Disable implicit namespace imports

[safern]

In preview7 SDK some namespace imports are defined by default. Most of our projects don't use implicit references as we are low in the stack and producing those references. This can cause issues when building with a preview7 SDK, i.e:

D:\repos\dotnet_runtime_3\artifacts\obj\System.IO.Pipelines\ref\net6.0-Debug\System.IO.Pipelines.ImplicitNamespaceImports.cs(5,29): error CS0234: The type or namespace name 'Linq' does not exist in the namespace 'System' (are you missing an assembly reference?) [D:\repos\dotnet_runtime_3\src\libraries\System.IO.Pipelines\ref\System.IO.Pipelines.csproj]

This was introduced to the SDK last week. dotnet/sdk@8594395

cc: @dotnet/runtime-infrastructure

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[stephentoub]

I'd convinced myself here:
#55828 (comment)
that libraries were ok. Your example proves otherwise. Do you know where my proof breaks down?

[safern]

that libraries were ok. Your example proves otherwise. Do you know where my proof breaks down?

Interesting. Let me look at what's going on that it is not working.

[safern]

Ah I see. In this case I'm hitting it in System.IO.Pipelines, which is an OOB. We only set DisableImplicitFrameworkReferences for inbox assemblies, for OOBs we set DisableImplicitAssemblyReferences. Which is our own custom thing, so that's why the SDK doesn't set the namespace imports to false:

runtime/eng/targetingpacks.targets

Lines 66 to 82 in d38a539

	<!-- DisableImplicitAssemblyReferences support. -->
	<Target Name="RemoveFrameworkReferences"
	BeforeTargets="_HandlePackageFileConflicts"
	AfterTargets="ResolveTargetingPackAssets"
	Condition="'$(DisableImplicitAssemblyReferences)' == 'true' and
	'$(TargetFrameworkIdentifier)' == '.NETCoreApp'">
	<!-- Point MicrosoftNetCoreAppRefPackRefDir to the acquired targeting pack to use it later for AssemblySearchPaths resolution. -->
	<PropertyGroup Condition="'$(UseLocalTargetingRuntimePack)' != 'true'">
	<_NetCoreAppTargetFrameworkIdentifier Condition="$([MSBuild]::VersionLessThan($(TargetFrameworkVersion), '5.0'))">netcoreapp</_NetCoreAppTargetFrameworkIdentifier>
	<_NetCoreAppTargetFrameworkIdentifier Condition="$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '5.0'))">net</_NetCoreAppTargetFrameworkIdentifier>
	<MicrosoftNetCoreAppRefPackRefDir>%(ResolvedFrameworkReference.TargetingPackPath)\ref\$(_NetCoreAppTargetFrameworkIdentifier)$(TargetFrameworkVersion.TrimStart('v'))\</MicrosoftNetCoreAppRefPackRefDir>
	</PropertyGroup>
	<ItemGroup>
	<Reference Remove="@(Reference)"
	Condition="'%(Reference.FrameworkReferenceName)' == '$(LocalFrameworkOverrideName)'" />
	</ItemGroup>
	</Target>

So I believe the right fix is to move this to the location where we set all these settings for libraries and set it to false only for that scenario.

[stephentoub]

Thanks.

[ViktorHofer]

LGTM 👍