Mocking in a different package breaks unexported methods

[adam-hanna]

Requested feature
Say you have the following project structure:

main.go
foo/
  interface.go

And inside /foo/interface.go you have:

// foo/interface.go
package foo

type IFoo interface {
  Bar() int
  baz()
}

And you try to mock it into your main (or any package other than foo):
$ mockgen -source=foo/interface.go -destination=mock.go -package=main

And then you try to use it:

// main.go
package main

import (
	"github.com/adam-hanna/mock-test/foo"
)

func main() {
	mockedFoo := MockIFoo{}
	doBar(&mockedFoo)
}

func doBar(f foo.IFoo) {
	f.Bar()
}

You will get the error: cannot use &mockedFoo (value of type *MockIFoo) as foo.IFoo value in argument to doBar: *MockIFoo does not implement foo.IFoo (unexported method baz)compilerInvalidIfaceAssign

Demo, here.

If you were to mock into the foo package, everything is fine, but this isn't always possible.

Why the feature is needed
This is needed if you are trying to mock a 3rd party library, and cannot modify their source code to add a mock. This happens to me all of the time when I'm writing tests and need to mock a 3rd party dependency.

(Optional) Proposed solution
The fix is quite simple. The created struct just needs to imbed the mocked interface. In my example:

// MockIFoo is a mock of IFoo interface.
type MockIFoo struct {
        foo.IFoo
	ctrl     *gomock.Controller
	recorder *MockIFooMockRecorder
}

I don't believe doing this will break any existing code.

We may also need to add a flag for the dependency location foo. For example, if foo wasn't local, but was at github.com/adam-hanna/foo we would need to add that to the imports.

[JacobOaks]

Thanks for the issue, but I think this is a duplicate of #64 so I'll close it. Please let me know if I'm mistaken.