Rustc suggesting to add a already existing trait bound, instead of relaxing an entirely different one

[onestacked]

I tried this code:

struct Victim<'a, T: Perpetrator + ?Sized>
where
  Self: Sized // rustc suggests adding this trait bound, even if it already exists, however this is absolutely not needed
{
  value: u8,
  perp: &'a T,
}

trait VictimTrait {
  type Ret;
  fn get(self) -> Self::Ret;
}

// Actual fix is here
impl<'a, T: Perpetrator /*+ ?Sized*/> VictimTrait for Victim<'a, T> {
  type Ret = u8;
  fn get(self) -> Self::Ret {
    self.value
  }
}

trait Perpetrator {
  fn getter<'a>(&'a self) -> Victim<'a, Self> {
    Victim {
      value: 0,
      perp: self,
    }
  }

  fn trigger(&self) {
    self.getter().get();
  }
}

This does not compile with the following error message:

error[[E0599]](https://doc.rust-lang.org/stable/error-index.html#E0599): the method `get` exists for struct `Victim<'_, Self>`, but its trait bounds were not satisfied
  --> src/main.rs:31:19
   |
1  | / struct Victim<'a, T: Perpetrator + ?Sized>
2  | | where
3  | |   Self: Sized // rustc sugests adding this trait bound, even if it already exists
4  | | {
5  | |   value: u8,
6  | |   perp: &'a T,
7  | | }
   | | -
   | | |
   | |_method `get` not found for this
   |   doesn't satisfy `Victim<'_, Self>: VictimTrait`
...
31 |       self.getter().get();
   |                     ^^^ method cannot be called on `Victim<'_, Self>` due to unsatisfied trait bounds
   |
note: trait bound `Self: Sized` was not satisfied
  --> src/main.rs:15:10
   |
15 | impl<'a, T: Perpetrator /*+ ?Sized*/> VictimTrait for Victim<'a, T> {
   |          ^                            -----------     -------------
   |          |
   |          unsatisfied trait bound introduced here
help: consider restricting the type parameter to satisfy the trait bound
   |
3  |   Self: Sized, Self: Sized // rustc sugests adding this trait bound, even if it already exists
   |              +++++++++++++

For more information about this error, try `rustc --explain E0599`.

This message is misleading, instruction you to add an (possibly) already existing trait bound instead of the actual fix, which is commented out in the code

Meta

This bug currently exists both in stable 1.61.0 and nightly 1.64.0-nightly(2022-06-25 20a6f3a).

playground link
gist link

[estebank]

I got a reasonable PR to suggest the appropriate fix, but removing the improper suggestion is gonna be hard because it's too naïve and flat out incorrect in cases like these where the obligation doesn't come directly from the impl'd trait:

error[E0599]: the method `get` exists for struct `Victim<'_, Self>`, but its trait bounds were not satisfied
  --> src/test/ui/trait-bounds/impl-derived-implicit-sized-bound.rs:31:19
   |
1  | / struct Victim<'a, T: Perpetrator + ?Sized>
2  | | where
3  | |   Self: Sized
4  | | {
5  | |   value: u8,
6  | |   perp: &'a T,
7  | | }
   | | -
   | | |
   | |_method `get` not found for this
   |   doesn't satisfy `Victim<'_, Self>: VictimTrait`
...
31 |       self.getter().get();
   |                     ^^^ method cannot be called on `Victim<'_, Self>` due to unsatisfied trait bounds
   |
note: trait bound `Self: Sized` was not satisfied
  --> src/test/ui/trait-bounds/impl-derived-implicit-sized-bound.rs:15:10
   |
15 | impl<'a, T: Perpetrator /*+ ?Sized*/> VictimTrait for Victim<'a, T> {
   |          ^                            -----------     -------------
   |          |
   |          unsatisfied trait bound introduced here
help: consider relaxing the type parameter's implicit `?Sized` bound
   |
15 | impl<'a, T: ?Sized + Perpetrator /*+ ?Sized*/> VictimTrait for Victim<'a, T> {
   |             ++++++++
help: consider restricting the type parameter to satisfy the trait bound
   |
3  |   Self: Sized, Self: Sized
   |              +++++++++++++

error: aborting due to previous error