Suggest `!` for bitwise negation when encountering a `~`

[bstrie]

Here's a program:

fn main() {
    let x = ~1;
}

Its output:

error: expected expression, found `~`
 --> <anon>:2:13
  |
2 |     let x = ~1;
  |             ^

error: aborting due to previous error

Given that this is our only deviation from C-style bitwise operator syntax, it would be nice to have a note of some kind to suggest ! here.

Ideally the note would only appear when the ~ looks like it's being applied as a unary operator on a numeric type, but if that would require too much machinery then something simpler will suffice.

[bstrie]

And no, I'm not concerned about potential confusion with the ancient owned pointer syntax. :P

[estebank]

If anyone wants to tackle this, you'll need to modify libsyntax/parse/parser.rs.

[F001]

I would like to have a try. It seems that I need to emit a struct_span_err with help() message. Is it enough?

[bstrie]

@F001 Worth asking on #rust-internals on irc.mozilla.org, or internals.rust-lang.org if you'd prefer something other than IRC.