proposal: sizeof: add bit sizes

[urandom2]

See the tail of the (open) CL 242018 for the (approved) proposal #29982. There are a number of use cases where bits, not bytes as unsafe.Sizeof and sizeof.Xxx speak in, are really useful and it is a shame that you have to manually derive them, const UintBits = sizeof.Uint * 8, as this is error prone. Instead we should expose constants for bit sizes too:

package sizeof

const (
	bitsSize = size * 8 // 32 or 64

	BitsBool       = 8
	BitsByte       = Uint8
	BitsComplex64  = 64
	BitsComplex128 = 128
	BitsFloat32    = 32
	BitsFloat64    = 64
	BitsInt        = bitsSize
	BitsUint8      = 8
	BitsInt16      = 16
	BitsInt32      = 32
	BitsInt64      = 64
	BitsRune       = BitsInt32
	BitsUint       = bitSize
	BitsUint8      = 8
	BitsUint16     = 16
	BitsUint32     = 32
	BitsUint64     = 64
	BitsUintptr    = bitSize
)

We could derive BitsXxx from Xxx * 8, but the current format seems more terse and intuitive. As well, the names could use a suffix, XxxBits, instead of the current prefix, since we get custom ordering for blocked constants, but to me, prefixes displays better and follows other standard library conventions: http.StatusXxx and xxx.ErrYyy.

Depending on the use case, we could instead export a constant or function for deriving these values. While the former is still prone to user error, it should be intuitive for users familiar with time.Duration. The latter seems like a blocker because functions cannot be compile time constant:

// Bits stores the number of bits in a byte.
//
// This is useful for converting byte type sizes into bits:
//	const UintBits = sizeof.Uint * sizeof.Bits
const Bits = 8

// Bits converts byte sizes into bits.
//
// This is useful for converting byte type sizes into bits:
//	var UintBits = sizeof.Bits(sizeof.Uint)
func Bits(bytes int) int { return bytes * 8 }

[urandom2]

@ianlancetaylor: can you add this to the proposals project?

[ianlancetaylor]

@arnottcr Done.

[rsc]

Putting on hold along with #29982.