包含空结构的名为“ _”（下划线）的字段的用途是什么？

I've seen two pieces of Go code using this pattern:

type SomeType struct{
  Field1 string
  Field2 bool
  _      struct{}    // <-- what is this?
}

Can anyone explain what this code accomplishes?

This technique enforces keyed fields when declaring a struct.

For example, the struct:

type SomeType struct {
  Field1 string
  Field2 bool
  _      struct{}
}

can only be declared with keyed fields:

// ALLOWED:
bar := SomeType{Field1: "hello", Field2: "true"}

// COMPILE ERROR:
foo := SomeType{"hello", true}

One reason for doing this is to allow additional fields to be added to the struct in the future without breaking existing code.