Do not report unused using/imports if not processing doc comments.

Also silently process doc comments when parsing them so we can report unused usings.
Fixes dotnet#2773

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -1890,7 +1890,7 @@ internal override void ReportUnusedImports(SyntaxTree filterTree, DiagnosticBag
                     cancellationToken.ThrowIfCancellationRequested();
 
                     SyntaxTree infoTree = info.Tree;
-                    if (filterTree == null || filterTree == infoTree)
+                    if ((filterTree == null || filterTree == infoTree) && infoTree.Options.DocumentationMode != DocumentationMode.None)
                     {
                         TextSpan infoSpan = info.Span;
                         if (!this.IsImportDirectiveUsed(infoTree, infoSpan.Start))

src/Compilers/CSharp/Portable/Compiler/DocumentationCommentCompiler.cs

@@ -392,8 +392,6 @@ private bool TryProcessDocumentationCommentTriviaNodes(
         {
             Debug.Assert(!docCommentNodes.IsDefaultOrEmpty);
 
-            bool haveWriter = _writer != null;
-
             bool processedDocComment = false; // Even if there are DocumentationCommentTriviaSyntax, we may not need to process any of them.
 
             ArrayBuilder<CSharpSyntaxNode> includeElementNodesBuilder = null;
@@ -413,13 +411,6 @@ private bool TryProcessDocumentationCommentTriviaNodes(
 
                 bool reportDiagnosticsForCurrentTrivia = trivia.SyntaxTree.ReportDocumentationCommentDiagnostics();
 
-                // If we're writing XML or we need to report diagnostics (either in this particular piece of trivia,
-                // or concerning undocumented [type] parameters), then we need to process this trivia node.
-                if (!(haveWriter || reportDiagnosticsForCurrentTrivia || reportParameterOrTypeParameterDiagnostics))
-                {
-                    continue;
-                }
-
                 if (!processedDocComment)
                 {
                     // Since we have to throw away all the parts if any part is bad, we need to write to an intermediate temp.

src/Compilers/CSharp/Test/Symbol/Compilation/GetUnusedImportDirectivesTests.cs

@@ -329,12 +329,8 @@ public void CrefCountsAsUse()
 /// <see cref='Console'/>
 public class C { }
 ";
-
-            // Not binding doc comments.
-            CreateCompilation(source).VerifyDiagnostics(
-                // (2,1): info CS8019: Unnecessary using directive.
-                // using System;
-                Diagnostic(ErrorCode.HDN_UnusedUsingDirective, "using System;"));
+            // Not reporting doc comment diagnostics. It is still a use.
+            CreateCompilation(source).VerifyDiagnostics();
 
             // Binding doc comments.
             CreateCompilationWithMscorlib40AndDocumentationComments(source).VerifyDiagnostics();
@@ -430,5 +426,34 @@ partial void M(ref int x) { }
                 //Diagnostic(ErrorCode.HDN_UnusedUsingDirective, "using System.Runtime.InteropServices;").WithLocation(1, 1)
                 );
         }
+
+        [Fact, WorkItem(2773, "https://github.com/dotnet/roslyn/issues/2773")]
+        public void UsageInDocComment()
+        {
+            var source = @"using X;
+
+/// <summary/>
+public class Program
+{
+    /// <summary>
+    /// <see cref=""Q""/>
+    /// </summary>
+    static void Main(string[] args)
+    {
+    }
+}
+
+namespace X
+{
+    /// <summary/>
+    public class Q { }
+}
+";
+            foreach (DocumentationMode documentationMode in Enum.GetValues(typeof(DocumentationMode)))
+            {
+                var compilation = CreateCompilation(source, parseOptions: TestOptions.Regular.WithDocumentationMode(documentationMode));
+                compilation.VerifyDiagnostics();
+            }
+        }
     }
 }

src/Compilers/CSharp/Test/Symbol/DocumentationComments/CrefTests.cs

@@ -4369,17 +4369,17 @@ public partial class D { }
 public partial class E { }
 ";
 
-            var tree1 = Parse(source1, options: TestOptions.RegularWithDocumentationComments.WithLanguageVersion(LanguageVersion.Latest));
-            var tree2 = Parse(source2, options: TestOptions.Regular.WithLanguageVersion(LanguageVersion.Latest));
+            var tree1 = Parse(source1, options: TestOptions.Regular.WithDocumentationMode(DocumentationMode.Diagnose).WithLanguageVersion(LanguageVersion.Latest));
+            var tree2 = Parse(source2, options: TestOptions.Regular.WithDocumentationMode(DocumentationMode.None).WithLanguageVersion(LanguageVersion.Latest));
 
             // This scenario does not exist in dev11, but the diagnostics seem reasonable.
             CreateCompilation(new[] { tree1, tree2 }).VerifyDiagnostics(
                 // (5,22): warning CS1591: Missing XML comment for publicly visible type or member 'D'
                 // public partial class D { }
-                Diagnostic(ErrorCode.WRN_MissingXMLComment, "D").WithArguments("D"),
+                Diagnostic(ErrorCode.WRN_MissingXMLComment, "D").WithArguments("D").WithLocation(5, 22),
                 // (7,22): warning CS1591: Missing XML comment for publicly visible type or member 'E'
                 // public partial class E { }
-                Diagnostic(ErrorCode.WRN_MissingXMLComment, "E").WithArguments("E"));
+                Diagnostic(ErrorCode.WRN_MissingXMLComment, "E").WithArguments("E").WithLocation(7, 22));
         }
 
