Add ArrayAssertion<TItem> so GenerateAssertion works with concrete array types

[Copilot]

• Analyze the issue: Assert.That(string[]) returns CollectionAssertion<string> (implementing IAssertionSource<IEnumerable<string>>), but [GenerateAssertion] on string[] generates extension on IAssertionSource<string[]> - type mismatch
• Create ArrayAssertion<TItem> class extending CollectionAssertionBase<TItem[], TItem> (implements IAssertionSource<TItem[]>)
• Update Assert.That(TItem[]) to return ArrayAssertion<TItem> instead of CollectionAssertion<TItem>
• Fix GetSimpleTypeName in source generator to handle array types (string[] → StringArray)
• Add snapshot .verified.txt files for ArrayTargetType test (all 4 frameworks)
• Add integration tests in TUnit.Assertions.Tests that call and verify the generated assertion
• Update PublicAPI snapshots to include ArrayAssertion<TItem> type
• Verify all 1788 assertion tests pass (no regressions)
• Code review completed

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Bug]: GenerateAssertion extension not available when targeting concrete collection</issue_title>
<issue_description>### Description

The following generated assertion is not available because of a type mismatch between the generated extension ContainsMessage(this IAssertionSource<string[]> source, ...) and the actual return type of Assert.That(), which is CollectionAssertion<string>.

public class UnitTest1
{
    [Test]
    public async Task Test1()
    {
        string[] items = new[] { "food", "bard" };
        await Assert.That(items).ContainsMessage("foo", false);
    }
}

public static partial class Assertions
{
    [EditorBrowsable(EditorBrowsableState.Never)]
    [GenerateAssertion(ExpectationMessage = "to contain message '{needle}'")]
    public static bool ContainsMessage(this string[] strings, string needle, bool exact = true)
    {
        return strings.Any(x => exact ? x == needle : x.Contains(needle));
    }
}

Expected Behavior

Generated assertion is available when assertion target is a concrete collection.

Actual Behavior

Assertion not available.

Steps to Reproduce

See above code.

TUnit Version

1.15.0

.NET Version

.NET 10

Operating System

Windows

IDE / Test Runner

dotnet CLI (dotnet test / dotnet run)

Error Output / Stack Trace

Additional Context

No response

IDE-Specific Issue?

• I've confirmed this issue occurs when running via dotnet test or dotnet run, not just in my IDE</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug]: GenerateAssertion extension not available when targeting concrete collection #4825

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[thomhurst]

@claude There should be new snapshots for the new test. And can we add a test that calls and uses the generated assertion?

[claude]

Claude finished @thomhurst's task in 4m 44s —— View job

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Summary

I've added an end-to-end usage test for the ArrayAssertion with [GenerateAssertion] feature. However, I encountered permission limitations that prevent me from completing the full task.

✅ Completed

• Read existing test files to understand structure
• Create comprehensive end-to-end usage test
• Committed the test locally

❌ Blocked - Requires Manual Steps

1. Push Permission Denied: I don't have permission to push to the repository
2. Cannot Run Tests: I need approval to run dotnet test and dotnet build commands
3. Snapshot Files Missing: The snapshot files need to be generated by running the tests

What I Created

New file: TUnit.Assertions.Tests/GeneratedAssertions/ArrayTargetGeneratedAssertionUsageTests.cs

This test file:

• Defines a [GenerateAssertion] method targeting string[]
• Actually uses the generated ContainsMessage assertion (not just checks that it generates)
• Includes three test cases:
  • Exact match (passes)
  • Partial match (passes)
  • Not found (fails with AssertionException)

The file is committed locally but needs to be pushed.

Next Steps (Manual)

From the repository root, run these commands:

# 1. Pull the latest changes (if needed)
git pull origin copilot/fix-generate-assertion-extension

# 2. Navigate to test directory
cd TUnit.Assertions.SourceGenerator.Tests

# 3. Build and run the ArrayTargetType test to generate snapshots
dotnet build
dotnet test --filter "FullyQualifiedName~ArrayTargetType"

# 4. Accept the generated .received.txt files as .verified.txt
for f in MethodAssertionGeneratorTests.ArrayTargetType*.received.txt; do 
    mv "$f" "${f%.received.txt}.verified.txt"
done

# 5. Go back to root and run all tests
cd ..
dotnet test

# 6. Stage and commit the snapshot files
git add TUnit.Assertions.SourceGenerator.Tests/*ArrayTargetType*.verified.txt
git commit -m "Accept snapshot files for ArrayTargetType test"

# 7. If I successfully pushed my changes, skip this. Otherwise:
git add TUnit.Assertions.Tests/GeneratedAssertions/ArrayTargetGeneratedAssertionUsageTests.cs
git commit -m "Add end-to-end usage test for ArrayAssertion with GenerateAssertion"

# 8. Push all changes
git push origin copilot/fix-generate-assertion-extension

Expected Snapshot Files

These files should be generated (one per target framework):

• MethodAssertionGeneratorTests.ArrayTargetType.DotNet8_0.verified.txt
• MethodAssertionGeneratorTests.ArrayTargetType.DotNet9_0.verified.txt
• MethodAssertionGeneratorTests.ArrayTargetType.DotNet10_0.verified.txt
• MethodAssertionGeneratorTests.ArrayTargetType.Net4_7.verified.txt

--- | Branch

[thomhurst]

@copilot There should be new snapshots for the new test. And can we add a test that calls and uses the generated assertion?

[Copilot]

@copilot There should be new snapshots for the new test. And can we add a test that calls and uses the generated assertion?

Added in 38152b5:

• Snapshot .verified.txt files for all 4 framework targets (DotNet8_0, DotNet9_0, DotNet10_0, Net4_7)
• Integration tests in GenerateAssertionArrayTests.cs that define a [GenerateAssertion] method on string[] and call it via Assert.That(items).ContainsMessage(...) — tests exact match, partial match, and failure cases (3 new tests, all passing)
• Also fixed GetSimpleTypeName to handle IArrayTypeSymbol so string[] maps to StringArray in generated class names
• Updated PublicAPI snapshots in 163af5b