perf: CreateExecutableTestFactory creates new lambdas on every property access

[thomhurst]

Summary

The CreateExecutableTestFactory property getter in TestMetadata<T> creates new lambda closures every time it is accessed, causing unnecessary heap allocations and GC pressure.

Evidence

From dotnet-trace profiling of the TUnit.PerformanceBenchmarks project (3700+ tests):

• <get_CreateExecutableTestFactory>b__2 shows 5.41% inclusive CPU time
• <set_InstanceFactory>b__0 shows 2.2% inclusive CPU time

Root Cause

In TUnit.Core/TestMetadata\1.cs` lines 62-116, the property getter creates new lambdas on every access:

public override Func<ExecutableTestCreationContext, TestMetadata, AbstractExecutableTest> CreateExecutableTestFactory
{
    get
    {
        if (InstanceFactory != null && InvokeTypedTest != null)
        {
            return (context, metadata) =>  // NEW LAMBDA EVERY ACCESS!
            {
                var typedMetadata = (TestMetadata<T>)metadata;
                Func<TestContext, Task<object>> createInstance = async testContext => { ... };
                Func<object, object?[], TestContext, CancellationToken, Task> invokeTest = async (...) => { ... };
                return new ExecutableTest(createInstance, invokeTest) { ... };
            };
        }
        throw new InvalidOperationException(...);
    }
}

Suggested Fix

Cache the delegate after first creation:

private Func<ExecutableTestCreationContext, TestMetadata, AbstractExecutableTest>? _cachedExecutableTestFactory;

public override Func<ExecutableTestCreationContext, TestMetadata, AbstractExecutableTest> CreateExecutableTestFactory
{
    get
    {
        if (_cachedExecutableTestFactory != null)
            return _cachedExecutableTestFactory;

        if (InstanceFactory != null && InvokeTypedTest != null)
        {
            _cachedExecutableTestFactory = (context, metadata) => { ... };
            return _cachedExecutableTestFactory;
        }
        throw new InvalidOperationException(...);
    }
}

Impact

• Reduces heap allocations per test by eliminating redundant lambda creation
• Expected improvement: Noticeable reduction in GC pressure for large test suites
• Affects every test execution path

Related Files

• TUnit.Core/TestMetadata\1.cs`