Uplift clippy::double_neg into rustc

[rbtcollins]

See this https://dtolnay.github.io/rust-quiz/17

I agree with our position on the inclusion of --/++ in Rust, however currently we just silently treat --i as -(-i), which means that users who are coming from C++ and C get absolutely no warning that their code is going to do something entirely different than they expect - and there's arguably no use case for an unbracketed - -i : its the same as a literal i.

I expected to see this happen:

A warning, or an error: "attempt to use unsupported post-increment operator. Use +=1 instead". Or something like that.

Instead, this happened: explanation

Meta

I'm not sure what rustc version the rust-quiz website is running.

rustc --version --verbose:

<version>

Backtrace

<backtrace>

[Aaron1011]

A concrete example:

fn main() {
    let a = 1;
    let _b = --a;
}

compiles without any warnings

[leonardo-m]

Clippy spots it:

warning: `--x` could be misinterpreted as pre-decrement by C programmers, is usually a no-op
 --> src/main.rs:3:14
  |
3 |     let _b = --a;
  |              ^^^
  |
  = note: `#[warn(clippy::double_neg)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#double_neg

So you're asking to move this lint from Clippy to rustc.
I think we could turn it into a rustc error.

[Flying-Toast]

I think I can tackle this one - should I start a PR?