E0308 Fallback to blanket impl due to lack of ?Sized creates a puzzling type error

Code

trait MyToOwned {
    type Owned;
    fn my_to_owned(&self) -> Self::Owned;
}

impl<T: ToOwned> MyToOwned for T {
    type Owned = <T as ToOwned>::Owned;
    fn my_to_owned(&self) -> Self::Owned {
        self.to_owned()
    }
}

fn main() {
    let a = "asd";
    let b: String = a.to_owned();
    let c: String = a.my_to_owned();
}

Current output

error[E0308]: mismatched types
  --> src/main.rs:16:21
   |
16 |     let c: String = a.my_to_owned();
   |            ------   ^^^^^^^^^^^^^^^- help: try using a conversion method: `.to_string()`
   |            |        |
   |            |        expected `String`, found `&str`
   |            expected due to this

Desired output

The ultimate solution is to add `?Sized`:

`impl<T: ToOwned + ?Sized> MyToOwned for T`

Rationale and extra context

Type error is technically correct, but it's very surprising in this context, and it's hard to see why the generic impl doesn't seem to work.

Perhaps the error could hint that a blanket impl has been used to satisfy T: ToOwned for &str instead of using impl for str:

https://doc.rust-lang.org/stable/std/borrow/trait.ToOwned.html#impl-ToOwned-for-T

Other cases

Rust Version

rustc 1.91.0-nightly (07d246fc6 2025-08-31)

Anything else?

No response

Reproduce the example from the issue in src/main.rs with rustc 1.91.0-nightly and confirm the current E0308 diagnostic. Trace the diagnostic's handling of the blanket ToOwned implementation and determine whether it can identify the missing ?Sized bound. Done means the compiler gives a useful hint explaining the selected blanket impl and suggesting ?Sized.