Fix #1681: Use discard assignment, if possible.

ICSharpCode.Decompiler.Tests/ILPrettyTestRunner.cs

@@ -154,6 +154,12 @@ namespace ICSharpCode.Decompiler.Tests
 			Run();
 		}
 
+		[Test]
+		public void Issue1681()
+		{
+			Run();
+		}
+
 		[Test]
 		public void Issue1454()
 		{

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -284,7 +284,9 @@ namespace ICSharpCode.Decompiler.Tests
 		[Test]
 		public void VariableNamingWithoutSymbols([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
 		{
-			RunForLibrary(cscOptions: cscOptions, decompilerSettings: new DecompilerSettings { UseDebugSymbols = false });
+			var settings = Tester.GetSettings(cscOptions);
+			settings.UseDebugSymbols = false;
+			RunForLibrary(cscOptions: cscOptions, decompilerSettings: settings);
 		}
 
 		[Test]

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/Issue1389.cs

@@ -12,7 +12,7 @@ namespace Issue1389
 
 		private static void UnusedResultOfIsinst()
 		{
-			bool flag = GetObject() is TypeCode;
+			_ = (GetObject() is TypeCode);
 		}
 
 		private static bool BoolResultOfIsinst()

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/Issue1681.cs

@@ -0,0 +1,15 @@
+namespace ICSharpCode.Decompiler.Tests.TestCases.ILPretty
+{
+	internal class BaseClass
+	{
+		public int importsClausePosition;
+	}
+	
+	internal class Issue1681 : BaseClass
+	{
+		public void Test()
+		{
+			_ = importsClausePosition;
+		}
+	}
+}

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/Issue1681.il

@@ -0,0 +1,42 @@
+// Metadata version: v4.0.30319
+.assembly extern mscorlib
+{
+  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )                         // .z\V.4..
+  .ver 4:0:0:0
+}
+.assembly ConsoleApp11
+{
+  .ver 1:0:0:0
+}
+.module ConsoleApp11.exe
+// MVID: {B973FCD6-A9C4-48A9-8291-26DDC248E208}
+.imagebase 0x00400000
+.file alignment 0x00000200
+.stackreserve 0x00100000
+.subsystem 0x0003       // WINDOWS_CUI
+.corflags 0x00020003    //  ILONLY 32BITPREFERRED
+// Image base: 0x000001C4B6C90000
+
+.class private auto ansi beforefieldinit ICSharpCode.Decompiler.Tests.TestCases.ILPretty.BaseClass
+       extends [mscorlib]System.Object
+{
+
+.field public int32 importsClausePosition
+
+}
+
+.class private auto ansi beforefieldinit ICSharpCode.Decompiler.Tests.TestCases.ILPretty.Issue1681
+       extends ICSharpCode.Decompiler.Tests.TestCases.ILPretty.BaseClass
+{
+
+.method public hidebysig instance void  Test() cil managed
+{
+  // Code size 18 (0x12)
+  .maxstack  8
+  ldarg.0
+	ldfld int32 class ICSharpCode.Decompiler.Tests.TestCases.ILPretty.BaseClass::importsClausePosition
+  pop
+	ret
+} // end of method Issue1681::Test
+
+}

ICSharpCode.Decompiler.Tests/TestCases/Pretty/VariableNaming.cs

@@ -10,12 +10,20 @@
 
 		private void Test(string text, C c)
 		{
+#if ROSLYN
+			_ = c.Name;
+#else
 			string name = c.Name;
+#endif
 		}
 
 		private void Test2(string text, C c)
 		{
+#if ROSLYN
+			_ = c.Text;
+#else
 			string text2 = c.Text;
+#endif
 		}
 	}
 }

ICSharpCode.Decompiler.Tests/TestCases/Pretty/VariableNamingWithoutSymbols.cs

@@ -10,12 +10,20 @@
 
 		private void Test(string text, C c)
 		{
+#if ROSLYN
+			_ = c.Name;
+#else
 			string name = c.Name;
+#endif
 		}
 
 		private void Test2(string text, C c)
 		{
+#if ROSLYN
+			_ = c.Text;
+#else
 			string text2 = c.Text;
+#endif
 		}
 	}
 }

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -188,6 +188,11 @@ namespace ICSharpCode.Decompiler.CSharp
 		}
 
 		internal bool HidesVariableWithName(string name)
+		{
+			return HidesVariableWithName(currentFunction, name);
+		}
+
+		internal static bool HidesVariableWithName(ILFunction currentFunction, string name)
 		{
 			return currentFunction.Ancestors.OfType<ILFunction>().Any(HidesVariableOrNestedFunction);
 

ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs

@@ -180,16 +180,23 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 				if (!IsValidInStatementExpression(stmt.Expression)) {
 					// fetch ILFunction
 					var function = stmt.Ancestors.SelectMany(a => a.Annotations.OfType<ILFunction>()).First(f => f.Parent == null);
-					// assign result to dummy variable
-					var type = stmt.Expression.GetResolveResult().Type;
-					var v = function.RegisterVariable(
-						VariableKind.StackSlot,
-						type,
-						AssignVariableNames.GenerateVariableName(function, type, stmt.Expression.Annotations.OfType<ILInstruction>().Where(AssignVariableNames.IsSupportedInstruction).FirstOrDefault())
-					);
-					stmt.Expression = new AssignmentExpression(
-						new IdentifierExpression(v.Name).WithRR(new ILVariableResolveResult(v, v.Type)),
-						stmt.Expression.Detach());
+					// if possible use C# 7.0 discard-assignment
+					if (context.Settings.Discards && !ExpressionBuilder.HidesVariableWithName(function, "_")) {
+						stmt.Expression = new AssignmentExpression(
+							new IdentifierExpression("_"), // no ResolveResult
+							stmt.Expression.Detach());
+					} else {
+						// assign result to dummy variable
+						var type = stmt.Expression.GetResolveResult().Type;
+						var v = function.RegisterVariable(
+							VariableKind.StackSlot,
+							type,
+							AssignVariableNames.GenerateVariableName(function, type, stmt.Expression.Annotations.OfType<ILInstruction>().Where(AssignVariableNames.IsSupportedInstruction).FirstOrDefault())
+						);
+						stmt.Expression = new AssignmentExpression(
+							new IdentifierExpression(v.Name).WithRR(new ILVariableResolveResult(v, v.Type)),
+							stmt.Expression.Detach());
+					}
 				}
 			}
 		}