Expose validation errors through the root API

internal/apicheck/main.go

@@ -70,10 +70,91 @@ func checkRepository(ctx context.Context, dir string) error {
 		if violations := checkExports(pkg); len(violations) != 0 {
 			return fmt.Errorf("facade %s has invalid public contracts:\n  %s", path, strings.Join(violations, "\n  "))
 		}
+		if path == modulePath {
+			if violations := checkValidationTypes(pkg); len(violations) != 0 {
+				return fmt.Errorf("facade %s has invalid validation diagnostics:\n  %s", path, strings.Join(violations, "\n  "))
+			}
+		}
 	}
 	return nil
 }
 
+func checkValidationTypes(pkg *types.Package) []string {
+	var violations []string
+	for _, name := range []string{"ValidationCheck", "ValidationFinding", "FileSizeMeasurement", "ValidationError"} {
+		object, ok := pkg.Scope().Lookup(name).(*types.TypeName)
+		if !ok {
+			violations = append(violations, fmt.Sprintf("missing facade-owned type %s", name))
+			continue
+		}
+		named, ok := types.Unalias(object.Type()).(*types.Named)
+		if !ok || named.Obj().Pkg() == nil || named.Obj().Pkg().Path() != pkg.Path() {
+			violations = append(violations, fmt.Sprintf("%s is not a facade-owned named type", name))
+		}
+	}
+	validationError, ok := pkg.Scope().Lookup("ValidationError").(*types.TypeName)
+	if !ok {
+		return violations
+	}
+	named, ok := types.Unalias(validationError.Type()).(*types.Named)
+	if !ok {
+		return violations
+	}
+	structure, ok := named.Underlying().(*types.Struct)
+	if !ok {
+		return append(violations, "ValidationError is not a struct")
+	}
+	fields := map[string]string{
+		"Check":       "ValidationCheck",
+		"Findings":    "[]ValidationFinding",
+		"Measurement": "*FileSizeMeasurement",
+	}
+	for fieldName, expected := range fields {
+		fieldIndex := -1
+		for index := 0; index < structure.NumFields(); index++ {
+			if structure.Field(index).Name() == fieldName && structure.Field(index).Exported() {
+				fieldIndex = index
+				break
+			}
+		}
+		if fieldIndex < 0 || !validationFieldMatches(structure.Field(fieldIndex).Type(), expected, pkg.Path()) {
+			violations = append(violations, fmt.Sprintf("ValidationError.%s must expose %s", fieldName, expected))
+		}
+	}
+	if cause := fieldByName(structure, "Cause"); cause == nil || !cause.Exported() || !types.Identical(cause.Type(), types.Universe.Lookup("error").Type()) {
+		violations = append(violations, "ValidationError.Cause must expose error")
+	}
+	return violations
+}
+
+func fieldByName(structure *types.Struct, name string) *types.Var {
+	for index := 0; index < structure.NumFields(); index++ {
+		if structure.Field(index).Name() == name {
+			return structure.Field(index)
+		}
+	}
+	return nil
+}
+
+func validationFieldMatches(fieldType types.Type, expected, packagePath string) bool {
+	switch expected {
+	case "[]ValidationFinding":
+		slice, ok := types.Unalias(fieldType).(*types.Slice)
+		if !ok {
+			return false
+		}
+		fieldType = slice.Elem()
+	case "*FileSizeMeasurement":
+		pointer, ok := types.Unalias(fieldType).(*types.Pointer)
+		if !ok {
+			return false
+		}
+		fieldType = pointer.Elem()
+	}
+	named, ok := types.Unalias(fieldType).(*types.Named)
+	return ok && named.Obj().Pkg() != nil && named.Obj().Pkg().Path() == packagePath && named.Obj().Name() == strings.TrimLeft(expected, "*[]")
+}
+
 func listPackages(ctx context.Context, dir string) ([]listedPackage, error) {
 	cmd := exec.CommandContext(ctx, "go", "list", "-deps", "-export", "-json", "./...")
 	cmd.Dir = dir