Add runtime patches to eliminate pre-builts

[eerhardt]

Related to dotnet/source-build#2421

[dagood]

• Don't build source generators that target older Roslyn versions

I don't know the context here, but to poke at it: are these source generators that ship in the framework? Would not building these prevent someone who is using source-build with a manually-lowered Roslyn version (acquired from NuGet.org) from building a repo that relies on the source generators? Or is that scenario impossible?

[eerhardt]

are these source generators that ship in the framework?

See dotnet/sdk#20355 for more information on what we are doing here.

In short, we are building our source-generators twice, each targeting different Roslyn versions:

• 3.11
• 4.0

The 4.0 targeted version ships in the framework (we are still building the 4.0 versions in source-build). And they both ship in the NuGet packages.

The reason we want to do this is so we can use new APIs introduced in 4.0, but keep our source-generators working in VS 2019 which uses the 3.x Roslyn version. The 3.x versions are slower and don't support new language features (e.g. file-scoped namespaces).

Would not building these prevent someone who is using source-build with a manually-lowered Roslyn version (acquired from NuGet.org) from building a repo that relies on the source generators? Or is that scenario impossible?

That scenario is impossible to build dotnet/runtime because we are using new language features that only came in Roslyn 4.0. It also would mean that the source generators that ship in the framework couldn't be built, since they need the new Roslyn APIs. Lastly, in order to target net6.0, you need to use the new Roslyn version.

[dagood]

That scenario is impossible to build dotnet/runtime

Maybe I'm not parsing this right, but I'm not talking about building dotnet/runtime with a lowered Roslyn version, I mean any user's project. dotnet new console + net6.0 + lowered Roslyn. Just exploring areas where it sounds like this change could cause a functional gap for source-built SDK users anywhere.

The 4.0 targeted version ships in the framework (we are still building the 4.0 versions in source-build). And they both ship in the NuGet packages.

Because we only ship the source-built framework, not nuget packages, to source-build SDK users, sounds like that means there's no way that SDK users can even tell we didn't build the 3.11 version. I think that means this is fine.

[eerhardt]

I mean any user's project. dotnet new console + net6.0 + lowered Roslyn

That is what my last sentence meant:

Lastly, in order to target net6.0, you need to use the new Roslyn version.

You can't build a dotnet new console template with an older version of Roslyn because the template uses new language features that older Roslyn versions don't support (like global usings).