Missing `&` in "use the fully qualified path for the potential candidates" suggestion

[dtolnay]

Given the following code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d14f1277f1ad02ee89b35d73b15d2ad6

struct Thing;

trait Method<T> {
    fn method(&self) -> T;
}

impl Method<i32> for Thing {
    fn method(&self) -> i32 { 0 }
}

impl Method<u32> for Thing {
    fn method(&self) -> u32 { 0 }
}

fn main() {
    let thing = Thing;
    thing.method();
}

The current output is:

error[E0283]: type annotations needed
  --> src/main.rs:17:11
   |
17 |     thing.method();
   |     ------^^^^^^--
   |     |     |
   |     |     cannot infer type for type parameter `T` declared on the trait `Method`
   |     this method call resolves to `T`
   |
note: multiple `impl`s satisfying `Thing: Method<_>` found
  --> src/main.rs:7:1
   |
7  | impl Method<i32> for Thing {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
...
11 | impl Method<u32> for Thing {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
help: use the fully qualified path for the potential candidates
   |
17 |     <Thing as Method<i32>>::method(thing);
   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
17 |     <Thing as Method<u32>>::method(thing);
   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Neither of the suggestions compiles because, unlike ., the suggested syntax does not perform autoref.

The correct output would observe that thing.method() is undergoing autoref and instead suggest:

<Thing as Method<i32>>::method(&thing);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<Thing as Method<u32>>::method(&thing);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[dtolnay]

Note: similarly I would expect <…>::method(&mut thing) for methods that involve autoref with a &mut self receiver.

[estebank]

@TaKO8Ki the second commit in #96347 uses multipart suggestions, which will likely make the "find where to put the &mut " easier. The "looking at the method definition self receiver and figure out the type of the expression to see if & or &mut should be added" would remain the same.

Edit: also, the same code needed to figure out the expr's type and check what's needed to make it match against the candidate can be used to skip the structured suggestion in the case of #96295. If the expression cannot be coerced/autoderefed to any of Self, &Self or &mut Self, then we don't supply a suggestion at all.