Permit stackalloc in nested contexts

[!INCLUDESpecletdisclaimer]

Champion issue: https://github.com/dotnet/csharplang/issues/1412

Stack allocation

We modify the section Stack allocation (§12.8.22 Stack allocation) of the C# language specification to relax the places when a stackalloc expression may appear. We delete

local_variable_initializer_unsafe
    : stackalloc_initializer
    ;

stackalloc_initializer
    : 'stackalloc' unmanaged_type '[' expression ']'
    ;

and replace them with

primary_no_array_creation_expression
    : stackalloc_initializer
    ;

stackalloc_initializer
    : 'stackalloc' unmanaged_type '[' expression? ']' array_initializer?
    | 'stackalloc' '[' expression? ']' array_initializer
    ;

Note that the addition of an array_initializer to stackalloc_initializer (and making the index expression optional) was an extension in C# 7.3 and is not described here.

The element type of the stackalloc expression is the unmanaged_type named in the stackalloc expression, if any, or the common type among the elements of the array_initializer otherwise.

The type of the stackalloc_initializer with element type K depends on its syntactic context:

• If the stackalloc_initializer appears directly as the local_variable_initializer of a local_variable_declaration statement or a for_initializer, then its type is K*.
• Otherwise its type is System.Span<K>.

Stackalloc Conversion

The stackalloc conversion is a new built-in implicit conversion from expression. When the type of a stackalloc_initializer is K*, there is an implicit stackalloc conversion from the stackalloc_initializer to the type System.Span<K>.