Get rid of the `std::util` module

[bstrie]

Having a util namespace is an anti-pattern. Go through all the remaining types and functions in std::util and either move them to std::prelude or delete them outright.

15:28 < bstrie> though I'm sad that Noncopyable lives in a module named "util"
15:28 < bstrie> maybe we can find a better home for it
15:28 < strcat> prelude
15:28 < strcat> just shove it in prelude
15:28 < strcat> and swap/replace too
15:28 < bstrie> is that all that's left?
15:28 < strcat> no but the other stuff is pretty useless
15:28 < bstrie> haha
15:28 < strcat> id and ignore
15:28 < strcat> |x| x
15:29 < strcat> and .drop() being allowed explicitly could zap ignore
15:29 < strcat> or let _ = x; dropping immediately
15:29 < strcat> also Void, but we can put that in prelude

[brson]

I'm not sure about putting things directly in the prelude. Currently it is only reexports. Defining things directly in the prelude isn't something we should do lightly. Furthermore, util is a grab bag of low-importance junk, while prelude is composed of the most important library definitions in Rust.

[thestinger]

@brson: I think swap and replace are important enough to be in prelude, you can't write owned data structures without them (we did used to have <->). If we do replace ! and the implicit bottom with an uninhabited enum (#7538) it would make sense to have Void there too.

[bblum]

Here is the full list of items exported from util:

fn id
fn ignore
fn with
fn swap
fn replace
struct NonCopyable
fn NonCopyable::new
enum Void
fn Void::uninhabited
fn unreachable

With the exception of wishing we had a more concise name for the noncopyable token, I would be fine with all of this being in prelude.

[bblum]

nominating backwards-compat

[thestinger]

We'll be able to drop ignore once either let _ = expr destroys the result right away or drop takes self by-value.

The id function isn't really much more concise than just doing |x| x so we could remove that.

I don't think with should be in prelude, but the rest are okay.

[catamorphism]

accepted for backwards-compatible

[edwardw]

Now the leftover in std::util has been down to:

fn id
fn swap
fn replace
struct NonCopyable
fn NonCopyable::drop
enum Void
fn Void::uninhabited

Seems to be the perfect candidate for a contributor wanna-be to try hand on. My question then is it okay to move all these to prelude, except that id is to be removed?

[flaper87]

I've a patch ready to remove NonCopyable as soon as #11768 lands.

[brson]

Perhaps we could make std::util private while reexporting this stuff from the prelude. That would preserve the organizational advantages of having all this stuffed tucked into its own file while still not exposing the ugly util module.

[brson]

Oops. Didn't mean to close.

[flaper87]

I agree this looks like an easy task for a newcomer. I'll tag it as E-easy for now.

[edwardw]

@flaper87, using NoPod marker I imagine? Then I shall try to take care of the rest.