Any difference in using an empty interface or an empty struct as a map's value?

Memory usage. For example, types struct{}, interface{}, and bool,

package main

import (
    "fmt"
    "unsafe"
)

func main() {
    var s struct{}
    fmt.Println(unsafe.Sizeof(s))
    var i interface{}
    fmt.Println(unsafe.Sizeof(i))
    var b bool
    fmt.Println(unsafe.Sizeof(b))
}

Output (bytes for 32-bit architecture):

0
8
1

Output (bytes for 64-bit architecture):

0
16
1

References:

Go Data Structures: Interfaces

The empty struct and empty interface, though syntactically similar, are actually opposites. An empty struct holds no data; an empty interface can hold any type of value. If I see a map[MyType]struct{}, I know immediately that no values will be stored, only keys. If I see a map[MyType]interface{}, my first impression will be that it is a heterogenous collection of values. Even if I see code storing nil in it, I won't know for sure that some other piece of code doesn't store something else in it.

In other words, using struct{} makes your code much more readable. It also saves a little memory, as described in the other answer, but that is just a fringe benefit of using the right tool for the job.

I would like to add additional detail about empty struct , as differences are already covered by andybalholm and peterSO .

Below is the example which shows usability of empty struct .

Creates an instance of rectangle struct by using a pointer address operator is denoted by & symbol.

package main

import "fmt"

type rectangle struct {
    length  int
    breadth int
    color   string
}

func main() {
    var rect1 = &rectangle{10, 20, "Green"} // Can't skip any value
    fmt.Println(rect1)

    var rect2 = &rectangle{}
    rect2.length = 10
    rect2.color = "Red"
    fmt.Println(rect2) // breadth skipped

    var rect3 = &rectangle{}
    (*rect3).breadth = 10
    (*rect3).color = "Blue"
    fmt.Println(rect3) // length skipped
}

Reference : https://www.golangprograms.com/go-language/struct.html

For a thorough read you can refer : https://dave.cheney.net/2014/03/25/the-empty-struct