Allow `const` fields in `mutable` structs

[Seelengrab]

Currently, const-per-field annotations are not allowed by the interface, but it would be nice to support this.

The way this would be implemented is by allowing the annotation through here:

FieldFlags.jl/src/FieldFlags.jl

Lines 154 to 165 in 6267ca5

	for ex in expr.args[3].args
	!(ex isa Expr) && continue
	(ex.head == :call
	&& length(ex.args) == 3
	&& first(ex.args) === :(:)) || continue # only Intx bitfields supported right now
	fieldname = ex.args[2]
	fieldname isa Symbol || throw(ArgumentError("Name of field is not a symbol: `$fieldname`"))
	fieldsize = ex.args[3]
	fieldsize isa Integer || throw(ArgumentError("Declared size of field `$fieldname` is not an integer literal!"))
	numbits += fieldsize
	push!(fields, fieldname => fieldsize)
	end

as well as simply not generating the various setindex! branches here:

FieldFlags.jl/src/FieldFlags.jl

Lines 239 to 256 in 6267ca5

	# only build the expression if we actually need to
	if mutable
	push!(setpropexpr.args, :(s === $(QuoteNode(fieldname))))
	ifbody = :(
	offsetshift = cast_extend_truncate($Ti, propertyoffset($T, s));
	shifted = Core.Intrinsics.shl_int(val, offsetshift);
	mask = Core.Intrinsics.not_int(Core.Intrinsics.shl_int(maskbase, fieldsize($T, s)));
	mask = Core.Intrinsics.not_int(Core.Intrinsics.shl_int(mask, propertyoffset($T, s)));
	cleareddata = Core.Intrinsics.and_int(getfield(x, :fields), mask);
	newdata = Core.Intrinsics.or_int(cleareddata, shifted);
	setfield!(x, :fields, newdata);
	return maskeddata;
	)
	push!(setpropexpr.args, ifbody)
	nsetprop = Expr(:elseif)
	push!(setpropexpr.args, nsetprop)
	setpropexpr = nsetprop
	end

Annotating a field with const currently doesn't error, but the field is ignored entirely, so adding this shouldn't break anything:

julia> @bitfield mutable struct Foo
           const a:2
           b:3
       end

julia> methods(Foo)
# 3 methods for type constructor:
 [1] Foo()
     @ none:0
 [2] Foo(t::Foo_fields)
     @ none:0
 [3] Foo(b::Union{Bool, Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8})
     @ none:0