fix: suppress nullability warnings after Should NotBeNull assertions

[mvanhorn]

Description

Add a focused suppressor regression using the reported pattern and the existing analyzer-test references for TUnit.Assertions.Should, proving that CS8602 is suppressed only after an awaited TUnit NotBeNull chain. Use that failing test to correct the existing NotBeNull chain recognition or receiver extraction in IsNotNullAssertionSuppressor, retaining semantic-symbol validation so similarly named third-party or user-defined Should() methods cannot suppress compiler diagnostics. Keep the established statement-order and expression-symbol matching behavior shared with Assert.That(...).IsNotNull() rather than introducing a second suppression algorithm.

TUnit's documented Should syntax promises that awaiting value.Should().NotBeNull() suppresses subsequent nullable-flow diagnostics for that value, but the reported nullable-string example still emits CS8602 on property access. The analyzer already has a Should-specific recognition path, while its suppressor test suite exercises only the Assert.That(value).IsNotNull() entry point. This leaves a concrete mismatch between the documented rule and the behavior shipped to Should users. The fix is limited to the nullability suppressor and its analyzer regression coverage; it does not change assertion runtime behavior or broadly mark every Should() receiver as non-null.

Closes#6673

Related Issue

Fixes#6673

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Checklist

Not applicable to this change.

Required

• I have read the Contributing Guidelines
• If this is a new feature, I started a discussion first and received agreement
• My code follows the project's code style (modern C# syntax, proper naming conventions)
• I have written tests that prove my fix is effective or my feature works

TUnit-Specific Requirements

• Dual-Mode Implementation: If this change affects test discovery/execution, I have implemented it in BOTH:
  • Source Generator path (TUnit.Core.SourceGenerator)
    Not run: no test command resolved in this workspace, so nothing was executed to pass.
  • Reflection path (TUnit.Engine)
    Not run: no test command resolved in this workspace, so nothing was executed to pass.
• Snapshot Tests: If I changed source generator output or public APIs:
  • I ran TUnit.Core.SourceGenerator.Tests and/or TUnit.PublicAPI tests
    Not run: no test command resolved in this workspace, so nothing was executed to pass.
  • I reviewed the .received.txt files and accepted them as .verified.txt
    Not run: no test command resolved in this workspace, so nothing was executed to pass.
  • I committed the updated .verified.txt files
    Not run: no test command resolved in this workspace, so nothing was executed to pass.
• Performance: If this change affects hot paths (test discovery, execution, assertions):
  • I minimized allocations and avoided LINQ in hot paths
  • I cached reflection results where appropriate
• AOT Compatibility: If this change uses reflection:
  • I added appropriate [DynamicallyAccessedMembers] annotations
    Not run: no test command resolved in this workspace, so nothing was executed to pass.
  • I verified the change works with dotnet publish -p:PublishAot=true
    Not run: no test command resolved in this workspace, so nothing was executed to pass.

Testing

• All existing tests pass (dotnet test)
  Not run: no test command resolved in this workspace, so nothing was executed to pass.
• I have added tests that cover my changes
• I have tested both source-generated and reflection modes (if applicable)
• A nullable string followed by await value.Should().NotBeNull() and then value.Length has its CS8602 diagnostic reported as suppressed. - The same nullable dereference without a preceding NotBeNull() assertion remains unsuppressed. - A custom or third-party Should().NotBeNull() chain with matching method names remains unsuppressed because its symbols are not TUnit's entry point.

Additional Notes

Nothing beyond what is described above.

Summary by CodeRabbit

• Bug Fixes

  • Improved nullability warning suppression for supported Should().NotBeNull() and Assert.That(...).IsNotNull() assertions.
  • Prevented suppression for unrelated or custom assertion extensions and for Should().BeNull().
  • Added reliable handling for supported assertions within chains, reducing false positives and missed suppressions.

• Tests

  • Added coverage for valid, invalid, unrelated, custom, and chained assertion scenarios across both supported assertion styles.

