diff --git a/src/QsCompiler/CommandLineTool/Commands/Build.cs b/src/QsCompiler/CommandLineTool/Commands/Build.cs
index 6ccd4b8b0d..ef31dbef64 100644
--- a/src/QsCompiler/CommandLineTool/Commands/Build.cs
+++ b/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
diff --git a/src/QsCompiler/CommandLineTool/Commands/Diagnose.cs b/src/QsCompiler/CommandLineTool/Commands/Diagnose.cs
index 51094dc6ce..14fc8486bc 100644
--- a/src/QsCompiler/CommandLineTool/Commands/Diagnose.cs
+++ b/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);
diff --git a/src/QsCompiler/CommandLineTool/Commands/Format.cs b/src/QsCompiler/CommandLineTool/Commands/Format.cs
index 684bdda5be..7a77aa2bf4 100644
--- a/src/QsCompiler/CommandLineTool/Commands/Format.cs
+++ b/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
diff --git a/src/QsCompiler/CommandLineTool/CompilationTracker.cs b/src/QsCompiler/CommandLineTool/CompilationTracker.cs
index c13ec95433..d46c54291d 100644
--- a/src/QsCompiler/CommandLineTool/CompilationTracker.cs
+++ b/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.
@@ -56,7 +56,7 @@ 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);
             }
@@ -64,7 +64,7 @@ internal static string GenerateKey(string parentName, string 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;
diff --git a/src/QsCompiler/CommandLineTool/Options.cs b/src/QsCompiler/CommandLineTool/Options.cs
index f637f3249e..7653d59ca8 100644
--- a/src/QsCompiler/CommandLineTool/Options.cs
+++ b/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>());
             }
diff --git a/src/QsCompiler/CompilationManager/Diagnostics.cs b/src/QsCompiler/CompilationManager/Diagnostics.cs
index d0c8beb27a..f512056eb7 100644
--- a/src/QsCompiler/CompilationManager/Diagnostics.cs
+++ b/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,
diff --git a/src/QsCompiler/Compiler/CompilationLoader.cs b/src/QsCompiler/Compiler/CompilationLoader.cs
index 71a7b8bc4f..de98c2002d 100644
--- a/src/QsCompiler/Compiler/CompilationLoader.cs
+++ b/src/QsCompiler/Compiler/CompilationLoader.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
 using System.Reflection;
@@ -17,6 +18,7 @@
 using Microsoft.Quantum.QsCompiler.ReservedKeywords;
 using Microsoft.Quantum.QsCompiler.Serialization;
 using Microsoft.Quantum.QsCompiler.SyntaxTree;
+using Microsoft.Quantum.QsCompiler.Transformations;
 using Microsoft.Quantum.QsCompiler.Transformations.BasicTransformations;
 using Microsoft.VisualStudio.LanguageServer.Protocol;
 using Newtonsoft.Json.Bson;
@@ -44,10 +46,10 @@ public enum CompilationTaskEventType
         public class CompilationTaskEventArgs : EventArgs
         {
             public CompilationTaskEventType Type;
-            public string ParentTaskName;
+            public string? ParentTaskName;
             public string TaskName;
 
-            public CompilationTaskEventArgs(CompilationTaskEventType type, string parentTaskName, string taskName)
+            public CompilationTaskEventArgs(CompilationTaskEventType type, string? parentTaskName, string taskName)
             {
                 this.ParentTaskName = parentTaskName;
                 this.TaskName = taskName;
@@ -75,12 +77,12 @@ public CompilationTaskEventArgs(CompilationTaskEventType type, string parentTask
         /// <summary>
         /// Used to raise a compilation task event.
         /// </summary>
-        public static event CompilationTaskEventHandler CompilationTaskEvent;
+        public static event CompilationTaskEventHandler? CompilationTaskEvent;
 
         /// <summary>
         /// If LoadAssembly is not null, it will be used to load the dlls that are search for classes defining rewrite steps.
         /// </summary>
-        public static Func<string, Assembly> LoadAssembly { get; set; }
+        public static Func<string, Assembly>? LoadAssembly { get; set; }
 
         /// <summary>
         /// Sorts the given list of step according to their relative priority give by getPriority.
@@ -98,7 +100,7 @@ public struct Configuration
             /// The name of the project. Used as assembly name in the generated dll.
             /// The name of the project with a suitable extension will also be used as the name of the generated binary file.
             /// </summary>
-            public string ProjectName;
+            public string? ProjectName;
 
             /// <summary>
             /// If set to true, the syntax tree rewrite step that replaces all generation directives
@@ -148,13 +150,13 @@ public struct Configuration
             /// Directory where the compiled binaries will be generated.
             /// No binaries will be written to disk unless this path is specified and valid.
             /// </summary>
-            public string BuildOutputFolder;
+            public string? BuildOutputFolder;
 
             /// <summary>
             /// Output path for the dll containing the compiled binaries.
             /// No dll will be generated unless this path is specified and valid.
             /// </summary>
-            public string DllOutputPath;
+            public string? DllOutputPath;
 
             /// <summary>
             /// If set to true, then referenced dlls will be loaded purely based on attributes in the contained C# code.
@@ -173,7 +175,7 @@ public struct Configuration
             /// (i.e. classes implementing IRewriteStep) and the corresponding output folder.
             /// The contained rewrite steps will be executed in the defined order and priority at the end of the compilation.
             /// </summary>
-            public IEnumerable<(string, string)> RewriteSteps;
+            public IEnumerable<(string, string?)> RewriteSteps;
 
