Fix source generator diagnostic locations for incremental caching

[Copilot]

DiagnosticInfo stored Roslyn Location objects but excluded them from equality comparisons. When the incremental generator cache returned cached values, stale locations from earlier compilations were reused, causing Roslyn to throw exceptions.

Changes

• Added LocationInfo record struct — Stores serializable location data (file path, text span, line span) instead of Roslyn Location objects
• Converted DiagnosticInfo to record struct — Uses LocationInfo? instead of Location?; record equality automatically includes all fields
• Reconstruct locations on demand — LocationInfo.ToLocation() creates fresh Location when building diagnostics

// Before: Location excluded from equality, causing stale cache hits
private readonly struct DiagnosticInfo
{
    public Location? Location { get; }
    public bool Equals(DiagnosticInfo other) =>
        Id == other.Id && MethodName == other.MethodName; // Location not compared!
}

// After: Record struct includes all fields in equality
private readonly record struct LocationInfo(string FilePath, TextSpan TextSpan, LinePositionSpan LineSpan);
private readonly record struct DiagnosticInfo(string Id, LocationInfo? Location, string MethodName);

Warning

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Source Generator returns diagnostics with locations outside the current compilation</issue_title>
<issue_description>Originally reported by @AArnott at dotnet/vscode-csharp#8887

Describe the bug
Roslyn is throwing an exception which is being caused by the diagnostics returned from the MCP source generator. The locations being returned for the diagnostics are not from the current Compilation. This is likely caused by not including the location in the equality check leading to returning locations from earlier iterations. See

csharp-sdk/src/ModelContextProtocol.Analyzers/XmlToDescriptionGenerator.cs

Line 533 in 774411b

// For incremental generator caching, we compare only the logical content, not the Location object

To Reproduce
Steps to reproduce the behavior:

1. Open the repo and branch at: https://github.com/coreai-microsoft/vs-mcp/tree/initial-mcp in VS Code
2. Position the caret at one of the many warnings.
3. Try to activate a Fix code action.

Expected behavior
It works.

Logs
The fix fails with the error given above.

C# output window pane

2026-01-14 19:56:42.717 [info] Locating .NET runtime version 10.0.0
2026-01-14 19:56:47.690 [info] Dotnet path: C:\Program Files\dotnet\dotnet.exe
2026-01-14 19:56:47.690 [info] Activating C# + C# Dev Kit...
2026-01-14 19:56:48.232 [info] [stdout] info: Program[0]
      Server started with process ID 223696

2026-01-14 19:56:48.879 [info] [stdout] {"pipeName":"\\\\.\\pipe\\e8d2b360"}

2026-01-14 19:56:48.879 [info] received named pipe information from server
2026-01-14 19:56:48.879 [info] client has connected to server
2026-01-14 19:56:48.940 [info] [Program] Language server initialized
2026-01-14 19:56:49.517 [info] [serviceBroker/connect] [WorkspaceProjectFactoryService] Project E:\src\Microsoft.DevDiv.VisualStudio.Mcp\test\Microsoft.DevDiv.VisualStudio.Mcp.Tests\Microsoft.DevDiv.VisualStudio.Mcp.Tests.csproj loaded by C# Dev Kit
2026-01-14 19:56:49.605 [info] [serviceBroker/connect] [WorkspaceProjectFactoryService] Project E:\src\Microsoft.DevDiv.VisualStudio.Mcp\src\Microsoft.DevDiv.VisualStudio.Mcp\Microsoft.DevDiv.VisualStudio.Mcp.csproj loaded by C# Dev Kit
2026-01-14 19:56:49.763 [info] [textDocument/_vs_getProjectContexts] [Microsoft.CodeAnalysis.MSBuild.BuildHostProcessManager] .NET BuildHost started from c:\Users\andarno\.vscode\extensions\ms-dotnettools.csharp-2.111.2-win32-x64\.roslyn\Microsoft.CodeAnalysis.LanguageServer.exe reloading to start from C:\Program Files\dotnet\dotnet.exe to match necessary SDK location.
2026-01-14 19:56:50.633 [warning] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Project C:\Users\andarno\AppData\Local\Temp\roslyn-canonical-misc\ce7648a5-c814-4034-87b9-bfe3f37db873\Canonical.csproj has unresolved dependencies
2026-01-14 19:56:50.731 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Successfully completed load of C:\Users\andarno\AppData\Local\Temp\roslyn-canonical-misc\ce7648a5-c814-4034-87b9-bfe3f37db873\Canonical.cs
2026-01-14 19:56:50.733 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restoring Canonical.cs: Running dotnet restore on C:\Users\andarno\AppData\Local\Temp\roslyn-canonical-misc\ce7648a5-c814-4034-87b9-bfe3f37db873\Canonical.cs
2026-01-14 19:56:50.735 [info] [textDocument/_vs_getProjectContexts] [Microsoft.CodeAnalysis.LanguageServer.DotnetCliHelper] Using dotnet executable configured on the PATH
2026-01-14 19:56:51.357 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restoring Canonical.cs:   Determining projects to restore...
2026-01-14 19:56:51.858 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restoring Canonical.cs:   Restored C:\Users\andarno\AppData\Local\Temp\roslyn-canonical-misc\ce7648a5-c814-4034-87b9-bfe3f37db873\Canonical.csproj (in 330 ms).
2026-01-14 19:56:51.905 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Restore complete
2026-01-14 19:56:51.906 [info] [textDocument/_vs_getProjectContexts] [LanguageServerProjectLoader] Completed (re)load of all projects in 00:00:02.3532891
2026-01-14 19:56:52.850 [info] [workspace/didChangeWatchedFiles] [Microsoft.CodeAnalysis.MSBuild.BuildHostProcessManager] .NET BuildHost started from c:\Users\andarno\.vscode\extensions\ms-dotnettools.csharp-2.111.2-win32-x64\.roslyn\Microsoft.CodeAnalysis.LanguageServer.exe reloading to start from C:\Program Files\dotnet\dotnet.exe to match necessary SDK location.
2026-01-14 19:56:53.621 [warning] [workspace/didChangeWatchedFiles] [LanguageServerProjectLoader] Project C:\Users\andarno\AppData\Local\Temp\roslyn-canonical-misc\ce7648a5-c814-4034-87b9-bfe3f37db873\Canonical.csproj has unresolved dependencies
2026-01-14 19:56:53.622 [info] [workspace/didChangeWatchedFiles] [LanguageServerProjectLoader] Successfully completed load of C:...

</details>



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- Fixes modelcontextprotocol/csharp-sdk#1152

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