         [Fact]

src/Compilers/Test/Utilities/CSharp/TestOptions.cs

@@ -8,9 +8,9 @@ namespace Microsoft.CodeAnalysis.CSharp.Test.Utilities
 {
     public static class TestOptions
     {
-        // Disable documentation comments by default so that we don't need to
+        // Disable diagnosing documentation comments by default so that we don't need to
         // document every public member of every test input.
-        public static readonly CSharpParseOptions Regular = new CSharpParseOptions(kind: SourceCodeKind.Regular, documentationMode: DocumentationMode.None).WithLanguageVersion(LanguageVersion.Latest);
+        public static readonly CSharpParseOptions Regular = new CSharpParseOptions(kind: SourceCodeKind.Regular, documentationMode: DocumentationMode.Parse).WithLanguageVersion(LanguageVersion.Latest);
         public static readonly CSharpParseOptions Script = Regular.WithKind(SourceCodeKind.Script);
         public static readonly CSharpParseOptions Regular6 = Regular.WithLanguageVersion(LanguageVersion.CSharp6);
         public static readonly CSharpParseOptions Regular7 = Regular.WithLanguageVersion(LanguageVersion.CSharp7);

src/Compilers/VisualBasic/Portable/Compilation/VisualBasicCompilation.vb

@@ -1585,7 +1585,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                     cancellationToken.ThrowIfCancellationRequested()
 
                     Dim infoTree As SyntaxTree = info.Tree
-                    If filterTree Is Nothing OrElse filterTree Is infoTree Then
+                    If (filterTree Is Nothing OrElse filterTree Is infoTree) AndAlso infoTree.Options.DocumentationMode <> DocumentationMode.None Then
                         Dim clauseSpans = info.ClauseSpans
                         Dim numClauseSpans = clauseSpans.Length
 

src/Compilers/VisualBasic/Test/Semantic/Semantics/GetUnusedImportDirectivesTests.vb

@@ -281,19 +281,17 @@ Imports System
 ''' <see cref='Console'/>
 Class A
 End Class
-    ]]></file>
+]]></file>
 </compilation>
 
             ' Without doc comments.
-            CreateCompilationWithMscorlib40(source, parseOptions:=New VisualBasicParseOptions(documentationMode:=DocumentationMode.None)).AssertTheseDiagnostics(
-                <errors>
-BC50001: Unused import statement.
-Imports System
-~~~~~~~~~~~~~~
-                </errors>, suppressInfos:=False)
+            CreateCompilationWithMscorlib40(source, parseOptions:=New VisualBasicParseOptions(documentationMode:=DocumentationMode.None)).AssertNoDiagnostics(suppressInfos:=False)
+
+            ' With doc comments parsed.
+            CreateCompilationWithMscorlib40(source, parseOptions:=New VisualBasicParseOptions(documentationMode:=DocumentationMode.Parse)).AssertNoDiagnostics(suppressInfos:=False)
 
-            ' With doc comments.
-            CreateCompilationWithMscorlib40(source, parseOptions:=New VisualBasicParseOptions(documentationMode:=DocumentationMode.Diagnose)).AssertTheseDiagnostics(<errors></errors>, suppressInfos:=False)
+            ' With doc comments diagnosed.
+            CreateCompilationWithMscorlib40(source, parseOptions:=New VisualBasicParseOptions(documentationMode:=DocumentationMode.Diagnose)).AssertNoDiagnostics(suppressInfos:=False)
         End Sub
 
         <Fact>
@@ -366,5 +364,36 @@ End Class
             Dim diagnostics = model.GetDiagnostics()
             AssertTheseDiagnostics(diagnostics, <errors></errors>)
         End Sub
+
+        <Fact, WorkItem(2773, "https://github.com/dotnet/roslyn/issues/2773")>
+        Public Sub UsageInDocComment()
+            Dim source =
+<compilation>
+    <file name="a.vb"><![CDATA[
+Imports X
+
+''' <summary/>
+Public Class Program
+
+    ''' <summary>
+    ''' <see cref="Q"/>
+    ''' </summary>
+    Public Sub Main()
+    End Sub
+
+End Class
+
+Namespace Global.X
+    ''' <summary/>
+    Public Class Q
+    End Class
+End Namespace
+]]></file>
+</compilation>
+            For Each documentationMode As DocumentationMode In [Enum].GetValues(GetType(DocumentationMode))
+                Dim compilation = CreateCompilationWithMscorlib40(source, parseOptions:=TestOptions.Regular.WithDocumentationMode(documentationMode))
+                compilation.AssertNoDiagnostics(suppressInfos:=False)
+            Next
+        End Sub
     End Class
 End Namespace

src/EditorFeatures/CSharpTest/RemoveUnnecessaryImports/RemoveUnnecessaryImportsTests.cs

@@ -1307,19 +1307,23 @@ public static void Main()
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnnecessaryImports)]
         public async Task TestReferenceInCref()
         {
-            // parsing doc comments as simple trivia; System is unnecessary
-            await TestInRegularAndScriptAsync(
+            // parsing doc comments as simple trivia; we don't know System is unnecessary
+            await TestMissingAsync(
 @"[|using System;
 /// <summary><see cref=""String"" /></summary>
 class C
 {
-}|]",
-@"/// <summary><see cref=""String"" /></summary>
+}|]", new TestParameters(Options.Regular.WithDocumentationMode(DocumentationMode.None)));
+
+            // fully parsing doc comments; System is necessary
+            await TestMissingAsync(
+@"[|using System;
+/// <summary><see cref=""String"" /></summary>
 class C
 {
-}");
+}|]", new TestParameters(Options.Regular.WithDocumentationMode(DocumentationMode.Parse)));
 
-            // fully parsing doc comments; System is necessary
+            // fully parsing and diagnosing doc comments; System is necessary
             await TestMissingAsync(
 @"[|using System;
 /// <summary><see cref=""String"" /></summary>