             /// <summary>
             /// If set to true, the post-condition for loaded rewrite steps is checked if the corresponding verification is implemented.
@@ -219,7 +221,7 @@ public struct Configuration
             /// If the ProjectName does not have an ending "proj", appends a .qsproj ending to the project name.
             /// Returns null if the project name is null.
             /// </summary>
-            internal string ProjectNameWithExtension =>
+            internal string? ProjectNameWithExtension =>
                 this.ProjectName == null ? null :
                 this.ProjectName.EndsWith("proj") ? this.ProjectName :
                 $"{this.ProjectName}.qsproj";
@@ -228,7 +230,7 @@ public struct Configuration
             /// If the ProjectName does have an extension ending with "proj", returns the project name without that extension.
             /// Returns null if the project name is null.
             /// </summary>
-            internal string ProjectNameWithoutExtension =>
+            internal string? ProjectNameWithoutExtension =>
                 this.ProjectName == null ? null :
                 Path.GetExtension(this.ProjectName).EndsWith("proj") ? Path.GetFileNameWithoutExtension(this.ProjectName) :
                 this.ProjectName;
@@ -366,7 +368,7 @@ internal bool Success(Configuration options) =>
         /// Returns a status NotRun if no such step was found or executed.
         /// Execution is considered successful if the precondition and transformation (if any) returned true.
         /// </summary>
-        public Status LoadedRewriteStep(string name, string source = null)
+        public Status LoadedRewriteStep(string name, string? source = null)
         {
             var uri = string.IsNullOrWhiteSpace(source) ? null : new Uri(Path.GetFullPath(source));
             bool MatchesQuery(int index) => this.externalRewriteSteps[index].Name == name && (source == null || this.externalRewriteSteps[index].Origin == uri);
@@ -389,7 +391,7 @@ public Status LoadedRewriteStep(string name, string source = null)
         /// <summary>
         /// Logger used to log all diagnostic events during compilation.
         /// </summary>
-        private readonly ILogger logger;
+        private readonly ILogger? logger;
 
         /// <summary>
         /// Configuration specifying the compilation steps to execute.
@@ -416,27 +418,27 @@ public Status LoadedRewriteStep(string name, string source = null)
         /// <summary>
         /// Contains the initial compilation built by the compilation unit manager after verification.
         /// </summary>
-        public readonly CompilationUnitManager.Compilation VerifiedCompilation;
+        public readonly CompilationUnitManager.Compilation? VerifiedCompilation;
 
         /// <summary>
         /// Contains the built compilation including the syntax tree after executing all configured rewrite steps.
         /// </summary>
-        public readonly QsCompilation CompilationOutput;
+        public readonly QsCompilation? CompilationOutput;
 
         /// <summary>
         /// Contains the absolute path where the binary representation of the generated syntax tree has been written to disk.
         /// </summary>
-        public readonly string PathToCompiledBinary;
+        public readonly string? PathToCompiledBinary;
 
         /// <summary>
         /// Contains the absolute path where the generated dll containing the compiled binary has been written to disk.
         /// </summary>
-        public readonly string DllOutputPath;
+        public readonly string? DllOutputPath;
 
         /// <summary>
         /// Contains the full Q# syntax tree after executing all configured rewrite steps, including the content of loaded references.
         /// </summary>
-        public IEnumerable<QsNamespace> GeneratedSyntaxTree =>
+        public IEnumerable<QsNamespace>? GeneratedSyntaxTree =>
             this.CompilationOutput?.Namespaces;
 
         /// <summary>
@@ -452,7 +454,7 @@ public Status LoadedRewriteStep(string name, string source = null)
         /// Uses the specified logger to log all diagnostic events.
         /// Throws an ArgumentNullException if either one of the given loaders is null or returns null.
         /// </summary>
-        public CompilationLoader(SourceLoader loadSources, ReferenceLoader loadReferences, Configuration? options = null, ILogger logger = null)
+        public CompilationLoader(SourceLoader loadSources, ReferenceLoader loadReferences, Configuration? options = null, ILogger? logger = null)
         {
             this.RaiseCompilationTaskStart(null, "OverallCompilation");
 
@@ -536,7 +538,7 @@ public CompilationLoader(SourceLoader loadSources, ReferenceLoader loadReference
 
             // executing the specified rewrite steps
 
-            var steps = new List<(int, Func<QsCompilation>)>();
+            var steps = new List<(int, Func<QsCompilation?>)>();
 
             if (this.config.ConvertClassicalControl)
             {
@@ -571,7 +573,7 @@ public CompilationLoader(SourceLoader loadSources, ReferenceLoader loadReference
             for (int j = 0; j < this.externalRewriteSteps.Length; j++)
             {
                 var priority = this.externalRewriteSteps[j].Priority;
-                Func<QsCompilation> Execute(int index) => () =>
+                Func<QsCompilation?> Execute(int index) => () =>
                     this.ExecuteAsAtomicTransformation(this.externalRewriteSteps[index], ref this.compilationStatus.LoadedRewriteSteps[index]);
                 steps.Add((priority, Execute(j)));
             }
@@ -630,7 +632,7 @@ Func<QsCompilation> Execute(int index) => () =>
         /// executing the compilation steps specified by the given options.
         /// Uses the specified logger to log all diagnostic events.
         /// </summary>
-        public CompilationLoader(IEnumerable<string> sources, IEnumerable<string> references, Configuration? options = null, ILogger logger = null)
+        public CompilationLoader(IEnumerable<string> sources, IEnumerable<string> references, Configuration? options = null, ILogger? logger = null)
             : this(load => load(sources), load => load(references), options, logger)
         {
         }
@@ -641,7 +643,7 @@ public CompilationLoader(IEnumerable<string> sources, IEnumerable<string> refere
         /// Uses the specified logger to log all diagnostic events.
         /// Throws an ArgumentNullException if the given loader is null or returns null.
         /// </summary>
-        public CompilationLoader(IEnumerable<string> sources, ReferenceLoader loadReferences, Configuration? options = null, ILogger logger = null)
+        public CompilationLoader(IEnumerable<string> sources, ReferenceLoader loadReferences, Configuration? options = null, ILogger? logger = null)
             : this(load => load(sources), loadReferences, options, logger)
         {
         }
@@ -652,7 +654,7 @@ public CompilationLoader(IEnumerable<string> sources, ReferenceLoader loadRefere
         /// Uses the specified logger to log all diagnostic events.
         /// Throws an ArgumentNullException if the given loader is null or returns null.
         /// </summary>
-        public CompilationLoader(SourceLoader loadSources, IEnumerable<string> references, Configuration? options = null, ILogger logger = null)
+        public CompilationLoader(SourceLoader loadSources, IEnumerable<string> references, Configuration? options = null, ILogger? logger = null)
             : this(loadSources, load => load(references), options, logger)
         {
         }
@@ -721,7 +723,7 @@ private void PrintResolvedFiles(IEnumerable<Uri> sourceFiles)
                 return;
             }
             var args = sourceFiles.Any()
-                ? sourceFiles.Select(f => f?.LocalPath).ToArray()
+                ? sourceFiles.Select(f => f.LocalPath).ToArray()
                 : new string[] { "(none)" };
             this.logger?.Log(InformationCode.CompilingWithSourceFiles, Enumerable.Empty<string>(), messageParam: Formatting.Indent(args).ToArray());
         }
@@ -763,22 +765,22 @@ private void PrintLoadedRewriteSteps(IEnumerable<RewriteSteps.LoadedStep> rewrit
         /// <summary>
         /// Raises a compilation task start event.
         /// </summary>
-        private void RaiseCompilationTaskStart(string parentTaskName, string taskName) =>
+        private void RaiseCompilationTaskStart(string? parentTaskName, string taskName) =>
             CompilationTaskEvent?.Invoke(this, new CompilationTaskEventArgs(CompilationTaskEventType.Start, parentTaskName, taskName));
 
