Provide a map file generation to use for remove unused references functionality

[ivanbasov]

This task is a part of dotnet/roslyn#6763

The build team will generate a file with a map of assemblies, packages which can be checked for usage/non-usage. MS Build will generated the file. If other environments are used, they have to provide this file on their own.

[ivanbasov]

@nguerrera and @livarcocc please help with scheduling.

[vatsalyaagrawal]

@rainersigwald please help with scheduling.

[JoeRobich]

A proposal for what the mapping could look like.

<Build ProjectPath="">
  <Projects>
    <Project Path="" IsRuntimeRequired="">
      <References>
        <ProjectReference Path="" />
        <PackageReference Name="" Version="" />
        <SdkReference Name="" />
        <FrameworkReference Name="" Version="" />
        <AssemblyReference AssemblyName="" />
      </References>
    </Project>
  </Projects>
  <Packages>
    <Package Name="" Version="" IsRuntimeRequired="">
      <References>
        <PackageReference Name="" Version="" />
        <AssemblyReference AssemblyName="" />
      </References>
  </Packages>
  <Sdks>
    <Sdk Name="" IsRuntimeRequired="">
      <References>
        <AssemblyReference AssemblyName="" />
      </References>
  </Sdks>
  <Frameworks>
    <Framework Name="" Version="" IsRuntimeRequired="">
      <References>
        <AssemblyReference AssemblyName="" />
      </References>
    </Framework>
  </Frameworks>
  <Assemblies>
    <Assembly AssemblyName="" Path="" IsRuntimeRequired="" />
  </Assemblies>
</Build>

Updated to include IsRuntimeRequired property which is used to indicate a reference is knowingly unused at compile time but necessary at runtime.

[davkean]

When we discussed this last time, this was going to be some metadata on the resulting resolved reference that pointed back to the original item spec + original item type. We would use the same data for when you copied a reference from one project to another or used a code fix to install a reference from another project.

[mjsabby]

Will this file contain assembyrefs that were walked as part of compilation but never ended up in the output assembly? So for example an assembly that only has type forwards?

[marcpopMSFT]

Update: There is an effort to see if the data needed for this can be pulled from the project.assets.json file so a map may not be needed from the SDK. We'll keep an eye on that and update this issue if that ends up being the case.

[stan-sz]

@marcpopMSFT - do you have any update for the effort?

[marcpopMSFT]

Adding @JonDouglas and @aortiz-msft to see if they recall if any effort on the nuget side for this made progress.