Use IOperation to detect implicit integer-to-enum conversions

[Copilot]

The analyzer only caught explicit casts to [Closed] enums, missing implicit conversions like Rgb rgb = 0.

Changes

• EnumClosedConversionAnalyzer: Migrated from RegisterSyntaxNodeAction(SyntaxKind.CastExpression) to RegisterOperationAction(OperationKind.Conversion) using IConversionOperation
• ClosedTests: Updated test to verify implicit conversions now trigger errors

Example

[StaticCs.Closed]
enum Rgb { Red, Green, Blue }

class C
{
    void M()
    {
        Rgb rgb1 = (Rgb)10;  // Previously: Error ✓
        Rgb rgb2 = 0;        // Previously: No error ✗  Now: Error ✓
    }
}

The IOperation API provides semantic analysis that captures both explicit and implicit conversions, eliminating the loophole.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• ytvvsblobprodcus310.vsblob.vsassets.io
  • Triggering command: /usr/bin/dotnet dotnet build (dns block)
  • Triggering command: /usr/bin/dotnet dotnet restore --no-dependencies /home/REDACTED/work/StaticCs/StaticCs/static-cs.sln --packages /home/REDACTED/work/StaticCs/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/StaticCs/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/StaticCs/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot (dns block)
  • Triggering command: /usr/bin/dotnet dotnet restore --no-dependencies /home/REDACTED/work/StaticCs/StaticCs/test/AotTest/AotTest.csproj --packages /home/REDACTED/work/StaticCs/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/StaticCs/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/StaticCs/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Consider using IOperation</issue_title>
<issue_description>By only looking for CastExpressionSyntaxes, you're missing the implicit 0 conversion case. Consider using IOperation to avoid missing this case and not having to deal with syntax.</issue_description>

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

@agocke https://github.com/agocke/static-cs/blob/d9ba7fcdfcb51850a8665e1442d8cf1bafb2a655/test/Program.cs#L20-L22

Haven't gotten around to fixing it yet :)</comment_new>

• Fixes Consider using IOperation #3

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