         /// <summary>
         /// Raises a compilation task end event.
         /// </summary>
-        private void RaiseCompilationTaskEnd(string parentTaskName, string taskName) =>
+        private void RaiseCompilationTaskEnd(string? parentTaskName, string taskName) =>
             CompilationTaskEvent?.Invoke(this, new CompilationTaskEventArgs(CompilationTaskEventType.End, parentTaskName, taskName));
 
         /// <summary>
         /// Executes the given rewrite step on the current CompilationOutput, and updates the given status accordingly.
         /// Sets the CompilationOutput to the transformed compilation if the status indicates success.
         /// </summary>
-        private QsCompilation ExecuteAsAtomicTransformation(RewriteSteps.LoadedStep rewriteStep, ref Status status)
+        private QsCompilation? ExecuteAsAtomicTransformation(RewriteSteps.LoadedStep rewriteStep, ref Status status)
         {
-            QsCompilation transformed = null;
+            QsCompilation? transformed = null;
             if (this.compilationStatus.Validation != Status.Succeeded)
             {
                 status = Status.NotRun;
@@ -799,7 +801,7 @@ private QsCompilation ExecuteAsAtomicTransformation(RewriteSteps.LoadedStep rewr
         /// Returns the unmodified CompilationOutput otherwise.
         /// Throws an ArgumentNullException if the given sequence of paths is null.
         /// </summary>
-        private QsCompilation ReplaceTargetSpecificImplementations(IEnumerable<string> paths, Uri rewriteStepOrigin, int nrReferences)
+        private QsCompilation? ReplaceTargetSpecificImplementations(IEnumerable<string> paths, Uri rewriteStepOrigin, int nrReferences)
         {
             if (paths == null)
             {
@@ -814,8 +816,7 @@ private QsCompilation ReplaceTargetSpecificImplementations(IEnumerable<string> p
                 try
                 {
                     var targetDll = Path.GetFullPath(path);
-                    var loadSucceeded = AssemblyLoader.LoadReferencedAssembly(targetDll, out var loaded, LogException);
-                    if (loadSucceeded)
+                    if (AssemblyLoader.LoadReferencedAssembly(targetDll, out var loaded, LogException))
                     {
                         return (NonNullable<string>.New(path), loaded.Namespaces);
                     }
@@ -830,7 +831,7 @@ private QsCompilation ReplaceTargetSpecificImplementations(IEnumerable<string> p
                 }
             }
 
-            var natives = paths.Select(LoadReferences).Where(loaded => loaded.HasValue).Select(loaded => loaded.Value).ToArray();
+            var natives = paths.SelectNotNull(LoadReferences).ToArray();
             var combinedSuccessfully = References.CombineSyntaxTrees(out var replacements, additionalAssemblies: nrReferences, onError: LogError, natives);
             if (!combinedSuccessfully)
             {
@@ -847,7 +848,7 @@ private QsCompilation ReplaceTargetSpecificImplementations(IEnumerable<string> p
         /// Catches and logs any thrown exception. Returns the status of the rewrite step.
         /// Throws an ArgumentNullException if the rewrite step to execute or the given compilation is null.
         /// </summary>
-        private Status ExecuteRewriteStep(RewriteSteps.LoadedStep rewriteStep, QsCompilation compilation, out QsCompilation transformed)
+        private Status ExecuteRewriteStep(RewriteSteps.LoadedStep rewriteStep, QsCompilation? compilation, out QsCompilation? transformed)
         {
             if (rewriteStep == null)
             {
@@ -858,7 +859,7 @@ private Status ExecuteRewriteStep(RewriteSteps.LoadedStep rewriteStep, QsCompila
                 throw new ArgumentNullException(nameof(compilation));
             }
 
-            string GetDiagnosticsCode(DiagnosticSeverity severity) =>
+            string? GetDiagnosticsCode(DiagnosticSeverity severity) =>
                 rewriteStep.Name == "CsharpGeneration" && severity == DiagnosticSeverity.Error ? Errors.Code(ErrorCode.CsharpGenerationGeneratedError) :
                 rewriteStep.Name == "CsharpGeneration" && severity == DiagnosticSeverity.Warning ? Warnings.Code(WarningCode.CsharpGenerationGeneratedWarning) :
                 rewriteStep.Name == "CsharpGeneration" && severity == DiagnosticSeverity.Information ? Informations.Code(InformationCode.CsharpGenerationGeneratedInfo) :
@@ -1019,7 +1020,7 @@ void LogError() => this.LogAndUpdate(
         /// Does *not* close the given memory stream.
         /// Throws an ArgumentNullException if the given memory stream is null.
         /// </summary>
-        private string GenerateBinary(MemoryStream serialization)
+        private string? GenerateBinary(MemoryStream serialization)
         {
             if (serialization == null)
             {
@@ -1058,7 +1059,7 @@ private string GenerateBinary(MemoryStream serialization)
         /// Does *not* close the given memory stream.
         /// Throws an ArgumentNullException if the given memory stream is null.
         /// </summary>
-        private string GenerateDll(MemoryStream serialization)
+        private string? GenerateDll(MemoryStream serialization)
         {
             if (serialization == null)
             {
@@ -1097,7 +1098,8 @@ bool CanBeIncluded(NonNullable<string> dll)
 
             try
             {
-                var referencePaths = GetSourceFiles.Apply(this.CompilationOutput.Namespaces) // we choose to keep only Q# references that have been used
+                var referencePaths = this.CompilationOutput?.Namespaces
+                    .Apply(ns => GetSourceFiles.Apply(ns)) // we choose to keep only Q# references that have been used
                     .Where(file => file.Value.EndsWith(".dll"));
                 var references = referencePaths.Select((dll, id) => (dll, CreateReference(dll.Value, id), CanBeIncluded(dll))).ToImmutableArray();
                 var csharpTree = MetadataGeneration.GenerateAssemblyMetadata(references.Where(r => r.Item3).Select(r => r.Item2));
@@ -1140,14 +1142,14 @@ bool CanBeIncluded(NonNullable<string> dll)
         /// Given the path to a Q# binary file, reads the content of that file and returns the corresponding compilation as out parameter.
         /// Throws the corresponding exception if the given path does not correspond to a suitable binary file.
         /// </summary>
-        public static bool ReadBinary(string file, out QsCompilation syntaxTree) =>
+        public static bool ReadBinary(string file, [NotNullWhen(true)] out QsCompilation? syntaxTree) =>
             ReadBinary(new MemoryStream(File.ReadAllBytes(Path.GetFullPath(file))), out syntaxTree);
 
