support passing []byte as TPM2B

[chrisfenner]

Capturing one more bit of feedback @matt-tsai gave us in his internship report -

Occasionally, someone sending a TPM2B to a TPM would benefit from not having had to deserialize or serialize it (e.g., the 2B came from a protobuf and can just be forwarded to the TPM command). Every TPM2B_X should support being instantiated either with its contents or a flat buffer. Go1.18 generics can help maintain type safety and compiler-time checking here.

[chrisfenner]

My first attempt at this was to turn TPM2BPublic into a type constraint:

type TPM2BPublic interface {
	[]byte | struct {
		// the public area
		PublicArea TPMTPublic `gotpm:"sized"`
	}
}

i.e., a TPM2BPublic is either a dumb byte array or a wrapped TPMTPublic

This throws the following compiler error:

interface contains type constraints (MisplacedConstraintIface)

Turns out that type constraints are intended for functions and not types: golang/go#54469 (comment)

There is probably a less clever way to solve this problem.

[chrisfenner]

Another attempt, this time parameterizing the 2B type itself:

type TPM2BPublic[Contents interface{ []byte | TPMTPublic }] struct {
	// the public area
	PublicArea Contents `gotpm:"sized"`
}

Structs can't use TPM2BPublic:

cannot use generic type TPM2BPublic[Contents interface{[]byte|TPMTPublic}] without instantiation (WrongTypeArgCount)