CMTK0005: report ImmutableArray<T>.Builder.Count set on a builder of value objects #269

Found in the weaving review of #229, beside CollectionsMarshal.SetCount.

ImmutableArray<T>.Builder.Count = n grows the builder with default items whenever n is larger than its count, as CollectionsMarshal.SetCount(list, n) does for a List<T>, and CMTK0005 does not report it:

var builder = ImmutableArray.CreateBuilder<OrderId>();
builder.Count = 3;                       // three default OrderIds, nothing reported
var ids = builder.MoveToImmutable();

After #229 the Analyzers README's row says "an array, span or list ... given a length", which a reader takes to include this.

Proposal

CMTK0005 reports an assignment to ImmutableArray<T>.Builder.Count (resolved by symbol) where T is a value object, Option, Result or [RequireCustomInitialization] struct, unless the value is a constant zero. A Warning reporting more needs no exception to decision 34. Check the other BCL builders and lists with a settable count while there.

Test

A CMTK0005 row for the setter, a zero control, across TypeArgumentKinds.