Why can't a stackalloc expression be assigned to a pointer field?

[jnm2]

This works:

char* temp = stackalloc char[1024];

var assemblyInfo = new ASSEMBLY_INFO
{
    pszCurrentAssemblyPathBuf = temp,
};

So why not this?

var assemblyInfo = new ASSEMBLY_INFO
{
    // ❌ CS8346 Conversion of a stackalloc expression of type 'char' to type 'char*' is not possible.
    //                          ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓
    pszCurrentAssemblyPathBuf = stackalloc char[1024],
};

ASSEMBLY_INFO.pszCurrentAssemblyPathBuf is declared as char*.

[333fred]

This is because stackalloc in C# behaves differently when it's a top-level expression vs a nested expression. Previous to C# 7.2, stackalloc was only allowed as top-level expression, and could not be nested. You can see remnants of this here: https://github.com/dotnet/csharplang/blob/master/spec/unsafe-code.md#stack-allocation. For legacy reasons, when you use stackalloc as a variable initializer today, you get a char* back as the natural type, and we add an implicit conversion to Span<T> if the thing you assign to is a Span<T>. However, for any other context, we do the safer thing and stackalloc always returns a Span<T>, instead of a T*. This means that in your nested object initializer, you're trying to assign a Span<char> to a char*, which is of course not valid.