Reduce code duplication.

src/Compilers/CSharp/Portable/Binder/Binder_Lookup.cs

@@ -1504,17 +1504,7 @@ private void AddMemberLookupSymbolsInfoInSubmissions(LookupSymbolsInfo result, T
                 {
                     foreach (var usingAlias in usingAliases)
                     {
-                        var aliasSymbol = usingAlias.Alias;
-                        var targetSymbol = aliasSymbol.GetAliasTarget(basesBeingResolved: null);
-                        if (originalBinder.CanAddLookupSymbolInfo(targetSymbol, options, null))
-                        {
-                            // Note: we do not mark nodes when looking up arities or names.  This is because these two
-                            // types of lookup are only around to make the public
-                            // SemanticModel.LookupNames/LookupSymbols work and do not count as usages of the directives
-                            // when the actual code is bound.
-
-                            result.AddSymbol(aliasSymbol, aliasSymbol.Name, 0);
-                        }
+                        AddAliasSymbolToResult(result, usingAlias.Alias, options, originalBinder);
                     }
                 }
 
@@ -1525,6 +1515,21 @@ private void AddMemberLookupSymbolsInfoInSubmissions(LookupSymbolsInfo result, T
             }
         }
 
+        /// <remarks>
+        /// Note: we do not mark nodes when looking up arities or names.  This is because these two
+        /// types of lookup are only around to make the public
+        /// SemanticModel.LookupNames/LookupSymbols work and do not count as usages of the directives
+        /// when the actual code is bound.
+        /// </remarks>
+        internal static void AddAliasSymbolToResult(LookupSymbolsInfo result, AliasSymbol aliasSymbol, LookupOptions options, Binder originalBinder)
+        {
+            var targetSymbol = aliasSymbol.GetAliasTarget(basesBeingResolved: null);
+            if (originalBinder.CanAddLookupSymbolInfo(targetSymbol, options, null))
+            {
+                result.AddSymbol(aliasSymbol, aliasSymbol.Name, 0);
+            }
+        }
+
         private static void AddMemberLookupSymbolsInfoInNamespace(LookupSymbolsInfo result, NamespaceSymbol ns, LookupOptions options, Binder originalBinder)
         {
             foreach (var symbol in GetCandidateMembers(ns, options, originalBinder))

src/Compilers/CSharp/Portable/Binder/Imports.cs

@@ -318,7 +318,8 @@ private void MarkImportDirective(CSharpSyntaxNode directive, bool callerIsSemant
 
         private static void MarkImportDirective(CSharpCompilation compilation, CSharpSyntaxNode directive, bool callerIsSemanticModel)
         {
-            if (directive != null && compilation != null && !callerIsSemanticModel)
+            Debug.Assert(compilation != null); // If any directives are used, then there must be a compilation.
+            if (directive != null && !callerIsSemanticModel)
             {
                 compilation.MarkImportDirectiveAsUsed(directive);
             }
@@ -598,25 +599,12 @@ internal void AddLookupSymbolsInfoInAliases(Binder originalBinder, LookupSymbols
         {
             foreach (var usingAlias in this.UsingAliases.Values)
             {
-                var usingAliasSymbol = usingAlias.Alias;
-                var usingAliasTargetSymbol = usingAliasSymbol.GetAliasTarget(basesBeingResolved: null);
-                if (originalBinder.CanAddLookupSymbolInfo(usingAliasTargetSymbol, options, null))
-                {
-                    result.AddSymbol(usingAliasSymbol, usingAliasSymbol.Name, 0);
-                }
+                Binder.AddAliasSymbolToResult(result, usingAlias.Alias, options, originalBinder);
             }
 
-            if (this.ExternAliases != null)
+            foreach (var externAlias in this.ExternAliases)
             {
-                foreach (var externAlias in this.ExternAliases)
-                {
-                    var externAliasSymbol = externAlias.Alias;
-                    var externAliasTargetSymbol = externAliasSymbol.GetAliasTarget(basesBeingResolved: null);
-                    if (originalBinder.CanAddLookupSymbolInfo(externAliasTargetSymbol, options, null))
-                    {
-                        result.AddSymbol(externAliasSymbol, externAliasSymbol.Name, 0);
-                    }
-                }
+                Binder.AddAliasSymbolToResult(result, externAlias.Alias, options, originalBinder);
             }
         }
 

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

@@ -1220,26 +1220,6 @@ private ImmutableArray<NamespaceOrTypeAndUsingDirective> BindExternalUsings()
             var externalUsings = ArrayBuilder<NamespaceOrTypeAndUsingDirective>.GetInstance();
             var uniqueTargets = PooledHashSet<NamespaceOrTypeSymbol>.GetInstance();
 
-            foreach (var previousUsing in previousSubmission.ExternalUsings)
-            {
-                var previousTarget = previousUsing.NamespaceOrType;
-                if (previousTarget.IsType)
-                {
-                    if (uniqueTargets.Add(previousTarget))
-                    {
-                        externalUsings.Add(previousUsing);
-                    }
-                }
-                else
-                { 
-                    var expandedNamespace = ExpandPreviousSubmissionNamespace((NamespaceSymbol)previousTarget, this.GlobalNamespace);
-                    if (uniqueTargets.Add(expandedNamespace))
-                    {
-                        externalUsings.Add(new NamespaceOrTypeAndUsingDirective(expandedNamespace, previousUsing.UsingDirective));
-                    }
-                }
-            }
-
             // The previous submission should not have had extern aliases (unsupported)
             // and using aliases are handled separately (during lookup) so we only need
             // to extract the regular usings.
@@ -1258,7 +1238,7 @@ private ImmutableArray<NamespaceOrTypeAndUsingDirective> BindExternalUsings()
             //}
             //else
             {
-                foreach (var previousUsing in previousSubmission.SubmissionImports.Usings)
+                foreach (var previousUsing in previousSubmission.ExternalUsings.Concat(previousSubmission.SubmissionImports.Usings))
                 {
                     var previousTarget = previousUsing.NamespaceOrType;
                     if (previousTarget.IsType)
@@ -1324,7 +1304,7 @@ private static NamespaceSymbol ExpandPreviousSubmissionNamespace(NamespaceSymbol
         /// <remarks>
         /// Always consider whether usings should be considered before accessing this (e.g. not when binding other usings).
         /// </remarks>
-        internal Dictionary<string, AliasAndUsingDirective> SubmissionUsingAliases => SubmissionImports?.UsingAliases;
+        internal ImmutableDictionary<string, AliasAndUsingDirective> SubmissionUsingAliases => SubmissionImports?.UsingAliases;
 
         private Imports SubmissionImports
         {