modules: describe both module filename styles without giving a clear preference

[RalfJung]

There's currently no clear consensus on which filename style to use for modules, and many people have a style tha mixes both options. So let's make the reference just describe the facts, without saying which variant is preferred.

Also see the discussion on IRLO.

[joshtriplett]

I think changing the expressed preference would require a consensus to change it. There was an intentional decision in 2018 to try to migrate towards the Rust 2018 style.

[RalfJung]

I didn't realize that this was an intentional decision. There's little other indication of a goal of pushing the ecosystem towards the 2018 style. Not even the compiler itself was ever systematically adjusted.

So, that would be a t-lang FCP then?

[RalfJung]

@rustbot labels +I-lang-nominated

[joshtriplett]

@RalfJung IIRC, in 2018 when we designed the new module system, it was at that time an intentional decision to pointedly present it as "the new way" by contrast with "the old way". We didn't particularly push people to switch.

I'm not suggesting that we would necessarily object to this proposal, but I do think it's fair to say that this proposal represents a change in recommendations, and it probably warrants at least a discussion.