Add analyzer for OSCondition (MSTEST0059)

[Copilot]

Adds a new code analyzer MSTEST0059 that detects usage of RuntimeInformation.IsOSPlatform or OperatingSystem.Is* methods with early return or Assert.Inconclusive in test methods and suggests using the [OSCondition] attribute instead.

Fixes #7011

Changes Made

• Analyzer: Created UseOSConditionAttributeInsteadOfRuntimeCheckAnalyzer that detects:

  • if (RuntimeInformation.IsOSPlatform(...)) { return; } patterns (suggests ConditionMode.Exclude)
  • if (!RuntimeInformation.IsOSPlatform(...)) { return; } patterns (suggests ConditionMode.Include)
  • if (OperatingSystem.IsWindows()) { return; } and similar patterns
  • Assert.Inconclusive calls within the if blocks
  • Supports Windows, Linux, OSX, FreeBSD, and other platforms
  • Only flags if statements at the very beginning of the method body

• Code Fixer: Created UseOSConditionAttributeInsteadOfRuntimeCheckFixer that:

  • Adds [OSCondition] attribute with the appropriate ConditionMode and OperatingSystems value
  • Removes the if statement from the method body

• Infrastructure:

  • Added diagnostic ID MSTEST0059 in DiagnosticIds.cs
  • Added type names for RuntimeInformation and OperatingSystem in WellKnownTypeNames.cs
  • Added resource strings and localization entries

• Tests: Comprehensive unit tests covering:

  • Negated and non-negated runtime checks
  • Different platforms (Windows, Linux, OSX, FreeBSD)
  • Single statement vs block returns
  • Assert.Inconclusive patterns
  • Edge cases (else branches, non-test methods, other statements)
  • Code fix behavior with trivias (leading/trailing/inline comments)
  • OperatingSystem.IsWindows(), OperatingSystem.IsLinux(), OperatingSystem.IsMacOS() etc.
  • If statements not at the beginning of method body (no diagnostic)
  • Methods with existing [OSCondition] attribute
  • Nested assertions (no diagnostic - only early returns/Assert.Inconclusive should be flagged)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add analyzer for OSCondition</issue_title>
<issue_description>## Summary

With #6994 merged an analyzer can help widespread the use of the new attribute.

</issue_description>

<agent_instructions>Add a new code analyzer that will detect in test methods the usage of early return or assert.inconclusive for calls to RuntimeInformation.IsOSPlatform and suggest instead to use [OSCondition] attribute. We should also handle !RuntimeInformation.... and also provide a codefix for this new analyzer.</agent_instructions>

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

• Fixes Add analyzer for OSCondition #7011

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[Copilot]

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[Evangelink]

@copilot let's add some test cases with trivias to see how the code fix reacts. For example

// Skip on windows
if (RuntimeInformation.IsOSPLatform(OSPlatform.Windows)

or with trivias after the if block

[Copilot]

@copilot let's add some test cases with trivias to see how the code fix reacts. For example

Added test cases with trivias in cbd04fa:

• WhenRuntimeCheckWithLeadingComment_Diagnostic: Comment before the if statement
• WhenRuntimeCheckWithTrailingComment_Diagnostic: Comment after the if block
• WhenRuntimeCheckWithCommentInsideBlock_Diagnostic: Comment inside the if block

[Evangelink]

@copilot we should still handle RuntimeInformation.IsOSPlatform in addition to the other constructs.

Let's also add some tests for a test method that would have some [OSCondition] attribute defined and still uses the RuntimeInformation early exit.

I'd love to also see some test for nesting like:

[TestMethod]
public void MyTest()
{
    if (RuntimeInformation.IsWindows())
    {
        Assert.IsTrue(true);
    }
}

[Copilot]

@copilot we should still handle RuntimeInformation.IsOSPlatform in addition to the other constructs.

Let's also add some tests for a test method that would have some [OSCondition] attribute defin...

Added the requested tests in 35943b5:

1. RuntimeInformation.IsOSPlatform is already handled alongside OperatingSystem.Is* methods
2. WhenMethodAlreadyHasOSConditionAttribute_Diagnostic: Tests that the code fix correctly adds a second [OSCondition] attribute when one already exists
3. WhenRuntimeCheckWithNestedAssertions_NoDiagnostic: Tests nesting with Assert.IsTrue(true) inside the if block - no diagnostic since it's not an early return
4. WhenOperatingSystemCheckWithNestedAssertions_NoDiagnostic: Same test but using OperatingSystem.IsWindows()
5. WhenRuntimeCheckWithNestedMultipleStatements_NoDiagnostic: Tests nesting with multiple statements inside the block - no diagnostic