Replace Nullable and polyfills with Meziantou.Polyfill

.editorconfig

@@ -85,7 +85,7 @@ dotnet_diagnostic.CA1837.severity = suggestion  # CA1837: Use 'Environment.Proce
 dotnet_diagnostic.CA1838.severity = suggestion  # CA1838: Avoid 'StringBuilder' parameters for P/Invokes
 dotnet_diagnostic.CA1845.severity = none        # CA1845: Use span-based 'string.Concat'
 dotnet_diagnostic.CA1846.severity = none        # CA1846: Prefer 'AsSpan' over 'Substring'
-dotnet_diagnostic.CA1847.severity = none        # CA1847: Use char literal for a single character lookup
+dotnet_diagnostic.CA1847.severity = suggestion  # CA1847: Use char literal for a single character lookup
 dotnet_diagnostic.CA1852.severity = suggestion  # CA1852: Seal internal types
 dotnet_diagnostic.CA1854.severity = suggestion  # CA1854: Prefer the 'IDictionary.TryGetValue(TKey, out TValue)' method
 dotnet_diagnostic.CA1859.severity = suggestion  # CA1859: Use concrete types when possible for improved performance

src/core/IronPython.Modules/IronPython.Modules.csproj

@@ -5,6 +5,16 @@
     <BaseAddress>885063680</BaseAddress>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <EmitCompilerGeneratedFiles>True</EmitCompilerGeneratedFiles>
+    <MeziantouPolyfill_IncludedPolyfills>
+      T:System.Diagnostics.CodeAnalysis.AllowNullAttribute;
+      T:System.Diagnostics.CodeAnalysis.DisallowNullAttribute;
+      T:System.Diagnostics.CodeAnalysis.MemberNotNullAttribute;
+      T:System.Diagnostics.CodeAnalysis.NotNullAttribute;
+      T:System.Diagnostics.CodeAnalysis.NotNullWhenAttribute;
+      T:System.Runtime.Versioning.SupportedOSPlatformAttribute;
+      M:System.String.EndsWith(System.Char);
+    </MeziantouPolyfill_IncludedPolyfills>
   </PropertyGroup>
 
   <ItemGroup Condition=" '$(IsFullFramework)' == 'true' ">
@@ -32,6 +42,13 @@
     <PackageReference Include="Microsoft.Win32.Registry" Version="4.7.0" />
   </ItemGroup>
 
+  <ItemGroup>
+    <PackageReference Include="Meziantou.Polyfill" Version="1.0.120">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
+
   <Import Project="$(AfterTargetFiles)" />
   <Target Name="AfterBuildEnds" AfterTargets="AfterBuild" DependsOnTargets="$(AfterTargets)" />
 

src/core/IronPython/IronPython.csproj

@@ -5,6 +5,21 @@
     <BaseAddress>879755264</BaseAddress>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <EmitCompilerGeneratedFiles>True</EmitCompilerGeneratedFiles>
+    <MeziantouPolyfill_IncludedPolyfills>
+      T:System.Diagnostics.CodeAnalysis.AllowNullAttribute;
+      T:System.Diagnostics.CodeAnalysis.DisallowNullAttribute;
+      T:System.Diagnostics.CodeAnalysis.MemberNotNullAttribute;
+      T:System.Diagnostics.CodeAnalysis.MemberNotNullWhenAttribute;
+      T:System.Diagnostics.CodeAnalysis.NotNullAttribute;
+      T:System.Diagnostics.CodeAnalysis.NotNullIfNotNullAttribute;
+      T:System.Diagnostics.CodeAnalysis.NotNullWhenAttribute;
+      T:System.Runtime.Versioning.SupportedOSPlatformAttribute;
+      T:System.Runtime.Versioning.UnsupportedOSPlatformAttribute;
+      M:System.String.StartsWith(System.Char);
+      M:System.String.EndsWith(System.Char);
+      M:System.String.Contains(System.Char);
+    </MeziantouPolyfill_IncludedPolyfills>
   </PropertyGroup>
 
   <ItemGroup>
@@ -50,7 +65,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Nullable" Version="1.3.1">
+    <PackageReference Include="Meziantou.Polyfill" Version="1.0.120">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>

src/core/IronPython/Modules/_fileio.cs

@@ -611,10 +611,5 @@ private void EnsureWritable() {
 
             #endregion
         }
-
-#if !NETCOREAPP
-        private static bool Contains(this string str, char value)
-            => str.IndexOf(value) != -1;
-#endif
     }
 }