         /// <summary>
         /// Given a stream with the content of a Q# binary file, returns the corresponding compilation as out parameter.
         /// Throws an ArgumentNullException if the given stream is null.
         /// </summary>
-        public static bool ReadBinary(Stream stream, out QsCompilation syntaxTree) =>
+        public static bool ReadBinary(Stream stream, [NotNullWhen(true)] out QsCompilation? syntaxTree) =>
             AssemblyLoader.LoadSyntaxTree(stream, out syntaxTree);
 
         /// <summary>
@@ -1157,7 +1159,7 @@ public static bool ReadBinary(Stream stream, out QsCompilation syntaxTree) =>
         /// Throws an ArgumentException if the given file id is incompatible with and id assigned by the Q# compiler.
         /// Throws the corresponding exception any of the path operations fails or if the writing fails.
         /// </summary>
-        public static string GeneratedFile(NonNullable<string> fileId, string outputFolder, string fileEnding, string content = null)
+        public static string GeneratedFile(NonNullable<string> fileId, string outputFolder, string fileEnding, string? content = null)
         {
             if (!CompilationUnitManager.TryGetUri(fileId, out var file))
             {
diff --git a/src/QsCompiler/Compiler/Compiler.csproj b/src/QsCompiler/Compiler/Compiler.csproj
index 2a5f429a9d..0aa3b0593f 100644
--- a/src/QsCompiler/Compiler/Compiler.csproj
+++ b/src/QsCompiler/Compiler/Compiler.csproj
@@ -6,6 +6,8 @@
     <PackageId>Microsoft.Quantum.Compiler</PackageId>
     <AssemblyName>Microsoft.Quantum.QsCompiler</AssemblyName>
     <AssemblyTitle>Microsoft Q# compiler library.</AssemblyTitle>
+    <Nullable>enable</Nullable>
+    <WarningsAsErrors>nullable</WarningsAsErrors>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/QsCompiler/Compiler/ExternalRewriteSteps.cs b/src/QsCompiler/Compiler/ExternalRewriteSteps.cs
index 934cfe74df..7dcfff6941 100644
--- a/src/QsCompiler/Compiler/ExternalRewriteSteps.cs
+++ b/src/QsCompiler/Compiler/ExternalRewriteSteps.cs
@@ -5,6 +5,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
 using System.Reflection;
@@ -12,8 +13,8 @@
 using Microsoft.Quantum.QsCompiler.Diagnostics;
 using Microsoft.Quantum.QsCompiler.ReservedKeywords;
 using Microsoft.Quantum.QsCompiler.SyntaxTree;
+using Microsoft.Quantum.QsCompiler.Transformations;
 using Microsoft.VisualStudio.LanguageServer.Protocol;
-using Lsp = Microsoft.VisualStudio.LanguageServer.Protocol;
 using Range = Microsoft.Quantum.QsCompiler.DataTypes.Range;
 
 namespace Microsoft.Quantum.QsCompiler
@@ -26,28 +27,30 @@ internal static class RewriteSteps
         internal class LoadedStep : IRewriteStep
         {
             internal readonly Uri Origin;
-            private readonly IRewriteStep selfAsStep;
+            private readonly IRewriteStep? selfAsStep;
             private readonly object selfAsObject;
 
-            private readonly MethodInfo[] interfaceMethods;
+            private readonly MethodInfo[]? interfaceMethods;
 
-            private MethodInfo InterfaceMethod(string name) =>
+            private MethodInfo? InterfaceMethod(string name) =>
                 // This choice of filtering the interface methods may seem a bit particular.
                 // However, unless you know what you are doing, please don't change it.
                 // If you are sure you know what you are doing, please make sure the loading via reflection works for rewrite steps
                 // implemented in both F# or C#, and whether they are compiled against the current compiler version or an older one.
                 this.interfaceMethods?.FirstOrDefault(method => method.Name.Split("-").Last() == name);
 
-            private object GetViaReflection(string name) =>
+            private object? GetViaReflection(string name) =>
                 this.InterfaceMethod($"get_{name}")?.Invoke(this.selfAsObject, null);
 
+            [return: MaybeNull]
             private T GetViaReflection<T>(string name) =>
                 (T)this.InterfaceMethod($"get_{name}")?.Invoke(this.selfAsObject, null);
 
             private void SetViaReflection<T>(string name, T arg) =>
-                this.InterfaceMethod($"set_{name}")?.Invoke(this.selfAsObject, new object[] { arg });
+                this.InterfaceMethod($"set_{name}")?.Invoke(this.selfAsObject, new object?[] { arg });
 
-            private T InvokeViaReflection<T>(string name, params object[] args) =>
+            [return: MaybeNull]
+            private T InvokeViaReflection<T>(string name, params object?[] args) =>
                 (T)this.InterfaceMethod(name)?.Invoke(this.selfAsObject, args);
 
             /// <summary>
@@ -75,7 +78,9 @@ internal LoadedStep(object implementation, Type interfaceType, Uri origin)
 
                 // The Name and Priority need to be fixed throughout the loading,
                 // so whatever their value is when loaded that's what these values well be as far at the compiler is concerned.
-                this.Name = this.selfAsStep?.Name ?? this.GetViaReflection<string>(nameof(IRewriteStep.Name));
+                this.Name = this.selfAsStep?.Name
+                    ?? this.GetViaReflection<string>(nameof(IRewriteStep.Name))
+                    ?? "(no name)";
                 this.Priority = this.selfAsStep?.Priority ?? this.GetViaReflection<int>(nameof(IRewriteStep.Priority));
             }
 
