Add Analyzer to disallow discarding locals e.g. `_ = result;`

fixes 70bd081a9, a2185d9ef

Common.ruleset

@@ -16,6 +16,9 @@
 		<!-- Default branch of switch expression should throw InvalidOperationException/SwitchExpressionException or not throw -->
 		<Rule Id="BHI1005" Action="Error" />
 
+		<!-- Do not discard local variables -->
+		<Rule Id="BHI1006" Action="Error" />
+
 		<!-- Don't call this.GetType() in sealed type, use typeof operator -->
 		<Rule Id="BHI1100" Action="Error" />
 

ExternalProjects/BizHawk.Analyzer/HawkSourceAnalyzer.cs

@@ -38,6 +38,14 @@ public class HawkSourceAnalyzer : DiagnosticAnalyzer
 		defaultSeverity: DiagnosticSeverity.Error,
 		isEnabledByDefault: true);
 
+	private static readonly DiagnosticDescriptor DiagNoDiscardingLocals = new(
+		id: "BHI1006",
+		title: "Do not discard local variables",
+		messageFormat: "RHS is a local, so this discard has no effect, and is at best unhelpful for humans",
+		category: "Usage",
+		defaultSeverity: DiagnosticSeverity.Error,
+		isEnabledByDefault: true);
+
 	private static readonly DiagnosticDescriptor DiagNoQueryExpression = new(
 		id: "BHI1003",
 		title: "Do not use query expression syntax",
@@ -58,11 +66,14 @@ public class HawkSourceAnalyzer : DiagnosticAnalyzer
 		DiagInterpStringIsDollarAt,
 		DiagNoAnonClasses,
 		DiagNoAnonDelegates,
+		DiagNoDiscardingLocals,
 		DiagNoQueryExpression,
 		DiagSwitchShouldThrowIOE);
 
 	public override void Initialize(AnalysisContext context)
 	{
+		static bool IsDiscard(AssignmentExpressionSyntax aes)
+			=> aes.OperatorToken.RawKind is (int) SyntaxKind.EqualsToken && aes.Left is IdentifierNameSyntax { Identifier.Text: "_" };
 		context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
 		context.EnableConcurrentExecution();
 		INamedTypeSymbol? invalidOperationExceptionSym = null;
@@ -83,6 +94,9 @@ public class HawkSourceAnalyzer : DiagnosticAnalyzer
 					case AnonymousObjectCreationExpressionSyntax:
 						snac.ReportDiagnostic(Diagnostic.Create(DiagNoAnonClasses, snac.Node.GetLocation()));
 						break;
+					case AssignmentExpressionSyntax aes when IsDiscard(aes) && snac.SemanticModel.GetSymbolInfo(aes.Right).Symbol?.Kind is SymbolKind.Local:
+						snac.ReportDiagnostic(Diagnostic.Create(DiagNoDiscardingLocals, snac.Node.GetLocation()));
+						break;
 					case InterpolatedStringExpressionSyntax ises:
 						if (ises.StringStartToken.Text[0] is '@') snac.ReportDiagnostic(Diagnostic.Create(DiagInterpStringIsDollarAt, ises.GetLocation()));
 						break;
@@ -113,6 +127,7 @@ public class HawkSourceAnalyzer : DiagnosticAnalyzer
 			SyntaxKind.AnonymousMethodExpression,
 			SyntaxKind.InterpolatedStringExpression,
 			SyntaxKind.QueryExpression,
+			SyntaxKind.SimpleAssignmentExpression,
 			SyntaxKind.SwitchExpressionArm);
 	}
 }

ExternalProjects/BizHawk.Analyzer/test_with_helloworld.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+set -e
+cd "$(dirname "$0")"
+./build_debug.sh
+cd ../../ExternalToolProjects/HelloWorld
+rm -fr bin obj
+./build_debug.sh

src/BizHawk.Client.Common/RomLoader.cs

@@ -796,7 +796,7 @@ namespace BizHawk.Client.Common
 					return false;
 				}
 
-				_ = game!; // shouldn't be null if `nextEmulator` isn't? just in case
+				if (game is null) throw new Exception("RomLoader returned core but no GameInfo"); // shouldn't be null if `nextEmulator` isn't? just in case
 			}
 			catch (Exception ex)
 			{

src/BizHawk.Client.EmuHawk/Api/ApiManager.cs

@@ -63,9 +63,7 @@ namespace BizHawk.Client.EmuHawk
 					tuple =>
 					{
 						var instance = tuple.Ctor.Invoke(tuple.CtorTypes.Select(t => avail[t]).ToArray());
-						var success = ServiceInjector.UpdateServices(serviceProvider, instance, mayCache: true);
-						Debug.Assert(success);
-						_ = success;
+						if (!ServiceInjector.UpdateServices(serviceProvider, instance, mayCache: true)) throw new Exception("ApiHawk impl. has required service(s) that can't be fulfilled");
 						return (IExternalApi) instance;
 					}));
 		}

src/BizHawk.Client.EmuHawk/tools/Lua/LuaLibraries.cs

@@ -66,9 +66,7 @@ namespace BizHawk.Client.EmuHawk
 					|| lib.GetCustomAttribute<LuaLibraryAttribute>(inherit: false)?.Released is not false)
 				{
 					var instance = (LuaLibraryBase)Activator.CreateInstance(lib, this, _apiContainer, (Action<string>)LogToLuaConsole);
-					var updateSuccess = ServiceInjector.UpdateServices(serviceProvider, instance, mayCache: true);
-					Debug.Assert(updateSuccess);
-					_ = updateSuccess;
+					if (!ServiceInjector.UpdateServices(serviceProvider, instance, mayCache: true)) throw new Exception("Lua lib has required service(s) that can't be fulfilled");
 
 					// TODO: make EmuHawk libraries have a base class with common properties such as this
 					// and inject them here