Fix missing static null analysis attribute errors on unsupported tfms.

src/MapTo/AccessModifier.cs

@@ -1,9 +0,0 @@
-namespace MapTo
-{
-    internal enum AccessModifier
-    {
-        Public,
-        Internal,
-        Private
-    }
-}

src/MapTo/Extensions/RoslynExtensions.cs

@@ -116,5 +116,16 @@ namespace MapTo.Extensions
 
         public static SyntaxNode? GetSyntaxNode(this ISymbol symbol) =>
             symbol.Locations.FirstOrDefault() is { } location ? location.SourceTree?.GetRoot().FindNode(location.SourceSpan) : null;
+
+        public static IEnumerable<INamedTypeSymbol> GetTypesByMetadataName(this Compilation compilation, string typeMetadataName)
+        {
+            return compilation.References
+                .Select(compilation.GetAssemblyOrModuleSymbol)
+                .OfType<IAssemblySymbol>()
+                .Select(assemblySymbol => assemblySymbol.GetTypeByMetadataName(typeMetadataName))
+                .Where(t => t != null)!;
+        }
+
+        public static bool TypeByMetadataNameExists(this Compilation compilation, string typeMetadataName) => GetTypesByMetadataName(compilation, typeMetadataName).Any();
     }
 }

src/MapTo/Models.cs

@@ -1,10 +1,25 @@
-using System.Collections.Immutable;
+using System;
+using System.Collections.Immutable;
 using MapTo.Extensions;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 
 namespace MapTo
 {
+    internal enum AccessModifier
+    {
+        Public,
+        Internal,
+        Private
+    }
+
+    internal enum NullStaticAnalysisState
+    {
+        Default,
+        Enabled,
+        Disabled
+    }
+
     internal record SourceCode(string Text, string HintName);
 
     internal record MappedProperty(
@@ -42,29 +57,25 @@ namespace MapTo
         AccessModifier GeneratedMethodsAccessModifier,
         bool GenerateXmlDocument,
         bool SupportNullableReferenceTypes,
-        bool SupportNullableStaticAnalysis,
-        LanguageVersion LanguageVersion)
+        bool SupportNullableStaticAnalysis)
     {
         internal static SourceGenerationOptions From(GeneratorExecutionContext context)
         {
-            var compilation = context.Compilation as CSharpCompilation;
-            var supportNullableReferenceTypes = false;
-            var supportNullableStaticAnalysis = false;
+            const string allowNullAttributeName = "System.Diagnostics.CodeAnalysis.AllowNullAttribute";
+            var supportNullableStaticAnalysis = context.GetBuildGlobalOption(propertyName: nameof(SupportNullableStaticAnalysis), NullStaticAnalysisState.Default);
+            var supportNullableReferenceTypes = context.Compilation.Options.NullableContextOptions is NullableContextOptions.Warnings or NullableContextOptions.Enable;
 
-            if (compilation is not  null)
-            {
-                supportNullableStaticAnalysis = compilation.LanguageVersion >= LanguageVersion.CSharp8;
-                supportNullableReferenceTypes = compilation.Options.NullableContextOptions == NullableContextOptions.Warnings || 
-                                                compilation.Options.NullableContextOptions == NullableContextOptions.Enable;
-            }
-            
             return new(
-                context.GetBuildGlobalOption(nameof(ConstructorAccessModifier), AccessModifier.Public),
-                context.GetBuildGlobalOption(nameof(GeneratedMethodsAccessModifier), AccessModifier.Public),
-                context.GetBuildGlobalOption(nameof(GenerateXmlDocument), true),
-                supportNullableReferenceTypes,
-                supportNullableStaticAnalysis,
-                compilation?.LanguageVersion ?? LanguageVersion.Default
+                ConstructorAccessModifier: context.GetBuildGlobalOption(propertyName: nameof(ConstructorAccessModifier), AccessModifier.Public),
+                GeneratedMethodsAccessModifier: context.GetBuildGlobalOption(propertyName: nameof(GeneratedMethodsAccessModifier), AccessModifier.Public),
+                GenerateXmlDocument: context.GetBuildGlobalOption(propertyName: nameof(GenerateXmlDocument), true),
+                SupportNullableReferenceTypes: supportNullableReferenceTypes,
+                SupportNullableStaticAnalysis: supportNullableStaticAnalysis switch
+                {
+                    NullStaticAnalysisState.Enabled => true,
+                    NullStaticAnalysisState.Disabled => false,
+                    _ => context.Compilation is CSharpCompilation { LanguageVersion: >= LanguageVersion.CSharp8 } cs && cs.TypeByMetadataNameExists(allowNullAttributeName)
+                }
             );
         }
 

test/TestConsoleApp/TestConsoleApp.csproj

@@ -2,9 +2,9 @@
 
     <PropertyGroup>
         <OutputType>Exe</OutputType>
-        <TargetFramework>net5.0</TargetFramework>
+        <TargetFramework>net471</TargetFramework>
         <LangVersion>latest</LangVersion>
-        <Nullable>disable</Nullable>
+        <Nullable>enable</Nullable>
     </PropertyGroup>
 
     <ItemGroup>

test/TestConsoleApp/ViewModels/UserViewModel.cs

@@ -18,7 +18,7 @@ namespace TestConsoleApp.ViewModels
 
         private class IdConverter : ITypeConverter<int, string>
         {
-            public string Convert(int source, object[] converterParameters) => $"{source:X}";
+            public string Convert(int source, object[]? converterParameters) => $"{source:X}";
         }
     }
 }