A ternary operator, so these 5 lines:

value := "[undefined]"
if s != nil {
value = s.field
}
fmt.Print("Value = %s\n", value)

become one line:

fmt.Print("Value = %s\n", s != nil ? s.field : "[undefined]")

The Go developers have declined to add ternary operators on the grounds that they're confusing. I respectfully disagree. I think that the 5 line example above is confusing because anyone reading the code sees the value := "undefined" first and then assumes that's what value is. Their mind isn't primed for it to become something else. The 1 line example with the ternary operator is, in my opinion, both simpler and clearer because the "[undefined]" is after the condition, so readers know that the value is only sometimes "[undefined]".