API proposal for "Using Alias to any Type"

[CyrusNajmabadi]

Championed proposal: dotnet/csharplang#4284
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/using-alias-types.md
Test Plan: #56323

Proposed API

using_directive
-  : 'global'? 'using' ('static' | name_equals)? name ';'
+  : 'global'? 'using' ('static' | ('unsafe'? name_equals))? type ';'
  ;

public sealed class UsingDirectiveSyntax
{
+    public SyntaxToken UnsafeToken { get; }
+    public TypeSyntax Type { get; }

-     public NameSyntax Name { get; }

+    /// <summary>Returns <see cref="Type"/> if it is a <see cref="NameSyntax"/>, null otherwise.</summary>
+    public NameSyntax? Name { get; }

+     // All the Update/With methods
}

public sealed class SyntaxFactory
{
+    public static UsingDirectiveSyntax UsingDirective(NameEqualsSyntax? alias, TypeSyntax type);
+    public static UsingDirectiveSyntax  UsingDirective(
+        SyntaxToken globalKeyword, SyntaxToken usingKeyword, SyntaxToken staticKeyword, SyntaxToken unsafeKeyword,
+        NameEqualsSyntax? alias, TypeSyntax type, SyntaxToken semicolonToken);
}

[333fred]

API Review

• Have we thought about what we'd need to do if we end up doing generics in alias names?
  • The part that would change is independent of this.
• Do we want an "expanded using alias" thing to avoid API breaks?
  • Would result in more pain for users than making Name nullable, as everywhere that is currently UsingDirectiveSyntax is now BaseUsingDirectiveSyntax
• Rename Type to be NamespaceOrType to be more clear
• Approved

[CyrusNajmabadi]

Closing out as completed.