Build target for seamless signature file includes

Directory.Build.targets

<Project>
 <PropertyGroup>
   <!-- Provide an escape hatch/opt-in to this feature -->
   <AutoIncludeExistingSignatures Condition="$(AutoIncludeExistingSignatures) == '' " >false</AutoIncludeExistingSignatures>
 </PropertyGroup>
  <Target Name="CollectFSharpSourcesAndSignatures" BeforeTargets="CoreCompile" Condition="$(AutoIncludeExistingSignatures)">
    <ItemGroup>
      <!-- Cache the current set of compile options -->
      <_Sources Include="@(Compile)" />
      <!-- clear out the Compile order because we need to insert the sigs before their associated source files -->
      <Compile Remove="@(Compile)" />
      <!-- N.B: the %(<array name>) syntax is a way of saying 'iterate over the collection and do X for each member of the collection' -->
      <!-- Include X.fsi if X.fsi exists. -->
      <Compile Include="$([System.IO.Path]::ChangeExtension(%(_Sources.Identity), &quot;fsi&quot;))" Condition="$([System.IO.File]::Exists($([System.IO.Path]::ChangeExtension(%(_Sources.Identity), &quot;fsi&quot;))))" />
      <!-- but don't forget to also include X.fs -->
      <Compile Include="%(_Sources.Identity)" />
    </ItemGroup>
  </Target>
</Project>