Why would I let an exported pointer variable point to an unexported variable, at package level?

Taking the standard library net/http for example, the DefaultClient is defined as:

var DefaultClient = &Client{}

However, the DefaultServeMux is defined as:

var DefaultServeMux = &defaultServeMux
var defaultServeMux ServeMux

What's the point of defining two variables for one object? What benefit does it bring over a simple var DefaultServeMux = &ServeMux{}?

The DefaultServeMux part is an optimization done by Brad Fitzpatrick. I will copy and reformat the conversations between him and Matthew Dempsky from the comments on the CL:

Matthew Dempsky:

> To make sure I understand, the problem is that doing
>
>
> var x = newFoo()
>
> requires newFoo()
> to be called even if x is unused

Brad Fitzpatrick:

> Yes, that generates:
>
>
> var x *T
>
> func init() {
> x = newFoo()
> }
>
>
> And the linker doesn't seem to ever remove init blocks, even if they only assign to things which are otherwise only ever read. Because maybe newFoo has side-effects too?

Matthew Dempsky:

> whereas
>
>
> var x = &y
> var y foo
>
>
> skips the explicit initialization code. So if x is otherwise unused, it can be dead code eliminated along with all of type foo's related code?

Brad Fitzpatrick:

> Yup

I have checked the size of the go binary compiled from the latest source code (commit 4f4a9c7fff), and I found that the change of that commit helps a lot, but var DefaultServeMux = &ServeMux{} is a little better. See the table below:

Update:

I tried to find out the difference between var DefaultServeMux = &defaultServeMux; var defaultServeMux ServeMux and var DefaultServeMux = &ServeMux{} at the time when that commit was created (it's back to go1.7). <del>But I failed to compile go1.12 on my machine, so I gave up</del> (see https://stackoverflow.com/a/76030876/1369400). What I have found is from go1.7 till now (devel go1.21-4f4a9c7fff), the two implementations are almost the same regarding the affect to the size of the go binary. Here is the result (my environment is go1.20.3 linux/amd64 and I compiled the binary with ./make.bash):