Suggest mutable self receiver if self is used mutably

[carbotaniuman]

Given the following code:

struct Lexer {
    parse_header: bool,
}
impl Lexer {
    fn set_include_mode(parse_header: bool) {
        self.parse_header = parse_header;
    }
}
<code>

The current output is:

error[E0424]: expected value, found module `self`
  --> src/preprocess/lex.rs:18:9
   |
17 |     fn set_include_mode(parse_header: bool) {
   |        ---------------- this function doesn't have a `self` parameter
18 |         self.parse_header = parse_header;
   |         ^^^^ `self` value is a keyword only available in methods with a `self` parameter
   |
help: add a `self` receiver parameter to make the associated `fn` a method
   |
17 |     fn set_include_mode(&self, parse_header: bool) {
   |                         ++++++

Ideally the output should look like:

error[E0424]: expected value, found module `self`
  --> src/preprocess/lex.rs:18:9
   |
17 |     fn set_include_mode(parse_header: bool) {
   |        ---------------- this function doesn't have a `self` parameter
18 |         self.parse_header = parse_header;
   |         ^^^^ `self` value is a keyword only available in methods with a `self` parameter
   |
help: add a `self` receiver parameter to make the associated `fn` a method
   |
17 |     fn set_include_mode(&mut self, parse_header: bool) {
   |                         +++++++++

[ChayimFriedman2]

If you'll try to apply the suggestion it'll error with

error[E0594]: cannot assign to `self.parse_header`, which is behind a `&` reference
 --> src/lib.rs:6:9
  |
5 |     fn set_include_mode(&self, parse_header: bool) {
  |                         ----- help: consider changing this to be a mutable reference: `&mut self`
6 |         self.parse_header = parse_header;
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be written

So this is not bad.

[estebank]

Triage: no change.