@@ -83,7 +88,7 @@ internal LoadedStep(object implementation, Type interfaceType, Uri origin)
 
             public int Priority { get; }
 
-            internal static Diagnostic ConvertDiagnostic(IRewriteStep.Diagnostic diagnostic, Func<DiagnosticSeverity, string> getCode = null)
+            internal static Diagnostic ConvertDiagnostic(IRewriteStep.Diagnostic diagnostic, Func<DiagnosticSeverity, string?>? getCode = null)
             {
                 var severity =
                     diagnostic.Severity == CodeAnalysis.DiagnosticSeverity.Error ? DiagnosticSeverity.Error :
@@ -108,10 +113,11 @@ internal static Diagnostic ConvertDiagnostic(IRewriteStep.Diagnostic diagnostic,
                 };
             }
 
-            public IDictionary<string, string> AssemblyConstants
+            public IDictionary<string, string?> AssemblyConstants
             {
                 get => this.selfAsStep?.AssemblyConstants
-                    ?? this.GetViaReflection<IDictionary<string, string>>(nameof(IRewriteStep.AssemblyConstants));
+                    ?? this.GetViaReflection<IDictionary<string, string?>>(nameof(IRewriteStep.AssemblyConstants))
+                    ?? ImmutableDictionary<string, string?>.Empty;
             }
 
             public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics
@@ -125,9 +131,9 @@ public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics
                     static bool IEnumerableInterface(Type t) => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(IEnumerable<>);
                     var enumerable = this.GetViaReflection(nameof(IRewriteStep.GeneratedDiagnostics)) as IEnumerable;
                     var itemType = enumerable?.GetType().GetInterfaces().FirstOrDefault(IEnumerableInterface)?.GetGenericArguments().FirstOrDefault();
-                    if (itemType == null)
+                    if (enumerable is null || itemType is null)
                     {
-                        return null;
+                        return Enumerable.Empty<IRewriteStep.Diagnostic>();
                     }
 
                     var diagnostics = ImmutableArray.CreateBuilder<IRewriteStep.Diagnostic>();
@@ -167,15 +173,15 @@ public bool ImplementsPostconditionVerification
                     ?? this.GetViaReflection<bool>(nameof(IRewriteStep.ImplementsPostconditionVerification));
             }
 
-            public bool Transformation(QsCompilation compilation, out QsCompilation transformed)
+            public bool Transformation(QsCompilation compilation, [NotNullWhen(true)] out QsCompilation? transformed)
             {
                 if (this.selfAsStep != null)
                 {
                     return this.selfAsStep.Transformation(compilation, out transformed);
                 }
-                var args = new object[] { compilation, null };
+                var args = new object?[] { compilation, null };
                 var success = this.InvokeViaReflection<bool>(nameof(IRewriteStep.Transformation), args);
-                transformed = success ? (QsCompilation)args[1] : compilation;
+                transformed = success ? args[1] as QsCompilation ?? compilation : compilation;
                 return success;
             }
 
@@ -183,9 +189,10 @@ public bool PreconditionVerification(QsCompilation compilation) =>
                 this.selfAsStep?.PreconditionVerification(compilation)
                 ?? this.InvokeViaReflection<bool>(nameof(IRewriteStep.PreconditionVerification), compilation);
 
-            public bool PostconditionVerification(QsCompilation compilation) =>
-                this.selfAsStep?.PostconditionVerification(compilation)
-                ?? this.InvokeViaReflection<bool>(nameof(IRewriteStep.PostconditionVerification), compilation);
+            public bool PostconditionVerification(QsCompilation? compilation) =>
+                !(compilation is null)
+                && (this.selfAsStep?.PostconditionVerification(compilation)
+                    ?? this.InvokeViaReflection<bool>(nameof(IRewriteStep.PostconditionVerification), compilation));
         }
 
         /// <summary>
@@ -199,15 +206,15 @@ public bool PostconditionVerification(QsCompilation compilation) =>
         /// </summary>
         internal static ImmutableArray<LoadedStep> Load(
             CompilationLoader.Configuration config,
-            Action<Diagnostic> onDiagnostic = null,
-            Action<Exception> onException = null)
+            Action<Diagnostic>? onDiagnostic = null,
+            Action<Exception>? onException = null)
         {
             if (config.RewriteSteps == null)
             {
                 return ImmutableArray<LoadedStep>.Empty;
             }
             static Assembly LoadAssembly(string path) => CompilationLoader.LoadAssembly?.Invoke(path) ?? Assembly.LoadFrom(path);
-            Uri WithFullPath(string file)
+            Uri? WithFullPath(string file)
             {
                 try
                 {
@@ -221,7 +228,7 @@ Uri WithFullPath(string file)
                 }
             }
 
