Test that Equals only enumerates once (#2463)

src/xunit.v3.assert.tests/Asserts/CollectionAssertsTests.cs

@@ -926,6 +926,14 @@ public static void Equivalence()
 
 			Assert.Equal(expected, actual);
 		}
+
+		[Fact]
+		public static void EnumeratesOnlyOnce()
+		{
+			var expected = new[] { 1, 2, 3, 4, 5 };
+			var actual = new RunOnceEnumerable<int>(expected);
+			Assert.Equal(expected, actual);
+		}
 	}
 
 	public class EqualDictionary
@@ -1351,6 +1359,26 @@ public static void PredicateTooManyMatches()
 		}
 	}
 
+	sealed class RunOnceEnumerable<T> : IEnumerable<T>
+	{
+		private readonly IEnumerable<T> _source;
+		private bool _called;
+
+		public RunOnceEnumerable(IEnumerable<T> source)
+		{
+			_source = source;
+		}
+
+		public IEnumerator<T> GetEnumerator()
+		{
+			Assert.False(_called, "GetEnumerator() was called more than once");
+			_called = true;
+			return _source.GetEnumerator();
+		}
+
+		IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+	}
+
 	sealed class SpyEnumerator<T> : IEnumerable<T>, IEnumerator<T>
 	{
 		IEnumerator<T>? innerEnumerator;