Fix namespace incremental parsing bug (#37770)

There was a previous parsing change (#32999) which modified namespace
parsing to allow modifiers and attributes on namespaces, to improve
error recovery.

This PR contained a bug because it didn't move the incremental parsing
check to before parsing attributes and modifiers, which should now
be included in incremental parsing to prevent changes from being dropped

Fixes #37665, #37664, #37663

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -378,11 +378,6 @@ internal CompilationUnitSyntax ParseCompilationUnitCore()
             SyntaxListBuilder<AttributeListSyntax> attributeLists,
             SyntaxListBuilder modifiers)
         {
-            if (this.IsIncrementalAndFactoryContextMatches && this.CurrentNodeKind == SyntaxKind.NamespaceDeclaration)
-            {
-                return (NamespaceDeclarationSyntax)this.EatNode();
-            }
-
             Debug.Assert(this.CurrentToken.Kind == SyntaxKind.NamespaceKeyword);
             var namespaceToken = this.EatToken(SyntaxKind.NamespaceKeyword);
 
@@ -2005,10 +2000,9 @@ private bool IsTypeDeclarationStart()
             }
         }
 
-        private static bool CanReuseMemberDeclaration(
-            CSharp.Syntax.MemberDeclarationSyntax member)
+        private static bool CanReuseMemberDeclaration(SyntaxKind kind)
         {
-            switch (member?.Kind())
+            switch (kind)
             {
                 case SyntaxKind.ClassDeclaration:
                 case SyntaxKind.StructDeclaration:
@@ -2025,6 +2019,7 @@ private bool IsTypeDeclarationStart()
                 case SyntaxKind.DestructorDeclaration:
                 case SyntaxKind.MethodDeclaration:
                 case SyntaxKind.ConstructorDeclaration:
+                case SyntaxKind.NamespaceDeclaration:
                     return true;
                 default:
                     return false;
@@ -2075,8 +2070,7 @@ private MemberDeclarationSyntax ParseMemberDeclarationOrStatementCore(SyntaxKind
             // don't reuse members if they were previously declared under a different type keyword kind
             if (this.IsIncrementalAndFactoryContextMatches)
             {
-                var member = this.CurrentNode as CSharp.Syntax.MemberDeclarationSyntax;
-                if (CanReuseMemberDeclaration(member))
+                if (CanReuseMemberDeclaration(CurrentNodeKind))
                 {
                     return (MemberDeclarationSyntax)this.EatNode();
                 }

src/Compilers/CSharp/Test/Syntax/IncrementalParsing/IncrementalParsingTests.cs

@@ -3,10 +3,7 @@
 using System;
 using System.Collections.Immutable;
 using System.Linq;
-using Microsoft.CodeAnalysis.CSharp.Symbols;
-using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
-using Microsoft.CodeAnalysis.Test.Utilities;
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Test.Utilities;
 using Xunit;
@@ -2774,6 +2771,112 @@ class B
             WalkTreeAndVerify(tree.GetCompilationUnitRoot(), fullTree.GetCompilationUnitRoot());
         }
 
+        [Fact]
+        [WorkItem(37663, "https://github.com/dotnet/roslyn/issues/37663")]
+        public void AssemblyAttributeBeforeNamespace()
+        {
+            var src = @"
+using System;
+using System.Linq;
+
+[assembly:]
+namespace N
+{ }";
+            var tree = SyntaxFactory.ParseSyntaxTree(src);
+            var text = tree.GetText();
+            var span = new TextSpan(src.IndexOf(":"), 1);
+            var change = new TextChange(span, "");
+            text = text.WithChanges(change);
+            tree = tree.WithChangedText(text);
+            var fullTree = SyntaxFactory.ParseSyntaxTree(text.ToString());
+            WalkTreeAndVerify(tree.GetCompilationUnitRoot(), fullTree.GetCompilationUnitRoot());
+        }
+
+        [WorkItem(37665, "https://github.com/dotnet/roslyn/issues/37665")]
+        [Fact]
+        public void AddBracketInUsingDirective()
+        {
+            var source =
+@"using System;
+namespace NS
+{
+    class A { }
+}
+";
+            var tree = SyntaxFactory.ParseSyntaxTree(source);
+            var text = tree.GetText();
+            var span = new TextSpan(source.IndexOf(";"), 0);
+            var change = new TextChange(span, "[");
+            text = text.WithChanges(change);
+            tree = tree.WithChangedText(text);
+            var fullTree = SyntaxFactory.ParseSyntaxTree(text.ToString());
+            WalkTreeAndVerify(tree.GetCompilationUnitRoot(), fullTree.GetCompilationUnitRoot());
+        }
+
+        [WorkItem(37665, "https://github.com/dotnet/roslyn/issues/37665")]
+        [Fact]
+        public void AddAttributeAfterUsingDirective()
+        {
+            var source =
+@"using System;
+namespace NS
+{
+    class A { }
+}
+";
+            var tree = SyntaxFactory.ParseSyntaxTree(source);
+            var text = tree.GetText();
+            var span = new TextSpan(source.IndexOf(";") + 1, 0);
+            var change = new TextChange(span, "[Obsolete]");
+            text = text.WithChanges(change);
+            tree = tree.WithChangedText(text);
+            var fullTree = SyntaxFactory.ParseSyntaxTree(text.ToString());
+            WalkTreeAndVerify(tree.GetCompilationUnitRoot(), fullTree.GetCompilationUnitRoot());
+        }
+
+        [WorkItem(37665, "https://github.com/dotnet/roslyn/issues/37665")]
+        [Fact]
+        public void AddTrailingModifierInUsingDirective()
+        {
+            var source =
+@"using System;
+namespace NS
+{
+    class A { }
+}
+";
+            var tree = SyntaxFactory.ParseSyntaxTree(source);
+            var text = tree.GetText();
+            var span = new TextSpan(source.IndexOf(";") + 1, 0);
+            var change = new TextChange(span, "public");
+            text = text.WithChanges(change);
+            tree = tree.WithChangedText(text);
+            var fullTree = SyntaxFactory.ParseSyntaxTree(text.ToString());
+            WalkTreeAndVerify(tree.GetCompilationUnitRoot(), fullTree.GetCompilationUnitRoot());
+        }
+
+        [WorkItem(37665, "https://github.com/dotnet/roslyn/issues/37665")]
+        [Fact]
+        public void AddTrailingModifierInUsingDirective_2()
+        {
+            var source =
+@"using System;publi
+namespace NS
+{
+    class A { }
+}
+";
+            var tree = SyntaxFactory.ParseSyntaxTree(source);
+            var text = tree.GetText();
+            var substring = "publi";
+            var span = new TextSpan(source.IndexOf(substring) + substring.Length, 0);
+            var change = new TextChange(span, "c");
+            text = text.WithChanges(change);
+            tree = tree.WithChangedText(text);
+            var fullTree = SyntaxFactory.ParseSyntaxTree(text.ToString());
+            WalkTreeAndVerify(tree.GetCompilationUnitRoot(), fullTree.GetCompilationUnitRoot());
+        }
+
         #endregion
 
         #region Helper functions