Shimming to an IDENT type

[natewernimont-wk]

I'm trying to shim an interface into something that knows how to properly serialize/deserialize that interface. Here is an example:

package foo

//go:generate go tool msgp -io=false -tests=false
//msgp:shim *Bar as:BarSerializer using:ToBarSerializer/unusedFn
//msgp:ignore BarSerializer Bar

type Bar interface{}

type Foo struct {
	Bar
}

type BarSerializer struct {
	Bar *Bar
}

func ToBarSerializer(b *Bar) BarSerializer {
	return BarSerializer{Bar: b}
}

func (b BarSerializer) MarshalMsg(o []byte) ([]byte, error) {
	// actual logic
	return o, nil
}

func (b BarSerializer) UnmarshalMsg(o []byte) ([]byte, error) {
	// actual logic
	*b.Bar = nil // set appropriately
	return o, nil
}

func (b BarSerializer) Msgsize() int {
	return 0
}

Using this generation, the shim is completely unused. I believe it is because the .Alias call here does not set .Convert = true when IDENT types are provided in the as. As such, the shim logic isn't used at all in marshal/unmarshal/size.

Unconditionally setting Convert to true for aliases fixes this problem and properly uses the shim.