Remove standard io chapter from the book

[steveklabnik]

This was originally used to set up the guessing game, but that no longer
exists. This version uses old_io, and updating it involves talking
about &mut and such, which we haven't covered yet. So, for now, let's
just remove it.

Fixes#23760

[rust-highfive]

r? @alexcrichton

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

Shouldn't this also modify SUMMARY.md?

[alexcrichton]

Other than that though r=me

[steveklabnik]

@bors: r=alexcrichton rollup

[bors]

📌 Commit 51ce2df has been approved by alexcrichton

[bors]

☔ The latest upstream changes (presumably #23796) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

🔒 Merge conflict

[steveklabnik]

@bors: r=alexcrichton rollup

[bors]

📌 Commit 9fb54f8 has been approved by alexcrichton

[KingOfThePirates]

Maybe having a "!This chapter will be revised to be less confusing" message at the top is better than no chapter at all?

As someone who is learning programming for the first time, I vote to reopen #23800. I would rather not know what that special mut code means or does, if it means I can use some code to use databases now in my games. Not being able to save accounts, characters, inventories, and game progress in the little, text-based games you make at this stage of programming is very frustrating. Yahoo answers solutions are 100x more confusing.

I think it is much better to learn a language as you make a program over and over again with new computer science topics and language idioms you learn.

Not being able to save data takes away all but tic-tac-toe.. not cool.

After further review, even #23800 doesn't do what I thought it was going to do. But at least I can get user input.

But when do we learn how to read and write files?

[bors]

⌛ Testing commit 9fb54f8 with merge fa3d778...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt

[steveklabnik]

But when do we learn how to read and write files?

Because this is being removed in the moment doesn't mean that it's gone forever. It just doesn't fit currently. Also, I'm working on docs for IO in general, so this will be in the API docs as well.

[KingOfThePirates]

Do you think you could provide us noobies, even if just here, for a copy and paste that reads a plain text file and stores each line as a different variable?

Example plain text file:	Example variables:
Gold Roger	character_name
12	character_level
Warrior	character_job
177 0	character_health character_mana

This may help write the docs for IO, too.