From 1263c3b73d0af78b7be0deaf91cc7ab995771006 Mon Sep 17 00:00:00 2001
From: Nikita Tsukanov <keks9n@gmail.com>
Date: Wed, 5 Jun 2019 13:44:36 +0300
Subject: [PATCH] Added support for F# way of passing `this` to static methods

---
 .../XamlCompilerTaskExecutor.Helpers.cs       | 32 +++++++++++++++++++
 .../XamlCompilerTaskExecutor.cs               | 10 +++---
 2 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/src/Avalonia.Build.Tasks/XamlCompilerTaskExecutor.Helpers.cs b/src/Avalonia.Build.Tasks/XamlCompilerTaskExecutor.Helpers.cs
index 05193172be..f94f10f792 100644
--- a/src/Avalonia.Build.Tasks/XamlCompilerTaskExecutor.Helpers.cs
+++ b/src/Avalonia.Build.Tasks/XamlCompilerTaskExecutor.Helpers.cs
@@ -4,6 +4,7 @@ using System.Linq;
 using Avalonia.Utilities;
 using Mono.Cecil;
 using Mono.Cecil.Cil;
+using Mono.Collections.Generic;
 using XamlIl.TypeSystem;
 
 namespace Avalonia.Build.Tasks
@@ -144,6 +145,37 @@ namespace Avalonia.Build.Tasks
             });
 
         }
+        
+
+        private static bool MatchThisCall(Collection<Instruction> instructions, int idx)
+        {
+            var i = instructions[idx];
+            // A "normal" way of passing `this` to a static method:
+            
+            // ldarg.0
+            // call void [Avalonia.Markup.Xaml]Avalonia.Markup.Xaml.AvaloniaXamlLoader::Load(object)
+            
+            if (i.OpCode == OpCodes.Ldarg_0 || (i.OpCode == OpCodes.Ldarg && i.Operand?.Equals(0) == true))
+                return true;
+
+            /* F# way of using `this` in constructor emits a monstrosity like this:
+                IL_01c7: ldarg.0
+                IL_01c8: ldfld class [FSharp.Core]Microsoft.FSharp.Core.FSharpRef`1<class FVim.Cursor> FVim.Cursor::this
+                IL_01cd: call instance !0 class [FSharp.Core]Microsoft.FSharp.Core.FSharpRef`1<class FVim.Cursor>::get_contents()
+                IL_01d2: call !!0 [FSharp.Core]Microsoft.FSharp.Core.LanguagePrimitives/IntrinsicFunctions::CheckThis<class FVim.Cursor>(!!0)
+                IL_01d7: call void [Avalonia.Markup.Xaml]Avalonia.Markup.Xaml.AvaloniaXamlLoader::Load(object)
+                
+                We check for the previous call to be Microsoft.FSharp.Core.LanguagePrimitives/IntrinsicFunctions::CheckThis
+                since it actually returns `this`
+            */
+            if (i.OpCode == OpCodes.Call
+                && i.Operand is GenericInstanceMethod gim
+                && gim.Name == "CheckThis"
+                && gim.DeclaringType.FullName == "Microsoft.FSharp.Core.LanguagePrimitives/IntrinsicFunctions")
+                return true;
+
+            return false;
+        }
     }
  
 }
diff --git a/src/Avalonia.Build.Tasks/XamlCompilerTaskExecutor.cs b/src/Avalonia.Build.Tasks/XamlCompilerTaskExecutor.cs
index df840464c1..c54d8e19a1 100644
--- a/src/Avalonia.Build.Tasks/XamlCompilerTaskExecutor.cs
+++ b/src/Avalonia.Build.Tasks/XamlCompilerTaskExecutor.cs
@@ -234,8 +234,7 @@ namespace Avalonia.Build.Tasks
                                 var i = method.Body.Instructions;
                                 for (var c = 1; c < i.Count; c++)
                                 {
-                                    if (i[c - 1].OpCode == OpCodes.Ldarg_0
-                                        && i[c].OpCode == OpCodes.Call)
+                                    if (i[c].OpCode == OpCodes.Call)
                                     {
                                         var op = i[c].Operand as MethodReference;
                                         
@@ -254,8 +253,11 @@ namespace Avalonia.Build.Tasks
                                             && op.Parameters[0].ParameterType.FullName == "System.Object"
                                             && op.DeclaringType.FullName == "Avalonia.Markup.Xaml.AvaloniaXamlLoader")
                                         {
-                                            i[c].Operand = trampoline;
-                                            foundXamlLoader = true;
+                                            if (MatchThisCall(i, c - 1))
+                                            {
+                                                i[c].Operand = trampoline;
+                                                foundXamlLoader = true;
+                                            }
                                         }
                                     }
                                 }