Improve error messages when the preparing builder fails

moor_generator/lib/src/analyzer/moor/create_table_reader.dart

@@ -210,10 +210,21 @@ class CreateTableReader {
   Future<UsedTypeConverter> _readTypeConverter(
       ColumnType sqlType, MappedBy mapper) async {
     final code = mapper.mapper.dartCode;
-    final type = await step.task.backend.resolveTypeOf(step.file.uri, code);
 
-    // todo report lint for any of those cases or when resolveTypeOf throws
+    DartType type;
+    try {
+      type = await step.task.backend.resolveTypeOf(step.file.uri, code);
+    } on CannotLoadTypeException catch (e) {
+      step.reportError(ErrorInMoorFile(span: mapper.span, message: e.msg));
+      return null;
+    }
+
     if (type is! InterfaceType) {
+      step.reportError(
+        ErrorInMoorFile(
+            span: mapper.span,
+            message: 'Must be an interface type (backed by a class)'),
+      );
       return null;
     }
 

moor_generator/lib/src/backends/backend.dart

@@ -30,7 +30,7 @@ abstract class BackendTask {
   Future<String> readMoor(Uri uri);
 
   Future<DartType> resolveTypeOf(Uri context, String dartExpression) {
-    throw UnsupportedError('Resolving dart expressions not supported');
+    throw CannotLoadTypeException('Resolving dart expressions not supported');
   }
 
   Future<AstNode> loadElementDeclaration(Element element) async {
@@ -59,3 +59,9 @@ class NotALibraryException implements Exception {
 
   NotALibraryException(this.uri);
 }
+
+class CannotLoadTypeException implements Exception {
+  final String msg;
+
+  CannotLoadTypeException(this.msg);
+}

moor_generator/lib/src/backends/build/build_backend.dart

@@ -76,7 +76,8 @@ class BuildBackendTask extends BackendTask {
         _resolve(context).changeExtension('.dart_in_moor');
 
     if (!await step.canRead(preparedHelperFile)) {
-      throw AssetNotFoundException(preparedHelperFile);
+      throw CannotLoadTypeException('Generated helper file not found. '
+          'Check the build log for earlier errors.');
     }
 
     final content = await step.readAsString(preparedHelperFile);

moor_generator/pubspec.yaml

@@ -41,7 +41,7 @@ dependencies:
 dev_dependencies:
   test: ^1.6.0
   build_runner: ^1.6.7
-  build_test: ^0.10.10
+  build_test: ^1.3.4
   json_serializable: ^4.0.0-nullsafety
 
 executables: