Disable implicit imports in tools

[kant2002]

Discovered in NativeAOT https://github.com/dotnet/runtimelab/issues/1334

Closes #55855

[jkotas]

We introduced these light weight Linq overloads to fix startup time of crossgen2. Is crossgen2 going to use the heavy weight Linq with this change? What is it going to do to the startup time?

[jkotas]

I am wondering whether we should have it disabled for the whole dotnet/runtime repo. It is very easy to fall into Linq performance trap by accident... cc @stephentoub

[stephentoub]

The SDK will be bringing in these:
https://github.com/dotnet/sdk/blob/55195f3c4ba3f582aa91ec824bd24a7ba3e73528/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.DefaultItems.props#L91-L97
I actually expected these would cause build failures once we started using a new enough SDK in runtime, since many of our assemblies don't or can't (because of layering) reference assemblies that define these namespaces, and my understanding is it effectively acts as if you directly added these using to each file, which would normally fail to compile in such cases. Either way, at a minimum we should reduce the list we automatically import (e.g. removing LINQ as you suggest), but I'd also be fine starting by disabling this entirely for the repo. If we do the latter, I expect in time we'll want to relax that, e.g. it'd be nice to eliminate lots of usings from the tops of most test files. There are other changes coming, e.g. file-scoped namespaces, that will likely cause us to overhaul lots of files in the repo as well.

[stephentoub]

It turns out we already disable it. We set:

runtime/src/libraries/Directory.Build.targets

Lines 54 to 56 in 405da67

	<DisableImplicitFrameworkReferences Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' and
	$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '$(NETCoreAppCurrentVersion)')) and
	('$(IsNETCoreAppRef)' == 'true' or '$(IsNETCoreAppSrc)' == 'true')">true</DisableImplicitFrameworkReferences>

which in turn sets DisableImplicitNamespaceImports:
https://github.com/dotnet/sdk/blob/714082d0e96cef8c3a5f5d04a08d18baaf036f21/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.targets#L42
which in turn sets DisableImplicitNamespaceImports_DotNet:
https://github.com/dotnet/sdk/blob/85943958e5a01d6bbd39a947aec2a4a37b8ba7cf/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.GenerateImplicitNamespaceImports.targets#L22

[kant2002]

But that affects just libraries, correct? And this PR still correct, since it is for tools.
Also curious, does rationale from #55855 (comment) applied to libs?

[stephentoub]

But that affects just libraries, correct? And this PR still correct, since it is for tools.

Yes, that's just libs, and this PR is still valid for tools.

Also curious, does rationale from #55855 (comment) applied to libs?

We're currently not interested in importing namespaces in that manner.

[jkotas]

Thank you!