Champion: Stackalloc array initializers (15.7)

[jcouv]

In addition to the currently allowed syntax (stackalloc int[3]) we should allow stack allocated arrays be created with initialization:

stackalloc int[3] { 1, 2, 3 }
stackalloc int[] { 1, 2, 3 }
stackalloc[] { 1, 2, 3 }

• Proposal added (https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.3/stackalloc-array-initializers.md)
• Discussed in LDM
• Decision in LDM
• Finalized (done, rejected, inactive)
• Spec'ed

Discussed in LDM in Jan 2018 (link to meeting notes TBD)
Prototype dotnet/roslyn#24249

[Thaina]

Alternatively I think we can have the same mechanism with Tuple extension indexer

I mean if we could have extension indexer we can extend Tuple<T,T,T,T,T,T,T,T> to allow indexing on it

var imitatingarray = (0,1,2,3,4,5,6,7);

imitatingarray[0]; // 0

[jcouv]

@Thaina The rationale for the proposed design is to match the syntax closely what is already allowed (such as new int[3] { 1, 2, 3 }) except with the keyword stackalloc instead of new so that transition is easier. It is less cognitive load (you don't have to remember two syntaxes and which is allowed when) and also less code churn when adopting stackalloc in an existing codebase.
Although the tuple approach is possible, it breaks away from habit and expectations.