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

[kornelski]

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