A couple of confusing diagnostics with &str slices

[durka]

https://play.rust-lang.org/?gist=089ca2d7214f6761051c&version=stable

let s = "abc";
let t = if true { s[..2] } else { s };       // confusing error message
let u: &str = if true { s[..2] } else { s }; // clear error message
let v = s[..2];                              // strange error message
let w: &str = s[..2];                        // clear error message

These are four attempts to take a slice of a &str, in two pairs. They are all wrong because indexing a &str returns a str, so you need an extra &. That's not the point. The point is without type annotations, the error diagnostics seem like they could use some improvement.

Especially the first one -- it sounds like it is saying my expression ought to be a str, but it already is. I think it's actually saying the associated type Index::Output ought to be a str, but the way the diagnostic is written, it sounds backwards.

Can either of these diagnostics be improved?

[steveklabnik]

Three of these have the same errors now: mismatched type between str and &str. The third has an error about Sized not being implemented.

[durka]

Maybe there could be a suggestion, if a variable is inferred to type T: !Sized, to add a & and make the variable &T instead?

[estebank]

Suggested output:

error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
 --> src/main.rs:3:5
  |
3 | let v = s[..2];                              // strange error message
  |     ^   ------ help: considering borrowing the slice: `&s[..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
help: considering borrowing the slice