What is an interface assertion?

Question

I have just came across this piece of code on this blog post

type Logger interface {
    Debug(msg string, keyvals ...interface{}) error
    Info(msg string, keyvals ...interface{}) error
    Error(msg string, keyvals ...interface{}) error
}

type tmLogger struct {
    srcLogger kitlog.Logger
}

// Interface assertions
var _ Logger = (*tmLogger)(nil) // What is this?

// ... interface definition ...

What is this "interface assertion"?

Answer 1

It assigns a nil pointer to a concrete type to a variable of the interface type. This is a common practice to prove that the concrete type fulfills the interface - if it doesn't, that line won't compile, giving an error that the concrete type can't be assigned to a variable of the interface type and why.

As @JimB noted, "interface assertion" is a term made up by the author. Go has no such term. This is, specifically, a type conversion, converting nil to a pointer to tmLogger, then assigning the typed nil pointer to a blank identifier variable of the interface type Logger. If *tmLogger does not satisfy Logger, the assignment won't compile; but, at runtime, this takes no memory because it's using a nil value.

Presumably the author uses this term more in the unit-testing sense of "assertion" than the "type assertion" sense - that line of code asserts that the type implements the interface, and if it doesn't, the line will fail.

Given that this is purely a testing practice, personally I put these checks in _test.go files so that they're included in unit test executions, excluded from the final binary, and clearly part of the test suite and not the application logic.