-            var specifiedPluginDlls = config.RewriteSteps.Select(step => (WithFullPath(step.Item1), step.Item2)).Where(step => step.Item1 != null).ToList();
+            var specifiedPluginDlls = config.RewriteSteps.SelectNotNull(step => WithFullPath(step.Item1)?.Apply(path => (path, step.Item2)));
             var (foundDlls, notFoundDlls) = specifiedPluginDlls.Partition(step => File.Exists(step.Item1.LocalPath));
             foreach (var file in notFoundDlls.Select(step => step.Item1).Distinct())
             {
diff --git a/src/QsCompiler/Compiler/FunctorGeneration.cs b/src/QsCompiler/Compiler/FunctorGeneration.cs
index dec2d358b8..a7ca06fc62 100644
--- a/src/QsCompiler/Compiler/FunctorGeneration.cs
+++ b/src/QsCompiler/Compiler/FunctorGeneration.cs
@@ -19,7 +19,7 @@ public static class CodeGeneration
         /// Given the argument tuple of a specialization, returns the argument tuple for its controlled version.
         /// Returns null if the given argument tuple is null.
         /// </summary>
-        private static QsTuple<LocalVariableDeclaration<QsLocalSymbol>> ControlledArg(QsTuple<LocalVariableDeclaration<QsLocalSymbol>> arg) =>
+        private static QsTuple<LocalVariableDeclaration<QsLocalSymbol>>? ControlledArg(QsTuple<LocalVariableDeclaration<QsLocalSymbol>> arg) =>
             arg != null
             ? SyntaxGenerator.WithControlQubits(
                 arg,
@@ -33,7 +33,7 @@ private static QsTuple<LocalVariableDeclaration<QsLocalSymbol>> ControlledArg(Qs
         /// Throws an ArgumentException if more than one specialization of that kind exist.
         /// Throws an ArgumentNullException if the sequence of specializations is null, or contains any entries that are null.
         /// </summary>
-        private static QsSpecialization GetSpecialization(this IEnumerable<QsSpecialization> specs, QsSpecializationKind kind)
+        private static QsSpecialization? GetSpecialization(this IEnumerable<QsSpecialization> specs, QsSpecializationKind kind)
         {
             if (specs == null || specs.Any(s => s == null))
             {
@@ -51,7 +51,7 @@ private static QsSpecialization GetSpecialization(this IEnumerable<QsSpecializat
         /// Return the argument tuple as well as the QsScope if the implementation is provided.
         /// Returns null if the given implementation is null or not provided.
         /// </summary>
-        private static (QsTuple<LocalVariableDeclaration<QsLocalSymbol>>, QsScope)? GetContent(this SpecializationImplementation impl) =>
+        private static (QsTuple<LocalVariableDeclaration<QsLocalSymbol>>, QsScope)? GetContent(this SpecializationImplementation? impl) =>
             impl is SpecializationImplementation.Provided content
                 ? (content.Item1, content.Item2)
                 : ((QsTuple<LocalVariableDeclaration<QsLocalSymbol>>, QsScope)?)null;
@@ -202,7 +202,7 @@ private static QsCallable BuildControlledAdjoint(this QsCallable callable)
         /// Returns a boolean indicating if the evaluation of all directives was successful.
         /// Throws an ArgumentNullException (that is not logged or ignored) if the given compilation is null.
         /// </summary>
-        public static bool GenerateFunctorSpecializations(QsCompilation compilation, out QsCompilation built, Action<Exception> onException = null)
+        public static bool GenerateFunctorSpecializations(QsCompilation compilation, out QsCompilation built, Action<Exception>? onException = null)
         {
             if (compilation == null)
             {
diff --git a/src/QsCompiler/Compiler/Logging.cs b/src/QsCompiler/Compiler/Logging.cs
index 72252a2f3d..5b5cf77a40 100644
--- a/src/QsCompiler/Compiler/Logging.cs
+++ b/src/QsCompiler/Compiler/Logging.cs
@@ -13,11 +13,11 @@ namespace Microsoft.Quantum.QsCompiler.Diagnostics
 {
     public interface ILogger
     {
-        void Log(ErrorCode item, IEnumerable<string> args, string source = null, LSP.Range range = null);
+        void Log(ErrorCode item, IEnumerable<string> args, string? source = null, LSP.Range? range = null);
 
-        void Log(WarningCode item, IEnumerable<string> args, string source = null, LSP.Range range = null);
+        void Log(WarningCode item, IEnumerable<string> args, string? source = null, LSP.Range? range = null);
 
-        void Log(InformationCode item, IEnumerable<string> args, string source = null, LSP.Range range = null, params string[] messageParam);
+        void Log(InformationCode item, IEnumerable<string> args, string? source = null, LSP.Range? range = null, params string[] messageParam);
 
         void Log(params Diagnostic[] messages);
 
@@ -39,7 +39,7 @@ public abstract class LogTracker : ILogger
 
         public LogTracker(
             DiagnosticSeverity verbosity = DiagnosticSeverity.Warning,
-            IEnumerable<int> noWarn = null,
+            IEnumerable<int>? noWarn = null,
             int lineNrOffset = 0)
         {
             this.Verbosity = verbosity;
@@ -75,7 +75,7 @@ protected internal virtual void OnException(Exception ex) =>
         /// Logs a diagnostic message based on the given error code,
         /// with the given source as the file for which the error occurred.
         /// </summary>
-        public void Log(ErrorCode code, IEnumerable<string> args, string source = null, LSP.Range range = null) =>
+        public void Log(ErrorCode code, IEnumerable<string> args, string? source = null, LSP.Range? range = null) =>
             this.Log(new Diagnostic
             {
                 Severity = DiagnosticSeverity.Error,
@@ -89,7 +89,7 @@ public void Log(ErrorCode code, IEnumerable<string> args, string source = null,
         /// Logs a a diagnostic message based on the given warning code,
         /// with the given source as the file for which the error occurred.
         /// </summary>
-        public void Log(WarningCode code, IEnumerable<string> args, string source = null, LSP.Range range = null) =>
+        public void Log(WarningCode code, IEnumerable<string> args, string? source = null, LSP.Range? range = null) =>
             this.Log(new Diagnostic
             {
                 Severity = DiagnosticSeverity.Warning,
@@ -104,7 +104,7 @@ public void Log(WarningCode code, IEnumerable<string> args, string source = null
         /// with any message parameters appended on a new line to the message defined by the information code.
         /// The given source is listed as the file for which the error occurred.
         /// </summary>
-        public void Log(InformationCode code, IEnumerable<string> args, string source = null, LSP.Range range = null, params string[] messageParam) =>
+        public void Log(InformationCode code, IEnumerable<string> args, string? source = null, LSP.Range? range = null, params string[] messageParam) =>
             this.Log(new Diagnostic
             {
                 Severity = DiagnosticSeverity.Information,
@@ -140,7 +140,7 @@ public void Log(params Diagnostic[] messages)
         /// Calls Print on the given diagnostic if the verbosity is sufficiently high.
         /// Does nothing if the given diagnostic is null.
         /// </summary>
-        private void Output(Diagnostic msg)
+        private void Output(Diagnostic? msg)
         {
             if (msg?.Severity <= this.Verbosity)
             {
@@ -198,7 +198,7 @@ public void Log(Exception ex)
 
     public static class Formatting
     {
-        public static IEnumerable<string> Indent(params string[] items) =>
+        public static IEnumerable<string>? Indent(params string[] items) =>
             items?.Select(msg => $"    {msg}");
 
         /// <summary>
diff --git a/src/QsCompiler/Compiler/PluginInterface.cs b/src/QsCompiler/Compiler/PluginInterface.cs
index 46ecb3f6fa..83d007a10f 100644
--- a/src/QsCompiler/Compiler/PluginInterface.cs
+++ b/src/QsCompiler/Compiler/PluginInterface.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using Microsoft.CodeAnalysis;
 using Microsoft.Quantum.QsCompiler.CompilationBuilder;
 using Microsoft.Quantum.QsCompiler.DataTypes;
@@ -67,13 +68,13 @@ public struct Diagnostic
             /// <summary>
             /// Diagnostic message to be displayed to the user.
             /// </summary>
-            public string Message { get; set; }
+            public string? Message { get; set; }
 
