Improve experience when using stack-only types in interactive code

[tmat]

This doesn't work since script variables are not stack allocated:

> var span = new byte[] { 1, 2, 3 }.AsSpan();
(1,1): error CS8345: Field or auto-implemented property cannot be of type 'Span<byte>' unless it is an instance member of a ref struct.

This is by design, the message could be better though (something like Script variables cannot be of type 'Span<byte>'.).

Similarly for ref locals:

> ref int x;
(1,10): error CS1003: Syntax error, '(' expected
(1,10): error CS1026: ) expected
> 

We should parse the declaration and report semantic error.

This doesn't work since top-level code is implicitly async:

> {
.     var span = new byte[] { 1, 2, 3 }.AsSpan();
. }
(2,5): error CS4012: Parameters or locals of type 'Span<byte>' cannot be declared in async methods or lambda expressions.

Is there a reason why this can't be allowed for variables that do not need to be hoisted? Is it just to simplify handling of these cases?

If we don't make this work we should report a better error message (e.g. Locals of type 'Span<byte>' cannot be declared outside of methods in scripts.).

BTW, one can still work with spans in interactive, they just need to be declared within a method:

> void F()
. {
.     var span = new byte[] { 1, 2, 3 }.AsSpan();
. 
. }

[tmat]

@agocke

Is there a reason why this can't be allowed for variables that do not need to be hoisted?

[tmat]

@MadsTorgersen FYI

[tmat]

Partially addressed by https://github.com/dotnet/roslyn/pull/72664