Surface the IL 'tail.' prefix as an inline comment in C# output

F# emits tail calls pervasively, but the 'tail.' prefix was dropped
entirely at the C# output stage, so the information never reached the
decompiled text. Render it inline as '/*tail.*/' before the call,
mirroring the existing 'constrained.' prefix comment in CallBuilder.

Fix #3817

Assisted-by: Claude:claude-opus-4-8:Claude Code

ICSharpCode.Decompiler.Tests/ILPrettyTestRunner.cs

@@ -135,6 +135,12 @@ namespace ICSharpCode.Decompiler.Tests
 			await Run();
 		}
 
+		[Test]
+		public async Task TailCall()
+		{
+			await Run();
+		}
+
 		[Test]
 		public async Task CS1xSwitch_Debug()
 		{

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/TailCall.cs

@@ -0,0 +1,11 @@
+internal class TailCall
+{
+	public static int Callee(int x)
+	{
+		return x;
+	}
+	public static int Caller(int x)
+	{
+		return /*tail.*/Callee(x);
+	}
+}

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/TailCall.il

@@ -0,0 +1,38 @@
+#define CORE_ASSEMBLY "System.Runtime"
+
+.assembly extern CORE_ASSEMBLY
+{
+  .publickeytoken = (B0 3F 5F 7F 11 D5 0A 3A )                         // .?_....:
+  .ver 4:0:0:0
+}
+
+.assembly TailCall { }
+
+.class private auto ansi beforefieldinit TailCall
+    extends [CORE_ASSEMBLY]System.Object
+{
+    .method public hidebysig static int32 Callee (int32 x) cil managed
+    {
+        .maxstack 8
+        ldarg.0
+        ret
+    }
+
+    .method public hidebysig static int32 Caller (int32 x) cil managed
+    {
+        .maxstack 8
+        ldarg.0
+        tail.
+        call int32 TailCall::Callee(int32)
+        ret
+    }
+
+    .method public hidebysig specialname rtspecialname
+        instance void .ctor () cil managed
+    {
+        .maxstack 8
+        ldarg.0
+        call instance void [CORE_ASSEMBLY]System.Object::.ctor()
+        ret
+    }
+}

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -238,8 +238,15 @@ namespace ICSharpCode.Decompiler.CSharp
 			{
 				return BuildStringConcat(inst.Method, operands).WithILInstruction(inst);
 			}
-			return Build(inst.OpCode, inst.Method, inst.Arguments, constrainedTo: inst.ConstrainedTo)
+			var result = Build(inst.OpCode, inst.Method, inst.Arguments, constrainedTo: inst.ConstrainedTo)
 				.WithILInstruction(inst);
+			if (inst.IsTail)
+			{
+				// Surface the IL 'tail.' prefix as an inline marker, e.g. '/*tail.*/Callee(x)'.
+				// F# emits tail calls pervasively, and the prefix is otherwise dropped entirely.
+				result.Expression.AddLeadingTrivia(new Comment("tail.", CommentType.MultiLine));
+			}
+			return result;
 		}
 
 		private ExpressionWithResolveResult BuildStringConcat(IMethod method, List<(ILInstruction Instruction, KnownTypeCode TypeCode)> operands)