Using an unsized value from slicing incorrectly should give a more helpful error message

[huonw]

fn main() {
    let y = "foo";
    let x = y[1..2];
}

<anon>:3:9: 3:10 error: the trait `core::marker::Sized` is not implemented for the type `str`
<anon>:3     let x = y[1..2];
                 ^

The fix is to write let x = &y[1..2]; but it's not the first thing that springs to mind given that error message, especially for a beginner.

cc #20783

[steveklabnik]

Triage: the error is slightly better:

hello.rs:3:9: 3:10 error: the trait `core::marker::Sized` is not implemented for the type `str` [E0277]
hello.rs:3     let x = y[1..2];
                   ^
hello.rs:3:9: 3:10 help: run `rustc --explain E0277` to see a detailed explanation
hello.rs:3:9: 3:10 note: `str` does not have a constant size known at compile-time
hello.rs:3:9: 3:10 note: all local variables must have a statically known size
error: aborting due to previous error

But still does not lead you to the fix.

[estebank]

Current output:

error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
 --> src/main.rs:3:9
  |
3 |     let x = y[1..2];
  |         ^   ------- help: consider borrowing here: `&y[1..2]`
  |         |
  |         `str` does not have a constant size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `str`
  = note: all local variables must have a statically known size