rustdoc rendering for stable-but-const-unstable `const fn` is misleading

[RalfJung]

I just spent 5min of horror after seeing this rustdoc page:

and then I realized that the constness of this function is unstable. rustdoc should either indicate that, or just not show the const at all for stable functions whose constness is still unstable.

Cc @rust-lang/rustdoc @rust-lang/wg-const-eval

[Nemo157]

It'd be nice to still show that it's unstably const, but I feel like my 2 minute prototype would be too easy to accidentally skim as a standard "unstable API" annotation. Is there some simple way we could link to the constification tracking issue while leaving it clear that the function is stable?

[GuillaumeGomez]

Not show the const on stable rendering if not stable?

[fee1-dead]

Why not like this?

(this is edited)

[GuillaumeGomez]

That works too!

[fee1-dead]

@rustbot claim