veb: add an early startup panic error message, when the result type of route handler methods *is not* veb.Result (fix #25970) (#25973)

Author: bradcypert

vlib/veb/parse.v

@@ -95,3 +95,23 @@ fn parse_form_from_request(request http.Request) !(map[string]string, map[string
 	}
 	return http.parse_form(request.data), map[string][]http.FileData{}
 }
+
+// has_route_attributes checks if a method has attributes that indicate it should be a route handler
+fn has_route_attributes(attrs []string) bool {
+	if attrs.len == 0 {
+		return false
+	}
+	for attr in attrs {
+		attru := attr.to_upper()
+		if attru in ['GET', 'POST', 'PUT', 'PATCH', 'DELETE', 'HEAD', 'OPTIONS'] {
+			return true
+		}
+		if attr.starts_with('/') {
+			return true
+		}
+		if attr.starts_with('host:') {
+			return true
+		}
+	}
+	return false
+}

vlib/veb/tests/invalid_return_type_test.v

@@ -0,0 +1,26 @@
+import veb
+
+pub struct App {}
+
+struct Context {
+	veb.Context
+}
+
+// Test that methods with route attributes but wrong return type are caught
+fn test_invalid_return_type_is_detected() {
+	mut app := &App{}
+
+	// This should fail because get_task has route attributes but returns !veb.Result
+	veb.generate_routes[App, Context](app) or {
+		assert err.msg().contains('invalid return type'), 'Expected error about invalid return type, got: ${err.msg()}'
+		assert err.msg().contains('get_task'), 'Expected error to mention method name, got: ${err.msg()}'
+		return
+	}
+
+	assert false, 'Expected generate_routes to return an error for invalid return type'
+}
+
+@['/test'; get]
+pub fn (app &App) get_task(mut ctx Context) !veb.Result {
+	return ctx.text('Hello')
+}

vlib/veb/veb.v

@@ -53,6 +53,11 @@ fn generate_routes[A, X](app &A) !map[string]Route {
 			}
 
 			routes[method.name] = route
+		} $else {
+			// If we have route attributes, but the wrong return type, return an error
+			if has_route_attributes(method.attrs) {
+				return error('method `${method.name}` has route attributes but invalid return type. Handler methods must return `veb.Result`, not `!veb.Result` or other types')
+			}
 		}
 	}
 	return routes