[coderabbitai]

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📒 Files selected for processing (2)

• src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs
• tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs

🚧 Files skipped from review as they are similar to previous changes (2)

• src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs
• tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs

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📝 Walkthrough

Walkthrough

The analyzer now validates TUnit assertion methods and containing types separately. It supports Assert.That(x).IsNotNull() and x.Should().NotBeNull(), including reduced extension methods and assertion chains. Tests cover valid and invalid suppression cases.

Changes

Nullability suppression

Layer / File(s)	Summary
Assertion symbol validation src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs	The suppressor validates TUnit symbols, rejects unrelated extensions, resolves reduced extension methods, and documents both assertion forms.
Suppression behavior tests tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs	Tests cover valid Should().NotBeNull() suppression, invalid assertions, unrelated extensions, custom extensions, and assertion chains.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk:⚪ Minimal · up to ccd6d

This change restores nullable-warning suppression after valid TUnit Should().NotBeNull() assertions while retaining validation against unrelated methods. No current merge-blocking risk is identified.

Poem

A rabbit checks each symbol,
TUnit marks the valid call,
Custom extensions stay outside,
Assertion chains remain clear,
CS8602 warnings fall.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 6.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 15 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the primary change: suppressing nullability warnings after Should().NotBeNull() assertions.
Linked Issues check	✅ Passed	The implementation addresses issue #6673 by enabling suppression after valid Should().NotBeNull() assertions. The tests also confirm that unrelated methods do not trigger suppression.
Out of Scope Changes check	✅ Passed	The changes remain within scope. Symbol validation, related assertion handling, documentation updates, and regression tests support the nullability suppression fix.

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[greptile-apps]

Greptile Summary

The PR fixes nullability suppression for awaited TUnit Should().NotBeNull() chains while preserving semantic validation of both assertion entry points and terminal methods.

• Recognizes TUnit NotBeNull within direct and fluent assertion chains.
• Rejects similarly named custom or unrelated assertion methods.
• Adds regression coverage for positive and negative suppression cases.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs	Adds semantic validation for TUnit terminal assertion methods and extracts the genuine Should() receiver for expression matching.
tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs	Adds coverage for TUnit Should chains, unrelated/custom methods, absent non-null assertions, and terminal-method validation.

_{Reviews (4): Last reviewed commit: "fix(analyzers): validate IsNotNull symbo..." | Re-trigger Greptile}

[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs`:
- Around line 211-216: Update IsNotNullAssertionSuppressor to validate the
selected invocation against the supported TUnit NotBeNull symbol before
suppressing diagnostics, rather than relying only on GetShouldReceiver’s
Should() validation. Preserve suppression for genuine TUnit NotBeNull calls and
add a regression test covering value.Should().Custom().NotBeNull() so unrelated
fluent methods do not suppress nullability warnings.
In `@tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs`:
- Around line 56-87: Update the affected IsNotNullAssertionSuppressor tests,
including Suppresses_CS8602_After_Should_NotBeNull_Assertion and the additional
cases, to execute explicitly in both source-generator and reflection modes.
Either add separate cases for each mode or configure the test harness to run
each case twice, while preserving the existing diagnostic expectations.

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• src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs
• tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs

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[github-actions]

Code Review

Summary: Fixes a real bug in IsNotNullAssertionSuppressor where nullability-warning suppression never actually worked for the await value.Should().NotBeNull() path (#6673).

Root cause analysis (verified): The old IsTUnitMethod compared symbol.GloballyQualifiedNonGeneric() directly against a fully-qualified method name. For Should(), which is an extension method invoked with instance syntax, the symbol returned by GetSymbolInfo is a reduced extension method symbol — its ContainingType resolves to the receiver type (e.g. string), not the declaring static class (ShouldExtensions). So the string comparison against "global::TUnit.Assertions.Should.ShouldExtensions.Should" could never succeed, meaning the Should-based suppression silently never fired. This matches the reported symptom exactly.

