Stop reporting unused usings in submissions

Since they can be used in subsequent submissions, they are always
considered to be used.

Fixes #4392

src/Compilers/CSharp/Test/Emit/Emit/ScriptTests.cs

@@ -582,9 +582,6 @@ public void ErrorInUsing()
                     // (1,7): error CS0246: The type or namespace name 'Unknown' could not be found (are you missing a using directive or an assembly reference?)
                     // using Unknown;
                     Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "Unknown").WithArguments("Unknown").WithLocation(1, 7),
-                    // (1,1): hidden CS8019: Unnecessary using directive.
-                    // using Unknown;
-                    Diagnostic(ErrorCode.HDN_UnusedUsingDirective, "using Unknown;").WithLocation(1, 1),
             };
 
             // Emit produces the same diagnostics as GetDiagnostics (below).

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

@@ -358,5 +358,26 @@ public void InfoAndWarnAsError()
                 // using System;
                 Diagnostic(ErrorCode.HDN_UnusedUsingDirective, "using System;").WithWarningAsError(false));
         }
+
+        [Fact]
+        public void UnusedUsingInteractive()
+        {
+            var tree = Parse("using System;", options: TestOptions.Interactive);
+            var comp = CSharpCompilation.CreateSubmission("sub1", tree, new[] { MscorlibRef_v4_0_30316_17626 });
+
+            comp.VerifyDiagnostics();
+        }
+
+        [Fact]
+        public void UnusedUsingScript()
+        {
+            var tree = Parse("using System;", options: TestOptions.Script);
+            var comp = CreateCompilationWithMscorlib(tree);
+
+            comp.VerifyDiagnostics(
+                // (2,1): info CS8019: Unnecessary using directive.
+                // using System;
+                Diagnostic(ErrorCode.HDN_UnusedUsingDirective, "using System;"));
+        }
     }
 }

src/Compilers/CSharp/Test/Symbol/Symbols/Source/ExternAliasTests.cs

@@ -111,10 +111,7 @@ public void ExternAliasInInteractive_Error()
             comp.VerifyDiagnostics(
                 // (1,1): error CS7015: 'extern alias' is not valid in this context
                 // extern alias Bar;
-                Diagnostic(ErrorCode.ERR_ExternAliasNotAllowed, "extern alias Bar;"),
-                // (1,1): info CS8020: Unused extern alias.
-                // extern alias Bar;
-                Diagnostic(ErrorCode.HDN_UnusedExternAlias, "extern alias Bar;"));
+                Diagnostic(ErrorCode.ERR_ExternAliasNotAllowed, "extern alias Bar;"));
         }
 
         [Fact]

src/Compilers/Core/Portable/Compilation/Compilation.cs

@@ -2098,7 +2098,8 @@ internal void MarkImportDirectiveAsUsed(SyntaxNode node)
 
         internal void MarkImportDirectiveAsUsed(SyntaxTree syntaxTree, int position)
         {
-            if (syntaxTree != null)
+            // Optimization: Don't initialize TreeToUsedImportDirectivesMap in submissions.
+            if (!IsSubmission && syntaxTree != null)
             {
                 var set = TreeToUsedImportDirectivesMap.GetOrAdd(syntaxTree, s_createSetCallback);
                 set.Add(position);
@@ -2107,8 +2108,13 @@ internal void MarkImportDirectiveAsUsed(SyntaxTree syntaxTree, int position)
 
         internal bool IsImportDirectiveUsed(SyntaxTree syntaxTree, int position)
         {
-            SmallConcurrentSetOfInts usedImports;
+            if (IsSubmission)
+            {
+                // Since usings apply to subsequent submissions, we have to assume they are used.
+                return true;
+            }
 
+            SmallConcurrentSetOfInts usedImports;
             return syntaxTree != null &&
                 TreeToUsedImportDirectivesMap.TryGetValue(syntaxTree, out usedImports) &&
                 usedImports.Contains(position);

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

@@ -300,5 +300,24 @@ Imports System
             ' With doc comments.
             CreateCompilationWithMscorlib(source, parseOptions:=New VisualBasicParseOptions(documentationMode:=DocumentationMode.Diagnose)).AssertTheseDiagnostics(<errors></errors>, suppressInfos:=False)
         End Sub
+
+        <Fact()>
+        Public Sub UnusedImportInteractive()
+            Dim tree = Parse("Imports System", options:=TestOptions.Interactive)
+            Dim compilation = VisualBasicCompilation.CreateSubmission("sub1", tree, {MscorlibRef_v4_0_30316_17626})
+            compilation.AssertNoDiagnostics(suppressInfos:=False)
+        End Sub
+
+        <Fact()>
+        Public Sub UnusedImportScript()
+            Dim tree = Parse("Imports System", options:=TestOptions.Script)
+            Dim compilation = CreateCompilationWithMscorlib(tree)
+            compilation.AssertTheseDiagnostics(
+                <errors>
+BC50001: Unused import statement.
+Imports System
+~~~~~~~~~~~~~~
+                </errors>, suppressInfos:=False)
+        End Sub
     End Class
 End Namespace