stackalloc expressions have limited syntax

[OmarTawfik]

Due to a CLR spec limitation, stackalloc instructions cannot be executed unless the evaluation stack is cleared of other items. This means that the following code will not be executed at run-time:

DoSomething(obj1, stackalloc int[10]);

For that to happen, we need to save (spill) all prior values on the evaluation stack to temps before executing this instruction. For now, in feature branch freatures/readonly-ref, the scope of this instruction is constricted to either (1) initialization statements or (2) ternary conditional operators.

[gafter]

This is tracked as a language issue at dotnet/csharplang#1412

[gafter]

This is relaxed (by spilling) in #28969