()
+	if _, ok := original.Type().(*types.Named); ok {
+		t = original.Type().Underlying()
 	}
-	_, ok := t.(T)
-	return ok
+
+	if t, ok := t.(T); ok {
+		return &t, true
+	}
+	return nil, false
 }
 
 func checkSignature(sig *types.Signature) (req, resp *types.Var, err error) {
@@ -190,11 +195,21 @@ func checkSignature(sig *types.Signature) (req, resp *types.Var, err error) {
 	if !types.AssertableTo(contextIfaceType(), params.At(0).Type()) {
 		return nil, nil, fmt.Errorf("first parameter must be of type context.Context but is %s", params.At(0).Type())
 	}
+
 	if params.Len() == 2 {
-		if !isType[*types.Struct](params.At(1).Type()) {
-			return nil, nil, fmt.Errorf("second parameter must be a struct but is %s", params.At(1).Type())
+		structParam := results.At(0)
+		if s, ok := typeCheck[*types.Struct](structParam); ok {
+			for i := 0; i < (*s).NumFields(); i++ {
+				fieldName := (*s).Field(i).Name()
+				if len(fieldName) > 0 && unicode.IsLower(rune(fieldName[0])) {
+					return nil, nil, fmt.Errorf("params field %s must be exported by starting with an uppercase letter", fieldName)
+				}
+			}
+		} else {
+			return nil, nil, fmt.Errorf("second parameter must be a struct but is %s", structParam.Type())
 		}
-		req = params.At(1)
+
+		req = structParam
 	}
 
 	if results.Len() > 2 {
@@ -207,8 +222,16 @@ func checkSignature(sig *types.Signature) (req, resp *types.Var, err error) {
 		return nil, nil, fmt.Errorf("must return an error but is %s", results.At(0).Type())
 	}
 	if results.Len() == 2 {
-		if !isType[*types.Struct](results.At(0).Type()) {
-			return nil, nil, fmt.Errorf("first result must be a struct but is %s", results.At(0).Type())
+		structResult := results.At(0)
+		if s, ok := typeCheck[*types.Struct](structResult); ok {
+			for i := 0; i < (*s).NumFields(); i++ {
+				fieldName := (*s).Field(i).Name()
+				if len(fieldName) > 0 && unicode.IsLower(rune(fieldName[0])) {
+					return nil, nil, fmt.Errorf("results field %s must be exported by starting with an uppercase letter", fieldName)
+				}
+			}
+		} else {
+			return nil, nil, fmt.Errorf("first result must be a struct but is %s", structResult.Type())
 		}
 		resp = results.At(0)
 	}