             /// <summary>
             /// Absolute path of the file where the code that caused the generation of the diagnostic is located.
             /// The source is null if the diagnostic is not caused by a piece of source code.
             /// </summary>
-            public string Source { get; set; }
+            public string? Source { get; set; }
 
             /// <summary>
             /// The stage during which the diagnostic was generated.
@@ -85,13 +86,13 @@ public struct Diagnostic
             /// Zero-based range in the source file of the code that caused the generation of the diagnostic.
             /// The range is null if the diagnostic is not caused by a piece of source code.
             /// </summary>
-            public Range Range { get; set; }
+            public Range? Range { get; set; }
 
             /// <summary>
             /// Initializes a new diagnostic.
             /// If a diagnostic generated by the Q# compiler is given as argument, the values are initialized accordingly.
             /// </summary>
-            public static Diagnostic Create(VS.Diagnostic d = null, Stage stage = Stage.Unknown) =>
+            public static Diagnostic Create(VS.Diagnostic? d = null, Stage stage = Stage.Unknown) =>
                 d == null ? default : new Diagnostic
                 {
                     Severity = d.Severity switch
@@ -123,7 +124,7 @@ public static Diagnostic Create(VS.Diagnostic d = null, Stage stage = Stage.Unkn
         /// Dictionary that will be populated by the Q# compiler when the rewrite step is loaded.
         /// It contains the assembly constants for the Q# compilation unit on which the rewrite step is acting.
         /// </summary>
-        public IDictionary<string, string> AssemblyConstants { get; }
+        public IDictionary<string, string?> AssemblyConstants { get; }
 
         /// <summary>
         /// Contains diagnostics generated by the rewrite step and intended for display to the user.
@@ -173,7 +174,7 @@ public static Diagnostic Create(VS.Diagnostic d = null, Stage stage = Stage.Unkn
         /// <param name="compilation">Q# compilation that satisfies the implemented precondition, if any.</param>
         /// <param name="transformed">Q# compilation after transformation. This value should not be null if the transformation succeeded.</param>
         /// <returns>Whether or not the transformation succeeded.</returns>
-        public bool Transformation(QsCompilation compilation, out QsCompilation transformed);
+        public bool Transformation(QsCompilation compilation, [NotNullWhen(true)] out QsCompilation? transformed);
 
         /// <summary>
         /// Verifies whether a given compilation satisfies the postcondition after executing the implemented transformation (if any).
diff --git a/src/QsCompiler/Compiler/Process.cs b/src/QsCompiler/Compiler/Process.cs
index 0bce9973a8..149f3f28c2 100644
--- a/src/QsCompiler/Compiler/Process.cs
+++ b/src/QsCompiler/Compiler/Process.cs
@@ -16,7 +16,7 @@ public static class ProcessRunner
         /// Returns true if the process completed within the specified time without throwing an exception, and false otherwise.
         /// Any thrown exception is returned as out parameter.
         /// </summary>
-        public static bool Run(Process process, StringBuilder output, StringBuilder error, out Exception ex, int timeout)
+        public static bool Run(Process process, StringBuilder output, StringBuilder error, out Exception? ex, int timeout)
         {
             using (var outputWaitHandle = new AutoResetEvent(false))
             using (var errorWaitHandle = new AutoResetEvent(false))
@@ -86,8 +86,8 @@ public static bool Run(
             out StringBuilder outstream,
             out StringBuilder errstream,
             out int exitCode,
-            out Exception ex,
-            IDictionary<string, string> envVariables = null,
+            out Exception? ex,
+            IDictionary<string, string>? envVariables = null,
             int timeout = 10000)
         {
             var process = new Process();
diff --git a/src/QsCompiler/Compiler/RewriteSteps/ClassicallyControlled.cs b/src/QsCompiler/Compiler/RewriteSteps/ClassicallyControlled.cs
index 01e2b236cf..77462e531a 100644
--- a/src/QsCompiler/Compiler/RewriteSteps/ClassicallyControlled.cs
+++ b/src/QsCompiler/Compiler/RewriteSteps/ClassicallyControlled.cs
@@ -19,9 +19,9 @@ internal class ClassicallyControlled : IRewriteStep
 
         public int Priority => RewriteStepPriorities.ControlFlowSubstitutions;
 
-        public IDictionary<string, string> AssemblyConstants { get; }
+        public IDictionary<string, string?> AssemblyConstants { get; }
 
-        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => null;
+        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => Enumerable.Empty<IRewriteStep.Diagnostic>();
 
         public bool ImplementsPreconditionVerification => true;
 
@@ -31,7 +31,7 @@ internal class ClassicallyControlled : IRewriteStep
 
         public ClassicallyControlled()
         {
-            this.AssemblyConstants = new Dictionary<string, string>();
+            this.AssemblyConstants = new Dictionary<string, string?>();
         }
 
         public bool PreconditionVerification(QsCompilation compilation)
diff --git a/src/QsCompiler/Compiler/RewriteSteps/ConjugationInlining.cs b/src/QsCompiler/Compiler/RewriteSteps/ConjugationInlining.cs
index 6de6fd5167..c4af9007b5 100644
--- a/src/QsCompiler/Compiler/RewriteSteps/ConjugationInlining.cs
+++ b/src/QsCompiler/Compiler/RewriteSteps/ConjugationInlining.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.Quantum.QsCompiler.SyntaxTree;
 
 namespace Microsoft.Quantum.QsCompiler.BuiltInRewriteSteps
@@ -15,9 +16,9 @@ internal class ConjugationInlining : IRewriteStep
 
         public int Priority => RewriteStepPriorities.InliningOfConjugations;
 
-        public IDictionary<string, string> AssemblyConstants { get; }
+        public IDictionary<string, string?> AssemblyConstants { get; }
 
-        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => null;
+        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => Enumerable.Empty<IRewriteStep.Diagnostic>();
 
         public bool ImplementsPreconditionVerification => false;
 
