Support adding new assembly level attributes with SyntaxGenerator when some already exist in the document

src/Workspaces/CSharp/Portable/CodeGeneration/CSharpSyntaxGenerator.cs

@@ -998,11 +998,11 @@ private SyntaxNode InsertAttributesInternal(SyntaxNode declaration, int index, I
             var existingAttributes = this.GetAttributes(declaration);
             if (index >= 0 && index < existingAttributes.Count)
             {
-                return this.InsertNodesBefore(declaration, existingAttributes[index], newAttributes);
+                return this.InsertNodesBefore(declaration, existingAttributes[index], WithRequiredTargetSpecifier(newAttributes, declaration));
             }
             else if (existingAttributes.Count > 0)
             {
-                return this.InsertNodesAfter(declaration, existingAttributes[existingAttributes.Count - 1], newAttributes);
+                return this.InsertNodesAfter(declaration, existingAttributes[existingAttributes.Count - 1], WithRequiredTargetSpecifier(newAttributes, declaration));
             }
             else
             {
@@ -1059,6 +1059,16 @@ private static SyntaxList<AttributeListSyntax> AsAssemblyAttributes(IEnumerable<
                     attributes.Select(list => list.WithTarget(SyntaxFactory.AttributeTargetSpecifier(SyntaxFactory.Token(SyntaxKind.AssemblyKeyword)))));
         }
 
+        private static SyntaxList<AttributeListSyntax> WithRequiredTargetSpecifier(SyntaxList<AttributeListSyntax> attributes, SyntaxNode declaration)
+        {
+            if (!declaration.IsKind(SyntaxKind.CompilationUnit))
+            {
+                return attributes;
+            }
+
+            return AsAssemblyAttributes(attributes);
+        }
+
         public override IReadOnlyList<SyntaxNode> GetAttributeArguments(SyntaxNode attributeDeclaration)
         {
             switch (attributeDeclaration.Kind())

src/Workspaces/CSharpTest/CodeGeneration/SyntaxGeneratorTests.cs

@@ -1588,6 +1588,14 @@ public void TestAddAttributes()
                     Generator.CompilationUnit(Generator.NamespaceDeclaration("n")),
                     Generator.Attribute("a")),
                 "[assembly: a]\r\nnamespace n\r\n{\r\n}");
+
+            VerifySyntax<CompilationUnitSyntax>(
+                Generator.AddAttributes(
+                    Generator.AddAttributes(
+                        Generator.CompilationUnit(Generator.NamespaceDeclaration("n")),
+                        Generator.Attribute("a")),
+                    Generator.Attribute("b")),
+                "[assembly: a]\r\n[assembly: b]\r\nnamespace n\r\n{\r\n}");
         }
 
         [Fact]

src/Workspaces/VisualBasic/Portable/CodeGeneration/VisualBasicSyntaxGenerator.vb

@@ -1637,6 +1637,15 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.CodeGeneration
         End Function
 
         Public Overrides Function GetAttributes(declaration As SyntaxNode) As IReadOnlyList(Of SyntaxNode)
+            If declaration.IsKind(SyntaxKind.CompilationUnit) Then
+                ' CompilationUnit syntaxes represent attribute lists in a way that we can't get a single AttributeList for all of the attributes in all cases.
+                ' However, some consumers of this API assume that all returned values are children of "declaration", so if there's one attribute list, we'll use
+                ' that value directly if possible.
+                Dim compilationUnit = DirectCast(declaration, CompilationUnitSyntax)
+                If compilationUnit.Attributes.Count = 1 Then
+                    Return compilationUnit.Attributes(0).AttributeLists
+                End If
+            End If
             Return Me.Flatten(declaration.GetAttributeLists())
         End Function
 
@@ -1649,9 +1658,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.CodeGeneration
             Dim existingAttributes = Me.GetAttributes(declaration)
 
             If index >= 0 AndAlso index < existingAttributes.Count Then
-                Return Me.InsertNodesBefore(declaration, existingAttributes(index), newAttributes)
+                Return Me.InsertNodesBefore(declaration, existingAttributes(index), WithRequiredTargetSpecifier(newAttributes, declaration))
             ElseIf existingAttributes.Count > 0 Then
-                Return Me.InsertNodesAfter(declaration, existingAttributes(existingAttributes.Count - 1), newAttributes)
+                Return Me.InsertNodesAfter(declaration, existingAttributes(existingAttributes.Count - 1), WithRequiredTargetSpecifier(newAttributes, declaration))
             Else
                 Dim lists = GetAttributeLists(declaration)
                 Return Me.WithAttributeLists(declaration, lists.AddRange(AsAttributeLists(attributes)))
@@ -1678,6 +1687,13 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.CodeGeneration
             End If
         End Function
 
+        Private Function WithRequiredTargetSpecifier(attributes As SyntaxList(Of AttributeListSyntax), declaration As SyntaxNode) As SyntaxList(Of AttributeListSyntax)
+            If Not declaration.IsKind(SyntaxKind.CompilationUnit) Then
+                Return attributes
+            End If
+            Return SyntaxFactory.List(attributes.Select(AddressOf WithAssemblyTargets))
+        End Function
+
         Public Overrides Function GetReturnAttributes(declaration As SyntaxNode) As IReadOnlyList(Of SyntaxNode)
             Return Me.Flatten(GetReturnAttributeLists(declaration))
         End Function

src/Workspaces/VisualBasicTest/CodeGeneration/SyntaxGeneratorTests.vb

@@ -2181,6 +2181,18 @@ End Class")
 "<Assembly:a>
 Namespace n
 End Namespace
+")
+
+            VerifySyntax(Of CompilationUnitSyntax)(
+                Generator.AddAttributes(
+                    Generator.AddAttributes(
+                        Generator.CompilationUnit(Generator.NamespaceDeclaration("n")),
+                        Generator.Attribute("a")),
+                    Generator.Attribute("b")),
+"<Assembly:a>
+<Assembly:b>
+Namespace n
+End Namespace
 ")
 
             VerifySyntax(Of DelegateStatementSyntax)(