rustdoc doesn't render constness of bare functions on nightly

[QuietMisdreavus]

On today's nightly docs for std::mem::size_of:

But if you click [src], you can see that it's supposed to be const:

Rustdoc is supposed to print the constness of functions on nightly. It currently works for associated functions (e.g. AtomicBool::new) but bare functions like size_of are currently not working.

[GuillaumeGomez]

Taking a look.