Add Analyzer (currently disabled) for target-typed `new` with `throw`

.global.editorconfig.ini

@@ -14,6 +14,8 @@ dotnet_diagnostic.BHI1004.severity = error
 dotnet_diagnostic.BHI1005.severity = error
 # Do not discard local variables
 dotnet_diagnostic.BHI1006.severity = error
+# Don't use target-typed new for throw expressions
+dotnet_diagnostic.BHI1007.severity = suggestion
 # Don't call this.GetType() in sealed type, use typeof operator
 dotnet_diagnostic.BHI1100.severity = error
 # Don't call this.GetType(), use typeof operator (or replace subtype check with better encapsulation)

ExternalProjects/BizHawk.Analyzer/NoTargetTypedThrowAnalyzer.cs

@@ -0,0 +1,58 @@
+namespace BizHawk.Analyzers;
+
+using System.Collections.Immutable;
+
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.CodeAnalysis.Operations;
+
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public sealed class NoTargetTypedThrowAnalyzer : DiagnosticAnalyzer
+{
+	private const string ERR_MSG_IMPLICIT = "Specify `Exception` (or a more precise type) explicitly";
+
+	private static readonly DiagnosticDescriptor DiagNoTargetTypedThrow = new(
+		id: "BHI1007",
+		title: "Don't use target-typed new for throw expressions",
+		messageFormat: "{0}",
+		category: "Usage",
+		defaultSeverity: DiagnosticSeverity.Warning,
+		isEnabledByDefault: true);
+
+	public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = ImmutableArray.Create(DiagNoTargetTypedThrow);
+
+	public override void Initialize(AnalysisContext context)
+	{
+		context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+		context.EnableConcurrentExecution();
+		context.RegisterOperationAction(
+			oac =>
+			{
+				var exceptionOp = ((IThrowOperation) oac.Operation).Exception;
+				if (exceptionOp is null) return; // re-`throw;`
+				void Fail(string message)
+					=> oac.ReportDiagnostic(Diagnostic.Create(DiagNoTargetTypedThrow, exceptionOp.Syntax.GetLocation(), message));
+				switch (exceptionOp.Kind)
+				{
+					case OperationKind.ObjectCreation:
+					case OperationKind.Invocation:
+					case OperationKind.PropertyReference:
+					case OperationKind.LocalReference:
+						return;
+					case OperationKind.Conversion:
+						if (((IConversionOperation) exceptionOp).Operand.Syntax
+							.IsKind(SyntaxKind.ImplicitObjectCreationExpression))
+						{
+							break;
+						}
+						return;
+					default:
+						Fail($"Argument to throw expression was of an unexpected kind: {exceptionOp.GetType().FullName}");
+						return;
+				}
+				Fail(ERR_MSG_IMPLICIT);
+			},
+			OperationKind.Throw);
+	}
+}

ExternalProjects/BizHawk.AnalyzersTests/BizHawk.Analyzer/NoTargetTypedThrowAnalyzerTests.cs

@@ -0,0 +1,43 @@
+namespace BizHawk.Tests.Analyzers;
+
+using System.Threading.Tasks;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+using Verify = Microsoft.CodeAnalysis.CSharp.Testing.CSharpAnalyzerVerifier<
+	BizHawk.Analyzers.NoTargetTypedThrowAnalyzer,
+	Microsoft.CodeAnalysis.Testing.DefaultVerifier>;
+
+[TestClass]
+public sealed class NoTargetTypedThrowAnalyzerTests
+{
+	[TestMethod]
+	public Task CheckMisuseOfXORAssignment()
+		=> Verify.VerifyAnalyzerAsync("""
+			using System;
+			public static class Cases {
+				// not present: throwing a local, which is apparently necessary in Win32 interop
+				private static void V()
+					=> throw ExceptionHelperProp;
+				private static void W()
+					=> throw ExceptionHelperMethod();
+				private static void X() {
+					try {
+						Z();
+					} catch (Exception) {
+						throw;
+					}
+				}
+				private static void Y()
+					=> throw new Exception();
+				private static void Z()
+					=> throw new NotImplementedException();
+				private static void A()
+					=> throw {|BHI1007:new()|};
+				private static Exception ExceptionHelperMethod()
+					=> new();
+				private static Exception ExceptionHelperProp
+					=> new();
+			}
+		""");
+}