Generate valid C# when std::string is only used for variables

Fixes https://github.com/mono/CppSharp/issues/1209.

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

src/Generator/Passes/TrimSpecializationsPass.cs

@@ -21,7 +21,6 @@ namespace CppSharp.Passes
             VisitOptions.VisitNamespaceEvents = false;
             VisitOptions.VisitNamespaceTemplates = false;
             VisitOptions.VisitNamespaceTypedefs = false;
-            VisitOptions.VisitNamespaceVariables = false;
             VisitOptions.VisitTemplateArguments = false;
         }
 
@@ -92,6 +91,21 @@ namespace CppSharp.Passes
             return true;
         }
 
+        public override bool VisitVariableDecl(Variable variable)
+        {
+            if (!base.VisitVariableDecl(variable))
+                return false;
+
+            if (variable.Access == AccessSpecifier.Public)
+            {
+                ASTUtils.CheckTypeForSpecialization(variable.Type,
+                    variable, AddSpecialization, Context.TypeMaps);
+                return true;
+            }
+
+            return true;
+        }
+
         private void AddSpecialization(ClassTemplateSpecialization specialization)
         {
             if (specializations.Contains(specialization))

tests/Encodings/Encodings.Tests.cs

@@ -7,12 +7,14 @@ public class EncodingsTests : GeneratorTestFixture
     [Test]
     public void TestFoo()
     {
-        var foo = new Foo();
         const string georgia = "საქართველო";
-        foo.Unicode = georgia;
-        Assert.That(foo.Unicode, Is.EqualTo(georgia));
+        Foo.Unicode = georgia;
+        Assert.That(Foo.Unicode, Is.EqualTo(georgia));
 
         // TODO: move this, it has nothing to do with Unicode, it's here only not to break the CLI branch
-        Assert.That(foo[0], Is.EqualTo(5));
+        using (var foo = new Foo())
+        {
+            Assert.That(foo[0], Is.EqualTo(5));
+        }
     }
 }

tests/Encodings/Encodings.cpp

@@ -1,5 +1,15 @@
 #include "Encodings.h"
 
+std::string Foo::Unicode = "";
+
+Foo::Foo()
+{
+}
+
+Foo::~Foo()
+{
+}
+
 // TODO: move this, it has nothing to do with Unicode, it's here only not to break the CLI branch
 int Foo::operator[](int i) const
 {
@@ -9,4 +19,4 @@ int Foo::operator[](int i) const
 int Foo::operator[](int i)
 {
     return 5;
-}
+}

tests/Encodings/Encodings.h

@@ -1,9 +1,13 @@
 #include "../Tests.h"
+#include <string>
 
 class DLL_API Foo
 {
 public:
-    const char* Unicode;
+    Foo();
+    ~Foo();
+
+    static std::string Unicode;
 
 	// TODO: VC++ does not support char16
 	// char16 chr16;