@@ -27,7 +28,7 @@ internal class ConjugationInlining : IRewriteStep
 
         public ConjugationInlining()
         {
-            this.AssemblyConstants = new Dictionary<string, string>();
+            this.AssemblyConstants = new Dictionary<string, string?>();
         }
 
         public bool PreconditionVerification(QsCompilation compilation)
diff --git a/src/QsCompiler/Compiler/RewriteSteps/FullPreEvaluation.cs b/src/QsCompiler/Compiler/RewriteSteps/FullPreEvaluation.cs
index 4c08854840..0dc7e5c83b 100644
--- a/src/QsCompiler/Compiler/RewriteSteps/FullPreEvaluation.cs
+++ b/src/QsCompiler/Compiler/RewriteSteps/FullPreEvaluation.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.Quantum.QsCompiler.SyntaxTree;
 
 namespace Microsoft.Quantum.QsCompiler.BuiltInRewriteSteps
@@ -15,9 +16,9 @@ internal class FullPreEvaluation : IRewriteStep
 
         public int Priority => RewriteStepPriorities.EvaluationOfClassicalComputations;
 
-        public IDictionary<string, string> AssemblyConstants { get; }
+        public IDictionary<string, string?> AssemblyConstants { get; }
 
-        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => null;
+        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => Enumerable.Empty<IRewriteStep.Diagnostic>();
 
         public bool ImplementsPreconditionVerification => false;
 
@@ -27,7 +28,7 @@ internal class FullPreEvaluation : IRewriteStep
 
         public FullPreEvaluation()
         {
-            this.AssemblyConstants = new Dictionary<string, string>();
+            this.AssemblyConstants = new Dictionary<string, string?>();
         }
 
         public bool PreconditionVerification(QsCompilation compilation)
diff --git a/src/QsCompiler/Compiler/RewriteSteps/FunctorGeneration.cs b/src/QsCompiler/Compiler/RewriteSteps/FunctorGeneration.cs
index 1903dded15..9d3b958ac4 100644
--- a/src/QsCompiler/Compiler/RewriteSteps/FunctorGeneration.cs
+++ b/src/QsCompiler/Compiler/RewriteSteps/FunctorGeneration.cs
@@ -16,9 +16,9 @@ internal class FunctorGeneration : IRewriteStep
 
         public int Priority => RewriteStepPriorities.GenerationOfFunctorSupport;
 
-        public IDictionary<string, string> AssemblyConstants { get; }
+        public IDictionary<string, string?> AssemblyConstants { get; }
 
-        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => null;
+        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => Enumerable.Empty<IRewriteStep.Diagnostic>();
 
         public bool ImplementsPreconditionVerification => true;
 
@@ -28,7 +28,7 @@ internal class FunctorGeneration : IRewriteStep
 
         public FunctorGeneration()
         {
-            this.AssemblyConstants = new Dictionary<string, string>();
+            this.AssemblyConstants = new Dictionary<string, string?>();
         }
 
         public bool PreconditionVerification(QsCompilation compilation)
diff --git a/src/QsCompiler/Compiler/RewriteSteps/IntrinsicResolution.cs b/src/QsCompiler/Compiler/RewriteSteps/IntrinsicResolution.cs
index 292a86b01f..6e9f4de3d7 100644
--- a/src/QsCompiler/Compiler/RewriteSteps/IntrinsicResolution.cs
+++ b/src/QsCompiler/Compiler/RewriteSteps/IntrinsicResolution.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.Quantum.QsCompiler.SyntaxTree;
 using Microsoft.Quantum.QsCompiler.Transformations.IntrinsicResolution;
 
@@ -16,9 +17,9 @@ internal class IntrinsicResolution : IRewriteStep
 
         public int Priority => 10; // Not used for built-in transformations like this
 
-        public IDictionary<string, string> AssemblyConstants { get; }
+        public IDictionary<string, string?> AssemblyConstants { get; }
 
-        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => null;
+        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => Enumerable.Empty<IRewriteStep.Diagnostic>();
 
         public bool ImplementsPreconditionVerification => false;
 
@@ -30,7 +31,7 @@ internal class IntrinsicResolution : IRewriteStep
 
         public IntrinsicResolution(QsCompilation environment)
         {
-            this.AssemblyConstants = new Dictionary<string, string>();
+            this.AssemblyConstants = new Dictionary<string, string?>();
             this.Environment = environment;
         }
 
diff --git a/src/QsCompiler/Compiler/RewriteSteps/Monomorphization.cs b/src/QsCompiler/Compiler/RewriteSteps/Monomorphization.cs
index 62be6f0a9f..1c4b387e16 100644
--- a/src/QsCompiler/Compiler/RewriteSteps/Monomorphization.cs
+++ b/src/QsCompiler/Compiler/RewriteSteps/Monomorphization.cs
@@ -18,9 +18,9 @@ internal class Monomorphization : IRewriteStep
 
         public int Priority => RewriteStepPriorities.TypeParameterElimination;
 
-        public IDictionary<string, string> AssemblyConstants { get; }
+        public IDictionary<string, string?> AssemblyConstants { get; }
 
-        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => null;
+        public IEnumerable<IRewriteStep.Diagnostic> GeneratedDiagnostics => Enumerable.Empty<IRewriteStep.Diagnostic>();
 
         public bool ImplementsPreconditionVerification => true;
 
@@ -30,7 +30,7 @@ internal class Monomorphization : IRewriteStep
 
         public Monomorphization()
         {
-            this.AssemblyConstants = new Dictionary<string, string>();
+            this.AssemblyConstants = new Dictionary<string, string?>();
         }
 
         public bool PreconditionVerification(QsCompilation compilation) => compilation.EntryPoints.Any();
diff --git a/src/QsCompiler/Transformations/Properties/AssemblyInfo.cs b/src/QsCompiler/Transformations/Properties/AssemblyInfo.cs
index c1001398a0..73d743a24a 100644
--- a/src/QsCompiler/Transformations/Properties/AssemblyInfo.cs
+++ b/src/QsCompiler/Transformations/Properties/AssemblyInfo.cs
@@ -4,5 +4,6 @@
 using System.Runtime.CompilerServices;
 
 [assembly: InternalsVisibleTo("Microsoft.Quantum.QsCompilationManager" + SigningConstants.PublicKey)]
+[assembly: InternalsVisibleTo("Microsoft.Quantum.QsCompiler" + SigningConstants.PublicKey)]
 [assembly: InternalsVisibleTo("Microsoft.Quantum.QsLanguageServer" + SigningConstants.PublicKey)]
 [assembly: InternalsVisibleTo("Tests.Microsoft.Quantum.QsCompiler" + SigningConstants.PublicKey)]