Remove `loop` keyword from the language

[liigo]

loop loops can be removed, because it doesn't do any special jobs than while true {} (which maybe simplified to while { }).

Do we really need two similar loops? (loop VS. while)

[liigo]

cc #2233: Remove while loops

[huonw]

loop does currently have a special job:

let mut a;
loop {
     a = 1;
     // ... break ...
}
do_something(a)

is valid but the while true equivalent is not: the compiler isn't sure that a is initialised. Of course, this is just a minor detail, we could special case the while true form like Java.

[alexcrichton]

Language changes such as this require and RFC rather than an issue in the rust repository, I would recommend following the procedure over at https://github.com/rust-lang/rfcs.

[pnkfelix]

Also, we definitely cannot remove loop-de-loops until #12643 is fixed.

[Thiez]

loop could not be replaced by while {stuff} because {stuff} would be interpreted as the condition (e.g. while { condition } { body }).

[liigo]

@alexcrichton Obviously requires an RFC. I'm much appreciate if someone has interest writing it. (I'm not a native-english user, too hard to write it for me, sorry.)

@pnkfelix @Thiez We can do more if we want to remove or replace it.

[jkleint]

I'd like to take up replacing the loop keyword with while true. Now that #12643 is fixed, this should be straightforward. It looks like we can recognize the two tokens while true in the parser and invoke the existing loop parsing machinery with very minimal changes. This allows us to remove loop before Rust 1.0 is frozen, and the internal implementation can be cleaned up at any later time.

I have a rough pass at an RFC up here:
https://github.com/jkleint/rfcs/blob/remove-loop-keyword/text/0000-remove-loop-keyword.md
I am requesting comments on my Request For Comments.

I am willing to do the work of replacing all occurrences of loop with while true in the source and documentation, but I will need help actually modifying the compiler. This is my first real change to Rust and first RFC, so I would love advice and input.

Also, if anyone can point me to an easy way to mechanically transform Rust source, I might make a tool to help with migration.

Thanks!

[liigo]

@jkleint thank you for working on this!