Unclear meaning of bitsize of an array

[ericlangedijk]

I encountered an unclear language thing, which - as a simple user - was surprising me.

pub const Letter = packed struct {
    machinechar: u5 = 0,
    is_blank: bool = false,
}
const Letters7 = [7]Letter;

The sizeOf Letters7 returns obviously 7, but bitSizeOf returns 54.

Where are the 2 missing bits?
The meaning of the bitsize is unclear in my opinion and should maybe be clarified somewhere (if possible).
As far as I understand the bitsize is the size which it would occupy in a packed struct.
However: Letters7 is not allowed by the compiler to reside inside a packed struct.
The reason that I am posting this issue is, is the question:
Is this a little inconsistency in the language?

[alexrp]

Arrays are not bit-packed. @bitSizeOf(Letter) == 6, so when you put these in an array, there will be 2 extra bits of padding per item.

[Rexicon226]

Should note that it's 2 extra bits of padding per item, except the last one. So (6 + 2) * 7 - 2 is 54.

[ericlangedijk]

Yes that I understand. Array items are not bitpacked, but still these last 2 bits are chopped off inside the bitSizeOf builtin function.
If this little array could reside in a packed struct it would be logical...

[190n]

Should @bitSizeOf even be allowed for types that can't appear in a packed struct or union?

[alexrp]

cc @mlugg

[mlugg]

Intended @bitSizeOf semantics are defined by the accepted, but not yet implemented, proposal #19755. The current semantics are indeed kinda weird in places; don't pay them too much attention.