Merge pull request #1122 from VSadov/StackArrInit

Stackalloc array initializers.

proposals/stackalloc-array-initializers.md

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+# Stackalloc array initializers.
+
+* [x] Proposed
+* [ ] Prototype: Not Started
+* [ ] Implementation: Not Started
+* [ ] Specification: Not Started
+
+## Summary
+[summary]: #summary
+
+Allow array initializer syntax to be used with `stackalloc` 
+
+## Motivation
+[motivation]: #motivation
+
+Ordinary arrays can have their elements initialized at creation time. It seems reasonable to allow that in `stackalloc` case.
+
+The question of why such syntax is not allowed with `stackalloc` arises fairly frequently.  
+See, for example, https://github.com/dotnet/csharplang/issues/1112  
+
+## Detailed design
+
+Ordinary arrays can be created through the following syntax:
+
+```C#
+new int[3]
+new int[3] { 1, 2, 3 }
+new int[] { 1, 2, 3 }
+new[] { 1, 2, 3 }
+```
+
+We should allow stack allocated arrays be created through:  
+
+```C#
+stackalloc int[3]				// currently allowed
+stackalloc int[3] { 1, 2, 3 }
+stackalloc int[] { 1, 2, 3 }
+stackalloc[] { 1, 2, 3 }
+```
+
+The semantics of all cases is roughly the same as with arrays.  
+For example: in the last case the element type is inferred from the initializer and must be an "unmanaged" type.
+
+NOTE: the feature is not dependent on the target being a `Span<T>`. It is just as applicable in `T*` case, so it does not seem reasonable to predicate it on `Span<T>` case.  
+
+## Translation ##
+
+The naive implementation could just initialize the array right after creation through a series of element-wise assignments.  
+
+Similarly to the case with arrays, it might be possible and desirable to detect cases where all or most of the elements are blittable types and use more efficient techniques by copying over the pre-created state of all the constant elements. 
+
+## Drawbacks
+[drawbacks]: #drawbacks
+
+## Alternatives
+[alternatives]: #alternatives
+
+This is a convenience feature. It is possible to just do nothing.
+
+## Unresolved questions
+[unresolved]: #unresolved-questions
+
+## Design meetings
+
+None yet.