Add nullable helpers and fix null warnings

src/OmniSharp.Roslyn.CSharp/Services/Completion/CompletionService.cs

@@ -23,6 +23,7 @@
 using CompletionItem = OmniSharp.Models.v1.Completion.CompletionItem;
 using CompletionTriggerKind = OmniSharp.Models.v1.Completion.CompletionTriggerKind;
 using CSharpCompletionService = Microsoft.CodeAnalysis.Completion.CompletionService;
+using Roslyn.Utilities;
 
 namespace OmniSharp.Roslyn.CSharp.Services.Completion
 {
@@ -66,8 +67,8 @@ public async Task<CompletionResponse> Handle(CompletionRequest request)
             var position = sourceText.GetTextPosition(request);
 
             var completionService = CSharpCompletionService.GetService(document);
-            Debug.Assert(completionService != null);
-            Debug.Assert(request.TriggerCharacter != null || request.CompletionTrigger != CompletionTriggerKind.TriggerCharacter);
+            RoslynDebug.Assert(completionService != null);
+            RoslynDebug.Assert(request.TriggerCharacter != null || request.CompletionTrigger != CompletionTriggerKind.TriggerCharacter);
 
             CompletionTrigger trigger = request.CompletionTrigger switch
             {
@@ -173,10 +174,10 @@ public async Task<CompletionResolveResponse> Handle(CompletionResolveRequest req
             }
 
             var completionService = CSharpCompletionService.GetService(document);
-            Debug.Assert(completionService != null);
+            RoslynDebug.Assert(completionService != null);
 
             var description = await completionService.GetDescriptionAsync(document, lastCompletionItem);
-            Debug.Assert(description != null);
+            RoslynDebug.Assert(description != null);
 
             var textBuilder = new StringBuilder();
             MarkdownHelpers.TaggedTextToMarkdown(description.TaggedParts, textBuilder, _formattingOptions, MarkdownFormat.FirstLineAsCSharp, out _);
@@ -250,7 +251,7 @@ public async Task<CompletionAfterInsertResponse> Handle(CompletionAfterInsertReq
             }
 
             var completionService = CSharpCompletionService.GetService(document);
-            Debug.Assert(completionService != null);
+            RoslynDebug.Assert(completionService != null);
 
             // Get a document with change from the completion inserted, so that we can resolve the completion and get the
             // final full change.

src/OmniSharp.Roslyn/Utilities/Debug.cs

@@ -0,0 +1,28 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+
+namespace Roslyn.Utilities
+{
+    internal static class RoslynDebug
+    {
+        /// <inheritdoc cref="Debug.Assert(bool)"/>
+        [Conditional("DEBUG")]
+        public static void Assert([DoesNotReturnIf(false)] bool b) => Debug.Assert(b);
+
+        /// <inheritdoc cref="Debug.Assert(bool, string)"/>
+        [Conditional("DEBUG")]
+        public static void Assert([DoesNotReturnIf(false)] bool b, string message)
+            => Debug.Assert(b, message);
+
+        [Conditional("DEBUG")]
+        public static void AssertNotNull<T>([NotNull] T value)
+        {
+            Assert(value is object, "Unexpected null reference");
+        }
+    }
+}

src/OmniSharp.Roslyn/Utilities/NullableAttributes.cs

@@ -0,0 +1,150 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// This was copied from https://github.com/dotnet/runtime/blob/39b9607807f29e48cae4652cd74735182b31182e/src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
+// and updated to have the scope of the attributes be internal.
+namespace System.Diagnostics.CodeAnalysis
+{
+#if !NETCOREAPP
+
+    /// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+    internal sealed class AllowNullAttribute : Attribute { }
+
+    /// <summary>Specifies that null is disallowed as an input even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+    internal sealed class DisallowNullAttribute : Attribute { }
+
+    /// <summary>Specifies that an output may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+    internal sealed class MaybeNullAttribute : Attribute { }
+
+    /// <summary>Specifies that an output will not be null even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+    internal sealed class NotNullAttribute : Attribute { }
+
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class MaybeNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter may be null.
+        /// </param>
+        public MaybeNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter will not be null even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class NotNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter will not be null.
+        /// </param>
+        public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+
+    /// <summary>Specifies that the output will be non-null if the named parameter is non-null.</summary>
+    [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)]
+    internal sealed class NotNullIfNotNullAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the associated parameter name.</summary>
+        /// <param name="parameterName">
+        /// The associated parameter name.  The output will be non-null if the argument to the parameter specified is non-null.
+        /// </param>
+        public NotNullIfNotNullAttribute(string parameterName) => ParameterName = parameterName;
+
+        /// <summary>Gets the associated parameter name.</summary>
+        public string ParameterName { get; }
+    }
+
+    /// <summary>Applied to a method that will never return under any circumstance.</summary>
+    [AttributeUsage(AttributeTargets.Method, Inherited = false)]
+    internal sealed class DoesNotReturnAttribute : Attribute { }
+
+    /// <summary>Specifies that the method will not return if the associated Boolean parameter is passed the specified value.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class DoesNotReturnIfAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified parameter value.</summary>
+        /// <param name="parameterValue">
+        /// The condition parameter value. Code after the method will be considered unreachable by diagnostics if the argument to
+        /// the associated parameter matches this value.
+        /// </param>
+        public DoesNotReturnIfAttribute(bool parameterValue) => ParameterValue = parameterValue;
+
+        /// <summary>Gets the condition parameter value.</summary>
+        public bool ParameterValue { get; }
+    }
+
+#endif
+
+#if !NETCOREAPP || NETCOREAPP3_1
+
+    /// <summary>Specifies that the method or property will ensure that the listed field and property members have not-null values.</summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+    internal sealed class MemberNotNullAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with a field or property member.</summary>
+        /// <param name="member">
+        /// The field or property member that is promised to be not-null.
+        /// </param>
+        public MemberNotNullAttribute(string member) => Members = new[] { member };
+
+        /// <summary>Initializes the attribute with the list of field and property members.</summary>
+        /// <param name="members">
+        /// The list of field and property members that are promised to be not-null.
+        /// </param>
+        public MemberNotNullAttribute(params string[] members) => Members = members;
+
+        /// <summary>Gets field or property member names.</summary>
+        public string[] Members { get; }
+    }
+
+    /// <summary>Specifies that the method or property will ensure that the listed field and property members have not-null values when returning with the specified return value condition.</summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+    internal sealed class MemberNotNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition and a field or property member.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter will not be null.
+        /// </param>
+        /// <param name="member">
+        /// The field or property member that is promised to be not-null.
+        /// </param>
+        public MemberNotNullWhenAttribute(bool returnValue, string member)
+        {
+            ReturnValue = returnValue;
+            Members = new[] { member };
+        }
+
+        /// <summary>Initializes the attribute with the specified return value condition and list of field and property members.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter will not be null.
+        /// </param>
+        /// <param name="members">
+        /// The list of field and property members that are promised to be not-null.
+        /// </param>
+        public MemberNotNullWhenAttribute(bool returnValue, params string[] members)
+        {
+            ReturnValue = returnValue;
+            Members = members;
+        }
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+
+        /// <summary>Gets field or property member names.</summary>
+        public string[] Members { get; }
+    }
+
+#endif
+}