x/tools/gopls: code action for extracting into interface

[js-sketch]

Consider the following program:

type bar struct {}
func (b *bar) x() {}
func (b *bar) y() {}
type Foo struct {
    bar *bar
}
func (f *Foo) baz() {
    f.b.y()
}

Now assume we would like to test foo and for this we'd want to mock bar. The recommended way of doing this is to extract the way that bar is used into an interface and use that in lieu of bar in foo.

type bar struct {}
func (b *bar) x() {}
func (b *bar) y() {}

type barI interface {
    y()
}
type Foo struct {
    bar barI
}
func (f *Foo) baz() {
    f.b.y()
}

Note that only y() was extracted.
Supporting such a code action in gopls would make life a lot easier for devs who write unit tests.
This is at least viable to implement for unexported members, as no new methods can be added in other packages which access this member.

[findleyr]

Thanks for the feature request. I agree that this could be a useful refactoring operation, and is certainly doable. It's also useful for more than just testing.