Rust's syntax highlighting breaks after `r#impl`

[alexr00]

From @rustka in microsoft/vscode#81085

Rust's syntax highlighting breaks after using impl keyword as a raw identifier.

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• VSCode version: 1.38.1
• OS version: Linux x64

Steps to reproduce:

1. Open the editor;
2. Set language mode to Rust;
3. Paste the following code:

use r#impl;
fn main() {}

4. Comment the first line by prefixing it with //;
   4.1. This failure could also be prevented by replacing r#impl with r#impl {}, but this is not the real solution for that case.
5. The syntax highlighting works properly back again.

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P.S.: I did test it on Atom editor (v1.40.1) and it seemed to work correctly.

[RpxdYTX]

Also, after a # the whole line (except some things) is white:

pub fn r#let (&self); // comments
// and              // are one of the
#[allow (dead_code)] // exceptions