Move unnecessary_string_interpolations tests (#4545)

test/rules/all.dart

@@ -155,6 +155,8 @@ import 'unnecessary_overrides_test.dart' as unnecessary_overrides;
 import 'unnecessary_parenthesis_test.dart' as unnecessary_parenthesis;
 import 'unnecessary_statements_test.dart' as unnecessary_statements;
 import 'unnecessary_string_escapes_test.dart' as unnecessary_string_escapes;
+import 'unnecessary_string_interpolations_test.dart'
+    as unnecessary_string_interpolations;
 import 'unnecessary_this_test.dart' as unnecessary_this;
 import 'unnecessary_to_list_in_spreads_test.dart'
     as unnecessary_to_list_in_spreads;
@@ -297,6 +299,7 @@ void main() {
   unnecessary_parenthesis.main();
   unnecessary_statements.main();
   unnecessary_string_escapes.main();
+  unnecessary_string_interpolations.main();
   unnecessary_this.main();
   unnecessary_to_list_in_spreads.main();
   unreachable_from_main.main();

test/rules/unnecessary_string_interpolations_test.dart

@@ -0,0 +1,92 @@
+// Copyright (c) 2023, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import '../rule_test_support.dart';
+
+main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(UnnecessaryStringInterpolationsTest);
+  });
+}
+
+@reflectiveTest
+class UnnecessaryStringInterpolationsTest extends LintRuleTest {
+  @override
+  String get lintRule => 'unnecessary_string_interpolations';
+
+  test_necessaryInterpolation_adjascentStrings() async {
+    await assertNoDiagnostics(r'''
+var a = '';
+var b = 'x' '$a';
+''');
+  }
+
+  test_necessaryInterpolation_nullableString() async {
+    await assertNoDiagnostics(r'''
+class Node {
+  String? x = '';
+  String f() => '$x';
+}
+''');
+  }
+
+  test_necessaryInterpolation_property() async {
+    await assertNoDiagnostics(r'''
+var a = '';
+var b = '${a.length}';
+''');
+  }
+
+  test_necessaryInterpolation_single() async {
+    await assertNoDiagnostics(r'''
+var a = '';
+var b = 'x$a';
+''');
+  }
+
+  test_necessaryInterpolation_triple() async {
+    await assertNoDiagnostics(r"""
+var a = '';
+var b = '''x$a''';
+""");
+  }
+
+  test_unnecessaryInterpolation_single() async {
+    await assertDiagnostics(r'''
+var a = '';
+var b = '$a';
+''', [
+      lint(20, 4),
+    ]);
+  }
+
+  test_unnecessaryInterpolation_substring() async {
+    await assertDiagnostics(r'''
+var a = '';
+var b = '${a.substring(1)}';
+''', [
+      lint(20, 19),
+    ]);
+  }
+
+  test_unnecessaryInterpolation_triple() async {
+    await assertDiagnostics(r"""
+var a = '';
+var b = '''$a''';
+""", [
+      lint(20, 8),
+    ]);
+  }
+
+  test_unnecessaryInterpolation_withBraces() async {
+    await assertDiagnostics(r'''
+var a = '';
+var b = '${a}';
+''', [
+      lint(20, 6),
+    ]);
+  }
+}