What is this variable declaration with underscore, inline interface and assignment? [duplicate]

Question

What does this snippet of Go code do?

var _ interface {
    add(string) error
} = &watcher{}

I believe &watcher{} returns two things, the first is discarded and the second is assigned to ... an interface? I found the code in fswatch on Github.

Answer 1

This construct will declare a variable with an blank identifier name with type given by a type literal; an interface definition in this case. What follows is in initializer expression - a pointer to a composite literal in this case.

Overall functionality of the snippet is to statically ensure that a *watcher satisfies the said interface as the _ variable is not materialized in any way and only any possible side effects of the initializer can be observed. Either static (as in this case) or dynamic (like e.g. calling a function which assigns at runtime to, say, some global vars, registers a handler, etc.)