Make builder not merge generic extension.

The previous behaviour was that if the user adds a build_extension,
the build would inject the generic build extension to cover all files.
This was useful to simplify the build.yaml file at the cost of
unexpected behaviour.

To avoid potencial unexpected behaviour,
if the user provides custom build_extensions we assume that the patterns
cover all files and therefore we do not merge the generic
build_extension.

Author: LuisDuarte1

FAQ.md

@@ -182,7 +182,9 @@ to the filename.
 To use `build_extensions` you can use `^` on the input string to match on the project root, and `{{}}` to capture the remaining path/filename.
 
 You can also have multiple build_extensions options, but they can't conflict with each other.
-For consistency, the output pattern must always end with `.mocks.dart` and the input pattern must always end with `.dart`
+For consistency, the output pattern must always end with `.mocks.dart` and the input pattern must always end with `.dart`.
+
+If you specify a build extension, you **MUST** ensure that your patterns cover all input files that you want generate mocks from. Failing to do so will lead to the unmatched file from not being generated at all.
 
 ```yaml
 targets:

build.yaml

@@ -10,8 +10,13 @@ targets:
           # to your desired path. The default behaviour is to the .mocks.dart file to be in the same
           # directory as the source .dart file. As seen below this is customizable, but the generated
           # file must always end in `.mocks.dart`.
+          #
+          # If you specify custom build_extensions you MUST ensure that they cover all input files
           build_extensions:
             '^example/build_extensions/{{}}.dart' : 'example/build_extensions/mocks/{{}}.mocks.dart'
+            '^example/example.dart' : 'example/example.mocks.dart'
+            '^example/iss/{{}}.dart' : 'example/iss/{{}}.mocks.dart'
+            '^test/end2end/{{}}.dart' : 'test/end2end/{{}}.mocks.dart'
 
 builders:
   mockBuilder:

lib/src/builder.dart

@@ -63,34 +63,26 @@ import 'package:source_gen/source_gen.dart';
 /// 'foo.mocks.dart' will be created.
 class MockBuilder implements Builder {
   @override
-  final Map<String, List<String>> buildExtensions = {
-    '.dart': ['.mocks.dart']
-  };
+  final Map<String, List<String>> buildExtensions;
 
-  MockBuilder({Map<String, List<String>>? buildExtensions}) {
-    this.buildExtensions.addAll(buildExtensions ?? {});
-  }
+  const MockBuilder(
+      {this.buildExtensions = const {
+        '.dart': ['.mocks.dart']
+      }});
 
   @override
   Future<void> build(BuildStep buildStep) async {
     if (!await buildStep.resolver.isLibrary(buildStep.inputId)) return;
     final entryLib = await buildStep.inputLibrary;
     final sourceLibIsNonNullable = entryLib.isNonNullableByDefault;
 
-    // While it can be acceptable that we get more than 2 allowedOutputs,
-    // because it's the general one and the user defined one. Having
-    // more, means that user has conflicting patterns so we should throw.
-    if (buildStep.allowedOutputs.length > 2) {
+    if (buildStep.allowedOutputs.length > 1) {
       throw ArgumentError('Build_extensions has conflicting outputs on file '
           '`${buildStep.inputId.path}`, it usually caused by missconfiguration '
           'on your `build.yaml` file');
     }
-    // if not single, we always choose the user defined one.
-    final mockLibraryAsset = buildStep.allowedOutputs.singleOrNull ??
-        buildStep.allowedOutputs
-            .where((element) =>
-                element != buildStep.inputId.changeExtension('.mocks.dart'))
-            .single;
+    final mockLibraryAsset = buildStep.allowedOutputs.single;
+
     final inheritanceManager = InheritanceManager3();
     final mockTargetGatherer =
         _MockTargetGatherer(entryLib, inheritanceManager);