First pass at replacing loop with while true.

[jkleint]

Proto-RFC here: https://github.com/jkleint/rfcs/blob/remove-loop-keyword/text/0000-remove-loop-keyword.md

This is my first foray into modifying the Rust compiler, so please let me know if I'm way off the mark. I'm not sure this is the right way to do things, it's just to get the conversation started and show it may be simple.

This piggybacks on while-expression parsing the same way as while let to recognize while true and call the existing loop parsing code.

I haven't been able to thoroughly test it, but it does seem to compile code like this:

fn main() {
    let x;
    while true { x = 1i; break; }
    println!("{}", x)
}

which previously did not compile (x possibly uninitialized).

I'm unsure of how to use the existing deprecation machinery for a keyword, so I have just put in placeholder warnings, but please advise on how do do that properly. I'm also not sure if the source span properly encompasses the whole while loop.

Fixes #12975.

[alexcrichton]

Currently language changes are required to go through the RFC process as outlined in the RFC repo's README.

In light of this, could you open an RFC for this change? In the meantime I'm going to close this PR against the rust repo to help clear out the queue.

[pcwalton]

This needs to go through the RFC process before submitting a PR. Closing.

[pcwalton]

Also even if we wanted to do this (which I don't want to) this implementation is nowhere near what we would need. I think you would want to get rid of ExprLoop entirely and add a method that applies a series of hacks to try to figure out whether a loop is endless to replace uses of it in the compiler. (You will at least need to consider things like ExprParen(ExprLit(true)).)

These hacks that you will have to add, by the way, are the reason I don't want to do this.