Add to_hstring overload for IStringable (#1271)

Author: jaigak

cppwinrt/code_writers.h

@@ -3253,6 +3253,7 @@ struct WINRT_IMPL_EMPTY_BASES produce_dispatch_to_overridable<T, D, %>
             w.write(strings::base_reference_produce);
             w.write(strings::base_deferral);
             w.write(strings::base_coroutine_foundation);
+            w.write(strings::base_stringable_to_hstring);
             w.write(strings::base_stringable_format);
             w.write(strings::base_stringable_streams);
         }

cppwinrt/cppwinrt.vcxproj

@@ -81,6 +81,7 @@
     <ClInclude Include="..\strings\base_string.h" />
     <ClInclude Include="..\strings\base_stringable_format.h" />
     <ClInclude Include="..\strings\base_stringable_streams.h" />
+    <ClInclude Include="..\strings\base_stringable_to_hstring.h" />
     <ClInclude Include="..\strings\base_string_input.h" />
     <ClInclude Include="..\strings\base_string_operators.h" />
     <ClInclude Include="..\strings\base_stringable_format_1.h" />

cppwinrt/cppwinrt.vcxproj.filters

@@ -169,7 +169,7 @@
     <ClInclude Include="..\strings\base_stringable_format.h">
       <Filter>strings</Filter>
     </ClInclude>
-     <ClInclude Include="..\strings\base_stringable_streams.h">
+    <ClInclude Include="..\strings\base_stringable_streams.h">
       <Filter>strings</Filter>
     </ClInclude>
     <ClInclude Include="..\strings\base_xaml_component_connector.h">
@@ -184,6 +184,9 @@
     <ClInclude Include="..\strings\base_xaml_component_connector_winui.h">
       <Filter>strings</Filter>
     </ClInclude>
+    <ClInclude Include="..\strings\base_stringable_to_hstring.h">
+      <Filter>strings</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <ResourceCompile Include="$(OutDir)version.rc" />

strings/base_stringable_to_hstring.h

@@ -0,0 +1,8 @@
+
+WINRT_EXPORT namespace winrt
+{
+    inline hstring to_hstring(Windows::Foundation::IStringable const& stringable)
+    {
+        return stringable.ToString();
+    }
+}

test/old_tests/UnitTests/to_hstring.cpp

@@ -4,6 +4,14 @@
 
 using namespace winrt;
 
+struct stringable : winrt::implements<stringable, winrt::Windows::Foundation::IStringable>
+{
+    winrt::hstring ToString()
+    {
+        return L"a stringable object";
+    }
+};
+
 namespace
 {
     void test_cases()
@@ -82,6 +90,10 @@ namespace
             hstring const c = to_hstring(b);
             REQUIRE(a == c);
         }
+        {
+            auto const obj = make<stringable>();
+            REQUIRE(to_hstring(obj) == obj.ToString());
+        }
     }
 }