Add Span.StackAlloc<T> method

[VBAndCs]

C# allows to allocate objects in the stack to be used with Span and Memory:
Span<byte> bytes = stackalloc byte[10];
I suggest to add a StackAlloc method to these classes to allow other languages like VB.NET an F# to do the same:
dim bytes = Span.StackAlloc(of byte)(10)

[FiniteReality]

This is likely impossible because stackalloc allocates to the current stack frame, which means that you'll be referencing freed/invalid memory as soon as your proposed API returns. To even have a chance at being implemented correctly, this would likely have to be an intrinsic and have compiler detection.

You'd be much better suggesting language features for VB.Net and F# to support stack-allocation a la C#. It would be a lot easier for them to implement it as a language feature than it would be to change the runtime and likely the compiler to detect this pattern.

[tannergooding]

This would not be impossible, it would just require the runtime to specially recognize the method and to treat it as part of the calling method (this is how the alloca() method works in C/C++ after all).

F# already has support for stackalloc today via their NativePtr.stackalloc<'T> method so you can already get a Span<T> around that stack-allocated memory by calling the appropriate constructor. F# also has inline methods which would allow you to create a helper which calls NativePtr.stackalloc and returns a Span<'T> wrapper over it.

I would think, if something like this was exposed, it would be better suited first as T* Unsafe.StackAlloc<T>(length). You could then just call the appropriate Span<T> constructor if required or consider a Span<T>.StackAlloc method separately.

[stephentoub]

Related: https://github.com/dotnet/corefx/issues/27985

[VBAndCs]

Thanks.
I tried to avoid requesting this feature in VB.NET lang because it will take too long if it ever happened. VB.NET team, or what left of it, of course is aware of new features of C#, so, why should we ask?
MS seems to left VB behind two years ago!

[tannergooding]

CC. @KathleenDollard

[FiniteReality]

@tannergooding TIL - However, it still would require runtime changes as you mention, which I'm guessing are harder to justify than language changes.

[Nukepayload2]

You can use MemoryMarshal.CreateSpan if you want to stackalloc in VB.NET .
https://github.com/Nukepayload2/ryu/blob/db558e00798ed48292f919264f7b99cd7361649e/visualbasic/D2s.vb#L610

[stephentoub]

Closing as a dup of #25423.