b is not a descriptive variable name. That's often okay in short functions, but it doesn't work well in this context. It's also typically associated with []byte type, not io.Reader. I see r is already taken and I don't think it's a good idea to reuse it.

I think it's better to either use a longer descriptive name here:

exportData := bytes.NewReader(a.ExportData)
packages[path], err = gcexportdata.Read(exportData, token.NewFileSet(), packages, path)

Which would mirror the gcexportdata.Write code below.

But I think this line is not that long, that it's even better to just inline it:

packages[path], err = gcexportdata.Read(bytes.NewReader(a.ExportData), token.NewFileSet(), packages, path)

Either of those is okay.

Done.

This line uses a very outdated style of initializing a new bytes.Buffer, no one writes it like that in anymore. bytes.NewBuffer documentation hints at this:

In most cases, new(Buffer) (or just declaring a Buffer variable) is sufficient to initialize a Buffer.

It's not surprising because this line was written 4 years ago.

I know it's not your code, but it's the line directly following your code, and I think it's not nice how just 4 lines above it is a different style is used to do the same thing:

exportData := new(bytes.Buffer)
// ... 3 lines
encodedFileSet := bytes.NewBuffer(nil)

Let's make this better by updating it so they both use the same style.

I would slightly prefer the var buf bytes.Buffer (and using & operator) style over buf := new(bytes.Buffer):

var exportData bytes.Buffer
if err := gcexportdata.Write(&exportData, nil, typesPkg); err != nil {
	return nil, fmt.Errorf("failed to write export data: %v", err)
}
var encodedFileSet bytes.Buffer
if err := fileSet.Write(json.NewEncoder(&encodedFileSet).Encode); err != nil {

But if you feel strongly about using new(bytes.Buffer) for both variables (and not using & operator), I'm okay with that too.

Also, I know this file has yet another instance of bytes.NewBuffer(nil), but it's fine to leave that alone, it's too far from the affected code and hence out of scope of this PR.

Done.