Helpful suggestions for incorrect address-of mutability

[TirushOne]

Added suggestions for & and &raw expressions used as function arguments when the function expects a mutable address.

I also did a small refactor, grouping all 'suggest mut' code into a single function called suggest_addr_mut, and removed suggest_ptr_null_mut. Its logic was moved into the new suggest_addr_mut.

Fixes #159490.

Example:

fn main() {
    let s = String::new();
    takes_raw(&raw const s);
    takes_ref(&s);
}

fn takes_raw(_: *mut String) {}
fn takes_ref(_: &mut String) {}

Diagnostics before:

error[E0308]: mismatched types
  |
3 |     takes_raw(&raw const s);
  |     --------- ^^^^^^^^^^^^ types differ in mutability
  |     |
  |     arguments to this function are incorrect
  |
  = note: expected raw pointer `*mut String`
             found raw pointer `*const String`
note: function defined here
 -->..\src\main.rs:7:4
  |
7 | fn takes_raw(_: *mut String) {}
  |    ^^^^^^^^^ --------------

error[E0308]: mismatched types
 --> ..\src\main.rs:4:15
  |
4 |     takes_ref(&s);
  |     --------- ^^ types differ in mutability
  |     |
  |     arguments to this function are incorrect
  |
  = note: expected mutable reference `&mut String`
                     found reference `&String`
note: function defined here
 --> ..\src\main.rs:8:4
  |
8 | fn takes_ref(_: &mut String) {}
  |    ^^^^^^^^^ --------------

Diagnostics after:

error[E0308]: mismatched types                                                                                                                        
 --> ..\src\main.rs:3:15
  |
3 |     takes_raw(&raw const s);
  |     --------- ^^^^^^^^^^^^ types differ in mutability
  |     |
  |     arguments to this function are incorrect
  |
  = note: expected raw pointer `*mut String`
             found raw pointer `*const String`
note: function defined here
 --> ..\src\main.rs:7:4
  |
7 | fn takes_raw(_: *mut String) {}
  |    ^^^^^^^^^ --------------
help: consider using `&raw mut` instead
  |
3 -     takes_raw(&raw const s);
3 +     takes_raw(&raw mut s);
  |

error[E0308]: mismatched types                                                                                                                        
 --> ..\src\main.rs:4:15
  |
4 |     takes_ref(&s);
  |     --------- ^^ types differ in mutability
  |     |
  |     arguments to this function are incorrect
  |
  = note: expected mutable reference `&mut String`
                     found reference `&String`
note: function defined here
 --> ..\src\main.rs:8:4
  |
8 | fn takes_ref(_: &mut String) {}
  |    ^^^^^^^^^ --------------
help: consider using `&mut` instead
  |
4 |     takes_ref(&mut s);
  |                +++

[rustbot]

r? @petrochenkov

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[petrochenkov]

Could you add a test case for &raw const -> &raw mut as well?
@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[chenyukang]

This code:

fn takes_raw(_: *mut String) {}

fn main() {
    let mut value = String::new();
    takes_raw(&value);
}

we can also suggest &mut here, but seems this PR missed it.

also please git squash the commits.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[TirushOne]

I made a dogs breakfast out of the git history for this PR, instead of untangling it I created a new PR with the suggested changes and assigned you to it @petrochenkov. You can find it here as #162106.