The fix is correct and well-targeted:

• Unreduces the symbol via symbol.ReducedFrom ?? symbol before comparing, then checks Name and ContainingType separately — this correctly recovers the original declaring type for extension methods while leaving the non-extension Assert.That path unaffected (ReducedFrom is null there, so behavior is unchanged).
• Semantic validation is preserved, so a third-party/user-defined Should().NotBeNull() with matching method names still won't suppress — verified by the new Does_Not_Suppress_CS8602_After_Unrelated_Should_NotBeNull_Assertion test, which defines a lookalike OtherLibrary.ShouldExtensions.Should() / Wrapper.NotBeNull() and asserts the warning stays unsuppressed.
• New tests also cover the not-suppressed-without-NotBeNull() case and both orderings of NotBeNull() within a chained assertion (.NotBeNull().And.Contain(...) and .Contain(...).And.NotBeNull()), exercising the existing chain-walking logic (FindInvocationInChain / GetShouldReceiver) against the fix.
• Scope is appropriately narrow: only the suppressor and its test file changed; no assertion runtime behavior, public API, or source-generator output touched, so no snapshot/dual-mode updates are needed here.

I manually traced FindInvocationInChain for both new chain test cases and confirmed it resolves the correct Should() call and receiver expression in each case, so the suppression/non-suppression outcomes in the new tests are consistent with the code, not just coincidentally matching.

No issues found. This is a solid, minimal, well-tested fix for the root cause rather than a workaround.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs`:
- Around line 191-196: Update GetAssertThatArgument to validate the terminal
IsNotNull invocation resolves to
TUnit.Assertions.Extensions.AssertionExtensions.IsNotNull before suppressing
CS8602, while retaining the existing Assert.That validation. Add a regression
test covering a custom IsNotNull extension that must not be suppressed.
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📒 Files selected for processing (2)

• src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs
• tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs

Included review availability: Your plan provides up to 8 included reviews per hour; 5 remain after this review.

[github-actions]

Code Review

Summary: This is a correct, well-targeted fix for #6673. The root cause — IsTUnitMethod comparing a raw ToDisplayString()/GloballyQualifiedNonGeneric() result for a reduced extension-method symbol (x.Should()) against a fully-qualified static-method name — could never match, since Roslyn's display formatting for reduced extension methods substitutes the receiver type in place of the declaring static class. That silently broke Should().NotBeNull() suppression while leaving Assert.That(x).IsNotNull() unaffected (not a reduced-extension call).

What's good:

• IsTUnitMethod now unreduces via symbol.ReducedFrom ?? symbol and compares Name + ContainingType separately instead of one opaque formatted string — this sidesteps the display-format quirk entirely and is a more robust way to identify a specific method going forward.
• The new GetShouldReceiver validates both ends of the chain (the terminal NotBeNull call and the Should() call) against their real declaring types, so a same-named third-party/user-defined Should()/NotBeNull() still won't trigger suppression — verified by the new Does_Not_Suppress_..._Unrelated_... and ..._Custom_... tests.
• Chain-order coverage (NotBeNull().And.Contain(...) vs Contain(...).And.NotBeNull()) exercises the existing FindInvocationInChain walk against the fix rather than just the happy path.
• Scope is appropriately narrow — only the suppressor and its tests changed, consistent with the PR description; no source-gen output or public API touched, so no snapshot updates are needed.

