Re-run WriteVerifyAttributes when the values change

The up-to-date check compared only $(MSBuildAllProjects) timestamps against the
generated file, but the content comes from $(SolutionDir), $(SolutionName) and
$(TargetFrameworks). So rebuilding with a different /p:SolutionDir or
/p:SolutionName, which is exactly what the DiscoverSolutionInfo warnings tell
users to pass, or building the same project from another solution, skipped the
target and CoreCompile and left stale metadata in the assembly for AttributeReader
and DerivePaths to read at test time. Only a full Rebuild or touching the csproj
cleared it.

The values are now written to a cache file with WriteOnlyWhenDifferent, and that
file is an input, so its timestamp moves only on a real change. Same pattern as
the SDK's GenerateAssemblyInfo.

Verified by building, then rebuilding with /p:SolutionName=OtherSolution: before
this the emitted Verify.SolutionName stayed at the old value, now it updates.

Author: SimonCropp

src/Verify/buildTransitive/Verify.props

@@ -90,14 +90,21 @@
     </PropertyGroup>
   </Target>
 
-  <Target Name="WriteVerifyAttributes"
+  <!-- The generated values come from properties, not from the files the up-to-date check can
+       see, so they are written to a cache file with WriteOnlyWhenDifferent: its timestamp
+       moves only when a value actually changes, which is what makes WriteVerifyAttributes
+       below re-run. This mirrors the SDK's GenerateAssemblyInfo.
+       Without it, only $(MSBuildAllProjects) timestamps were compared, so rebuilding with a
+       different /p:SolutionDir or /p:SolutionName (exactly what the DiscoverSolutionInfo
+       warnings above tell users to pass), or building the same project from another
+       solution, skipped the target and left stale Verify.SolutionDirectory and
+       Verify.SolutionName metadata in the assembly until a full Rebuild. -->
+  <Target Name="WriteVerifyAttributesCache"
           Condition="$(Language) == 'VB' or $(Language) == 'C#' or $(Language) == 'F#' or $(Language) == 'X#'"
-          DependsOnTargets="DiscoverSolutionInfo"
-          BeforeTargets="BeforeCompile;CoreCompile"
-          Inputs="$(MSBuildAllProjects)"
-          Outputs="$(IntermediateOutputPath)$(VerifyAttributesFile)">
+          DependsOnTargets="DiscoverSolutionInfo">
     <PropertyGroup>
       <VerifyAttributesFilePath>$(IntermediateOutputPath)$(VerifyAttributesFile)</VerifyAttributesFilePath>
+      <VerifyAttributesCacheFile>$(IntermediateOutputPath)Verify.Attributes.cache</VerifyAttributesCacheFile>
       <!-- IntermediateOutputPath is relative to the project for the default layout, but absolute for
            others (eg artifacts output), so combine rather than concatenate. -->
       <VerifyIntermediateDirectory>$([System.IO.Path]::Combine('$(MSBuildProjectDirectory)', '$(IntermediateOutputPath)'))</VerifyIntermediateDirectory>
@@ -132,6 +139,21 @@
       <!-- Ensure not part of Compile, as a workaround for https://github.com/dotnet/sdk/issues/114 -->
       <Compile Remove="$(VerifyAttributesFilePath)" />
     </ItemGroup>
+    <MakeDir Directories="$(IntermediateOutputPath)" />
+    <WriteLinesToFile File="$(VerifyAttributesCacheFile)"
+                      Lines="@(VerifyAttributes->'%(_Parameter1)=%(_Parameter2)')"
+                      Overwrite="true"
+                      WriteOnlyWhenDifferent="true" />
+    <ItemGroup>
+      <FileWrites Include="$(VerifyAttributesCacheFile)" />
+    </ItemGroup>
+  </Target>
+  <Target Name="WriteVerifyAttributes"
+          Condition="$(Language) == 'VB' or $(Language) == 'C#' or $(Language) == 'F#' or $(Language) == 'X#'"
+          DependsOnTargets="WriteVerifyAttributesCache"
+          BeforeTargets="BeforeCompile;CoreCompile"
+          Inputs="$(MSBuildAllProjects);$(IntermediateOutputPath)Verify.Attributes.cache"
+          Outputs="$(IntermediateOutputPath)$(VerifyAttributesFile)">
     <WriteCodeFragment AssemblyAttributes="@(VerifyAttributes)"
                        Language="$(Language)"
                        OutputFile="$(VerifyAttributesFilePath)">