Enable C# nullable reference types in Compiler project

src/QsCompiler/CommandLineTool/Commands/Build.cs

@@ -206,7 +206,7 @@ public static int Run(BuildOptions options, ConsoleLogger logger)
             {
                 ProjectName = options.ProjectName,
                 AssemblyConstants = assemblyConstants,
-                TargetPackageAssemblies = options.TargetSpecificDecompositions,
+                TargetPackageAssemblies = options.TargetSpecificDecompositions ?? Enumerable.Empty<string>(),
                 RuntimeCapabilities = options.RuntimeCapabilites,
                 SkipMonomorphization = options.RuntimeCapabilites == RuntimeCapabilities.Unknown,
                 GenerateFunctorSupport = true,
@@ -226,7 +226,11 @@ public static int Run(BuildOptions options, ConsoleLogger logger)
                 CompilationLoader.CompilationTaskEvent += CompilationTracker.OnCompilationTaskEvent;
             }
 
-            var loaded = new CompilationLoader(options.LoadSourcesOrSnippet(logger), options.References, loadOptions, logger);
+            var loaded = new CompilationLoader(
+                options.LoadSourcesOrSnippet(logger),
+                options.References ?? Enumerable.Empty<string>(),
+                loadOptions,
+                logger);
             if (options.PerfFolder != null)
             {
                 try

src/QsCompiler/CommandLineTool/Commands/Diagnose.cs

@@ -274,7 +274,7 @@ public static int Run(DiagnoseOptions options, ConsoleLogger logger)
             var loadOptions = new CompilationLoader.Configuration
             {
                 AssemblyConstants = assemblyConstants,
-                TargetPackageAssemblies = options.TargetSpecificDecompositions,
+                TargetPackageAssemblies = options.TargetSpecificDecompositions ?? Enumerable.Empty<string>(),
                 RuntimeCapabilities = options.RuntimeCapabilites,
                 SkipMonomorphization = options.RuntimeCapabilites == RuntimeCapabilities.Unknown,
                 GenerateFunctorSupport = true,
@@ -285,7 +285,11 @@ public static int Run(DiagnoseOptions options, ConsoleLogger logger)
                 EnableAdditionalChecks = true,
                 ExposeReferencesViaTestNames = options.ExposeReferencesViaTestNames
             };
-            var loaded = new CompilationLoader(options.LoadSourcesOrSnippet(logger), options.References, loadOptions, logger);
+            var loaded = new CompilationLoader(
+                options.LoadSourcesOrSnippet(logger),
+                options.References ?? Enumerable.Empty<string>(),
+                loadOptions,
+                logger);
             if (loaded.VerifiedCompilation == null)
             {
                 return ReturnCode.Status(loaded);

src/QsCompiler/CommandLineTool/Commands/Format.cs

@@ -175,11 +175,18 @@ ImmutableDictionary<Uri, string> LoadSources(SourceFileLoader loadFromDisk) =>
                 options.LoadSourcesOrSnippet(logger)(loadFromDisk)
                     .ToImmutableDictionary(entry => entry.Key, entry => UpdateArrayLiterals(entry.Value)); // manually replace array literals
 
-            var loaded = new CompilationLoader(LoadSources, options.References, logger: logger); // no rewrite steps, no generation
+            // no rewrite steps, no generation
+            var loaded =
+                new CompilationLoader(LoadSources, options.References ?? Enumerable.Empty<string>(), logger: logger);
             if (ReturnCode.Status(loaded) == ReturnCode.UNRESOLVED_FILES)
             {
                 return ReturnCode.UNRESOLVED_FILES; // ignore compilation errors
             }
+            else if (loaded.VerifiedCompilation is null)
+            {
+                logger.Log(ErrorCode.QsGenerationFailed, Enumerable.Empty<string>());
+                return ReturnCode.CODE_GENERATION_ERRORS;
+            }
 
             // TODO: a lot of the formatting logic defined here and also in the routines above
             // is supposed to move into the compilation manager in order to be available for the language server to provide formatting

src/QsCompiler/CommandLineTool/CompilationTracker.cs

@@ -26,7 +26,7 @@ private class CompilationTask
             /// <summary>
             /// Represents the name of the parent compilation task.
             /// </summary>
-            public readonly string ParentName;
+            public readonly string? ParentName;
 
             /// <summary>
             /// Represents the name of the compilation task.
@@ -56,15 +56,15 @@ private class CompilationTask
             /// <summary>
             /// Generates a key that uniquely identifies a task in the compilation process based on the task's name and its parent's name.
             /// </summary>
-            internal static string GenerateKey(string parentName, string name)
+            internal static string GenerateKey(string? parentName, string name)
             {
                 return string.Format("{0}.{1}", parentName ?? "ROOT", name);
             }
 
             /// <summary>
             /// Creates a compilation task object and starts its stopwatch.
             /// </summary>
-            public CompilationTask(string parentName, string name)
+            public CompilationTask(string? parentName, string name)
             {
                 this.ParentName = parentName;
                 this.Name = name;

src/QsCompiler/CommandLineTool/Options.cs

@@ -298,17 +298,17 @@ public static bool IsCodeSnippet(NonNullable<string> fileId) =>
         /// </summary>
         internal CompilationLoader.SourceLoader LoadSourcesOrSnippet(ILogger logger) => loadFromDisk =>
         {
-            bool inputIsEmptyOrNull = this.Input == null || !this.Input.Any();
-            if (this.CodeSnippet == null && !inputIsEmptyOrNull)
+            var input = this.Input ?? Enumerable.Empty<string>();
+            if (this.CodeSnippet == null && input.Any())
             {
-                return loadFromDisk(this.Input);
+                return loadFromDisk(input);
             }
-            else if (this.CodeSnippet != null && inputIsEmptyOrNull)
+            else if (this.CodeSnippet != null && !input.Any())
             {
                 return new Dictionary<Uri, string> { { SNIPPET_FILE_URI, AsSnippet(this.CodeSnippet, this.WithinFunction) } }.ToImmutableDictionary();
             }
 
-            if (inputIsEmptyOrNull)
+            if (!input.Any())
             {
                 logger?.Log(ErrorCode.MissingInputFileOrSnippet, Enumerable.Empty<string>());
             }

src/QsCompiler/CompilationManager/Diagnostics.cs

@@ -129,7 +129,7 @@ internal static string Warning(int code) =>
 
         // warnings 70**
 
-        public static Diagnostic LoadWarning(WarningCode code, IEnumerable<string> args, string source) =>
+        public static Diagnostic LoadWarning(WarningCode code, IEnumerable<string> args, string? source) =>
             new Diagnostic
             {
                 Severity = DiagnosticSeverity.Warning,
@@ -164,7 +164,7 @@ internal static string Error(int code) =>
 
         // errors 70**
 
-        public static Diagnostic LoadError(ErrorCode code, IEnumerable<string> args, string source) =>
+        public static Diagnostic LoadError(ErrorCode code, IEnumerable<string> args, string? source) =>
             new Diagnostic
             {
                 Severity = DiagnosticSeverity.Error,