casting variable with type `&Unsized` to `&dyn Trait` incorrectly says that the variable is unsized.

[hkalbasi]

Code

use std::fmt::Debug;

fn f(x: &dyn Debug) {
    println!("{:?}", x)
}

fn main() {
    let x: &[u8] = &[1, 2, 3];
    f(x);
}

Current output

Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/lib.rs:9:7
  |
9 |     f(x);
  |       ^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[u8]`
  = note: required for the cast from `[u8]` to the object type `dyn Debug`
help: consider borrowing the value, since `&[u8]` can be coerced into `dyn Debug`
  |
9 |     f(&x);
  |       +

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (lib) due to previous error

Desired output

No response

Rationale and extra context

This error confused me as it was in a context that I thought the type of x is actually [u8], but it was &[u8] and the problem was about dyn cast.

Other cases

No response

Anything else?

No response

[Zoybean]

I can see how this pointer could have been especially misleading, given x is &[u8] and therefore Sized:

9 |     f(x);
  |       ^ doesn't have a size known at compile-time

Maybe if it instead said:

...
9 |     f(x);
  |       ^ casting this value from `&[u8]` to the object type `&dyn Debug`
...

but that might risk being too much repetition of similar-but-distinct terms ([u8],&[u8],dyn Debug,&dyn Debug) in the context of the full error