Ensure C# language version is 9.0+

Instead of forcing `preview` as the lang version, we now detect the whole range of valid values for the `LangVersion` property that users might configure, as documented in https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/langversion-compiler-option, and show an error if it's not `preview`, `latest`, `latestMajor` or is less than `9.0`.

Fixes #26

Author: kzu

src/ThisAssembly.Prerequisites/ThisAssembly.Prerequisites.csproj

@@ -10,7 +10,7 @@
     <Description>Ensures that referencing project satisfies the prerequisites for ThisAssembly, namely:
 
 * MSBuild 16.8+: contains the Roslyn support for source generators
-* C# language: it's the only language supported by Roslyn source generators at the moment.
+* C# 9.0 or greater language: it's the only language supported by Roslyn source generators at the moment.
 </Description>
   </PropertyGroup>
 

src/ThisAssembly.Prerequisites/ThisAssembly.Prerequisites.props

@@ -2,8 +2,6 @@
 
   <PropertyGroup>
     <MSBuildShortVersion>$(MSBuildVersion.TrimEnd('0123456789').TrimEnd('.'))</MSBuildShortVersion>
-    <!-- Remove when 16.8/C# 9 is stable -->
-    <LangVersion Condition="'$(MSBuildShortVersion)' == '16.8'">Preview</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>

src/ThisAssembly.Prerequisites/ThisAssembly.Prerequisites.targets

@@ -3,8 +3,13 @@
   <Target Name="EnsureSourceGenerators" BeforeTargets="BeforeCompile;CoreCompile">
     <Error Text="ThisAssembly requires MSBuild 16.8+ or .NET SDK 5.0+."
            Condition="$(MSBuildShortVersion) &lt; '16.8'" />
-    <Error Text="ThisAssembly uses Roslyn source generators, which are only supported in C# at the moment."
-           Condition="'$(Language)' != 'C#'" />
+    <!-- See https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/langversion-compiler-option -->
+    <Error Text="ThisAssembly uses Roslyn source generators, which are only supported in C# 9.0 or greater at the moment."
+           Condition="'$(Language)' != 'C#' OR 
+                ('$(LangVersion)' != 'preview' AND
+                 '$(LangVersion)' != 'latest' AND
+                 '$(LangVersion)' != 'latestMajor' AND
+                 '$(LangVersion)' &lt; '9.0')" />
   </Target>
 
 </Project>