Visitor.cs

public class Visitor : SymbolVisitor
{
    // TODO: Pooling
    internal StringBuilder Builder { get; } = new();
    private readonly bool _includeGlobalNamespace;

    private void AppendName(string text)
    {
        if (SyntaxFacts.GetKeywordKind(text) != SyntaxKind.None)
        {
            Builder.Append('@');
        }

        Builder.Append(text);
    }

    public override void VisitArrayType(IArrayTypeSymbol symbol)
    {
        Visit(symbol.ElementType);
        Builder.Append("[]");
    }

    public override void VisitDynamicType(IDynamicTypeSymbol symbol)
    {
        Builder.Append("dynamic");
    }

    public override void VisitNamedType(INamedTypeSymbol symbol)
    {
        if (GetSpecialTypeName(symbol) is string specialName)
        {
            Builder.Append(specialName);
            return;
        }

        if (symbol.IsGenericType && symbol.OriginalDefinition.SpecialType == SpecialType.System_Nullable_T)
        {
            Visit(symbol.TypeArguments[0]);
            Builder.Append('?');
            return;
        }

        Visit(symbol.ContainingSymbol);
        if (symbol.ContainingSymbol is INamespaceSymbol { IsGlobalNamespace: true })
        {
            if (_includeGlobalNamespace)
            {
                Builder.Append("global::");
            }
        }
        else
        {
            Builder.Append('.');
        }

        AppendName(symbol.Name);

        if (symbol.IsGenericType)
        {
            Builder.Append('<');
            for (int i = 0; i < symbol.TypeArguments.Length; i++)
            {
                Visit(symbol.TypeArguments[i]);
                if (i < symbol.TypeArguments.Length - 1)
                {
                    Builder.Append(", ");
                }
            }

            Builder.Append('>');
        }
    }

    private static string? GetSpecialTypeName(INamedTypeSymbol symbol)
    {
        return symbol.SpecialType switch
        {
            SpecialType.System_SByte => "sbyte",
            SpecialType.System_Int16 => "short",
            SpecialType.System_Int32 => "int",
            SpecialType.System_Int64 => "long",
            SpecialType.System_Byte => "byte",
            SpecialType.System_UInt16 => "ushort",
            SpecialType.System_UInt32 => "uint",
            SpecialType.System_UInt64 => "ulong",
            SpecialType.System_Single => "float",
            SpecialType.System_Double => "double",
            SpecialType.System_Decimal => "decimal",
            SpecialType.System_Char => "char",
            SpecialType.System_Boolean => "bool",
            SpecialType.System_String => "string",
            SpecialType.System_Object => "object",
            _ => null,
        };
    }

    public override void VisitNamespace(INamespaceSymbol symbol)
    {
        Visit(symbol.ContainingSymbol);
        if (symbol.IsGlobalNamespace)
        {
            return;
        }

        if (symbol.ContainingNamespace?.IsGlobalNamespace == true)
        {
            if (_includeGlobalNamespace)
            {
                Builder.Append("global::");
            }
        }
        else
        {
            Builder.Append('.');
        }

        AppendName(symbol.Name);
    }

    public override void VisitTypeParameter(ITypeParameterSymbol symbol)
    {
        AppendName(symbol.Name);
    }
}