[release/9.0-rc2] Mark the System.Formats.Nrbf assembly as [Experimental] with SYSLIB5005

[github-actions]

Backport of #107905 to release/9.0-rc2

/cc @jeffhandley

Customer Impact

• Customer reported
• Found internally

Our testing of System.Formats.Nrbf has continued through .NET 9 RC1 and RC2, and we're still making changes to it. We expect a lot of customers will use these APIs as part of their BinaryFormatter migrations, which might result in feedback on the API shape after .NET 9 ships. To afford the opportunity to change the API shape based on feedback, we are marking the entire assembly with the [Experimental] attribute. Once we have sufficient customer adoption and feedback on the API, we will remove that attribute and mark the APIs as stable.

Despite having the [Experimental] attribute applied, the package and API will remain fully supported in the .NET 9 lifecycle. We just might change the API shape in .NET 10.

Regression

• Yes
• No

This is a new feature in .NET 9. It shipped without [Experimental] in .NET 9 RC1, so adding the attribute in RC2 will cause new SYSLIB5005 diagnostics to surface where the API is referenced. We expect customers to suppress SYSLIB5005 project-wide as we did in dotnet/wpf#9792 and dotnet/winforms#12156.

Testing

• Build and unit tests passing in dotnet/runtime.
• The locally built package was installed into .NET 9, .NET 6, and .NET Framework 4.8 projects that referenced this API. In each scenario, the SYSLIB5005 diagnostic showed up as expected in VS 17.11.3 and in command-line builds.
• The dotnet/wpf and dotnet/winforms repos were tested with this local build as well, and those builds were clean with the suppression PRs linked above.

Risk

Medium. This is disruptive to customers who have already adopted System.Formats.Nrbf using an earlier build. The total download count is at 1.5K. But for those customers, they will be prompted with the diagnostic and can suppress it.

By merging the dotnet/wpf and dotnet/winforms suppressions first, we aim to avoid any code flow issues from the new diagnostic.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-policy-service]

Tagging subscribers to 'binaryformatter-migration': @adamsitnik, @bartonjs, @jeffhandley, @terrajobst

[carlossanlop]

/ba-g errors known