Honor MSBuild diagnostic severity settings for NBGV1001

[Copilot]

C# Doctor reports NBGV1001 despite project-local suppression or downgrade settings because cached version computation runs in a separate project without those settings.

• Severity propagation: Forward NoWarn and MSBuildWarningsAsMessages/AsErrors/NotAsErrors to the cached project, preserving semicolon-delimited lists.
• Suppression: Honor NoWarn when emitting NBGV1001 without changing placeholder values.
• Coverage and documentation: Cover regular and design-time builds in both computation modes, including exact case-insensitive code matching; document the opt-out.

Suppress the warning in a project or Directory.Build.props:

<PropertyGroup>
  <NoWarn>$(NoWarn);NBGV1001</NoWarn>
</PropertyGroup>

Use MSBuildWarningsAsMessages instead to retain the diagnostic as a message.

• Fixes [Question] NBGV1001 opt-out with vscode's C# Doctor #1558

[AArnott]

@copilot

[Copilot]

@copilot

Addressed in 73287a7. Forwarded severity lists now contain only canonical NBGV1001 or empty; tests verify different project lists still compute the version once. Comma/mixed NoWarn lists also work without C# targets.

I kept task-side suppression: Microsoft’s NuGet tasks explicitly filter NoWarn; TaskLoggingHelper has no generic suppression API. MSBuild handles downgrade/promotion.

Release build and cloud tests passed (1,374 passed, 13 skipped); CodeQL found no alerts.

[AArnott]

@copilot

[AArnott]

Where do we honor MSBuildWarningsAsMessages?

[Copilot]

@copilot

MSBuild handles the downgrade when GetBuildVersion calls Log.LogWarning with code NBGV1001; no task-side MSBuildWarningsAsMessages check is needed.

MSBuildTargetCaching.targets lines 15–21 filter and forward that property, and line 36 passes it to PrivateP2PCaching.proj. In-project mode already has the property. PlaceholderWarningHonorsProjectSeverity covers both modes and regular/design-time builds, asserting no warning and a retained BuildMessageEventArgs (lines 71–83).

The screenshot identifies PrivateP2PCaching.proj, which is why forwarding the setting to that project is essential.