Does a `_` receiver on a value (non-pointer) still copy the value?

Question

My question is whether or not a copy of a value is made when a method is invoked where _ is the receiver.

type Foo struct {
  // Many fields, making a large struct
}

func (_ Foo) Test(v *T) int {
  // Here we can't use the receiver but the method is still needed
}

So I'm wondering if Go implementations will still copy the Foo value when Test() is invoked, even though it's impossible to actually mutate the receiver value.

var f Foo
f.Test() // Is this making a copy?

I would also wonder about the case of a pointer, which is automatically dereferenced by default.

var f = new(Foo)
f.Test() // Is this making a copy?

I tried looking at the assembly, and I think it may be making the copy, but I just don't know enough to be sure.

---

For details on the situation:

This is an odd case where I can't use a pointer. The code is machine generated and required to cause a type to fulfill an interface while doing some initialization on the v parameter. (The generated code has metadata about the Foo that gets set on v .)

So if I make the receiver a pointer, the interface won't be fulfilled for "value" instances. This method will be invoked once for each instance, and instances can sometimes be large and/or created in great numbers, which is why I would like to avoid an unnecessary copy.

Answer 1

According to this blog post , the caller allocates stack elements for return values and the callee populates them.

This leads me to believe that the value is copied and then discarded.

That or a specialized callee would have to be generated in the case of _ receiver