One non-blocking suggestion for a future pass: the fix now hardcodes two source-generator-emitted fully-qualified type names (global::TUnit.Assertions.Should.Extensions.ShouldAssertionExtensions, global::TUnit.Assertions.Should.ShouldExtensions) and still resolves them via ToDisplayString() string comparison on every candidate invocation. Two things worth considering later:

1. Fragility: ShouldAssertionExtensions is a name assembled at generation time ("Should" + containerName in ShouldExtensionGenerator.EmitContainer) from an internal grouping key. If that grouping algorithm ever changes which container NotBeNull lands in, this suppressor silently stops firing again, with only the analyzer test suite as a safety net — the same failure mode this PR just fixed. A more resilient anchor would be a well-known marker (e.g. resolving the type via compilation.GetTypeByMetadataName(...) once, or tagging the generated entry points with an internal attribute) rather than a string literal that has to stay in lockstep with generator internals.
2. Perf: since DiagnosticSuppressors run per-keystroke in the IDE, and WasAssertedNotNull re-scans every prior statement per diagnostic, doing ToDisplayString() (allocates) on every candidate invocation adds up. Resolving the two well-known INamedTypeSymbols once (e.g. via compilation.GetTypeByMetadataName) and comparing with SymbolEqualityComparer.Default would be both cheaper and immune to future display-format surprises like the one this PR just fixed.

Neither point blocks this PR — the existing pattern predates it and the fix is correct as written — but since this PR already touched IsTUnitMethod's contract, it's a reasonable follow-up.

No other issues found. Tests are focused and the negative cases are the ones that actually matter here.

[github-actions]

Code Review

Verdict: Correct, well-targeted fix. I traced the root cause independently and it checks out.

Root cause confirmed:IsTUnitMethod previously compared symbol.GloballyQualifiedNonGeneric() (a formatted display string) of the reduced extension-method symbol for x.Should() against a hardcoded static-method name. Roslyn's display formatting for a reduced extension method substitutes the receiver type in place of the declaring static class, so that comparison could never succeed — meaning Should().NotBeNull() suppression was silently dead code while Assert.That(x).IsNotNull() (not a reduced-extension invocation) worked fine. That matches issue #6673 exactly.

Fix verified against actual generated code, not just plausible-looking string literals:

• global::TUnit.Assertions.Should.ShouldExtensions.Should matches the hand-written Should() overloads in src/TUnit.Assertions.Should/ShouldExtensions.cs.
• global::TUnit.Assertions.Should.Extensions.ShouldAssertionExtensions matches the source-generator's naming scheme ("Should" + containerName, where containerName is the containing type of the source IsNotNull method — i.e. AssertionExtensions) in ShouldExtensionGenerator.EmitContainer. I confirmed this by reading the generator directly rather than trusting the PR description.
• Unreducing via symbol.ReducedFrom ?? symbol before comparing Name/ContainingType sidesteps the display-format quirk entirely — a more robust identification strategy than string-matching a formatted symbol.

Prior CodeRabbit finding is resolved: the earlier review flagged that GetAssertThatArgument didn't validate the terminal IsNotNull invocation's symbol (only the Assert.That call). The final commit (fix(analyzers): validate IsNotNull symbol) added that check — current HEAD now validates both ends of the Assert.That(x).IsNotNull() chain, symmetric with the Should()/NotBeNull() validation. Confirmed by reading the current file state, not just the diff.

Test coverage is appropriately adversarial, not just happy-path: custom IsNotNull/NotBeNull extensions on TUnit's own source types, an unrelated third-party Should()/NotBeNull() with matching names, BeNull() (no suppression expected), and both chain orderings (NotBeNull().And.Contain(...) vs Contain(...).And.NotBeNull()). These are exactly the cases that would silently regress if the symbol validation were loosened again.

Scope is appropriately narrow: only the suppressor and its tests changed; no source-gen output or public API touched, so no snapshot updates were needed.

Non-blocking follow-up (not new — surfaced in the prior automated review, still true): the fix hardcodes two generator-emitted fully-qualified names and re-resolves them via ToDisplayString() on every candidate invocation in ReportSuppressions, which runs per-keystroke in the IDE. Resolving the two well-known INamedTypeSymbols once via compilation.GetTypeByMetadataName(...) and comparing with SymbolEqualityComparer.Default would be cheaper and immune to a future rename of the generator's container-naming scheme silently breaking suppression again (the same failure mode this PR fixes). Worth a follow-up issue, not a blocker here.

No other issues found.

[thomhurst]

Thanks!