Diagnostic hints emitted on unresolved method calls that suggest qualified associated function calls don't account for more complex self types

[fmease]

Uplifted from #118911 (comment) and further noticed while reviewing #119957.

With “more complex self types” I mean self types like Box<Self>, Rc<Self> etc. & feature(arbitrary_self_types), basically anything other than Self, &&...Self and &mut &mut... Self.

---

Given:

struct S;

trait TraitA { fn f(this: Box<Self>); }
trait TraitB { fn f(this: Box<Self>); }

impl TraitA for S { fn f(this: Box<Self>) {} }
impl TraitB for S { fn f(this: Box<Self>) {} }

fn main() {
    Box::new(S).f(); //~ ERROR no method named `f` found for struct `Box<S>` in the current scope [E0599]
}

We currently output:

error[E0599]: no method named `f` found for struct `Box<S>` in the current scope
  --> src/main.rs:10:17
   |
10 |     Box::new(S).f();
   |                 ^ this is an associated function, not a method
   |
   = note: found the following associated functions; to be used as methods, functions must have a `self` parameter
note: candidate #1 is defined in the trait `TraitA`
  --> src/main.rs:3:16
   |
3  | trait TraitA { fn f(this: Box<Self>); }
   |                ^^^^^^^^^^^^^^^^^^^^^^
note: candidate #2 is defined in the trait `TraitB`
  --> src/main.rs:4:16
   |
4  | trait TraitB { fn f(this: Box<Self>); }
   |                ^^^^^^^^^^^^^^^^^^^^^^
help: disambiguate the associated function for candidate #1
   |
10 |     <Box<S> as TraitA>::f(Box::new(S));
   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
help: disambiguate the associated function for candidate #2
   |
10 |     <Box<S> as TraitB>::f(Box::new(S));
   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

However, <Box<S> as $Trait>::f(Box::new(S)) is not correct ($Trait ∊ {TraitA, TraitB}).
We should either suggest

  help: disambiguate the associated function for candidate #1
     |
- 10 |     <Box<S> as TraitA>::f(Box::new(S));
-    |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ 10 |     TraitA::f(Box::new(S));
+    |     ~~~~~~~~~~~~~~~~~~~~~~
  help: disambiguate the associated function for candidate #2
     |
- 10 |     <Box<S> as TraitB>::f(Box::new(S));
-    |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ 10 |     TraitB::f(Box::new(S));
+    |     ~~~~~~~~~~~~~~~~~~~~~~

or

  help: disambiguate the associated function for candidate #1
     |
- 10 |     <Box<S> as TraitA>::f(Box::new(S));
-    |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ 10 |     <S as TraitA>::f(Box::new(S));
+    |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  help: disambiguate the associated function for candidate #2
     |
- 10 |     <Box<S> as TraitB>::f(Box::new(S));
-    |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ 10 |     <S as TraitB>::f(Box::new(S));
+    |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

---

Given:

struct A;

trait M {
    fn f(_: Box<Self>);
}

impl M for A {
    fn f(_: Box<Self>) {}
}

fn main() {
    Box::new(A).f(); //~ ERROR no method named `f` found for struct `Box<A>` in the current scope [E0599]
}

We should output the following:

  error[E0599]: no method named `f` found for struct `Box<A>` in the current scope
    --> src/main.rs:12:17
     |
  12 |     Box::new(A).f();
     |     ------------^--
     |     |           |
     |     |           this is an associated function, not a method
-    |     help: use associated function syntax instead: `Box<A>::f()`
+    |     help: use associated function syntax instead: `A::f(Box::new(A))`
     |
     = note: found the following associated functions; to be used as methods, functions must have a `self` parameter
  note: the candidate is defined in the trait `M`
    --> src/main.rs:4:5
     |
  4  |     fn f(_: Box<Self>);
     |     ^^^^^^^^^^^^^^^^^^^
  

[fmease]

cc @Young-Flash

[estebank]

CC @compiler-errors

print_disambiguation_help needs some reworking, I'm not sure what the right setup is to have the trait ref reference the right Self type in the face of arbitrary self types, but it shouldn't be too much work to fix.