Don't suggest extra clone when converting cloned slice to Vec

[sunjay]

Let's say you have the following (simplified) code: (Rust Playground)

fn main() {
    let items = vec![1, 2, 3];
    let ref_items: &[i32] = &items;
    let items_clone: Vec<i32> = ref_items.clone();
}

This results in the following error:

error[E0308]: mismatched types
 --> src/main.rs:4:33
  |
4 |     let items_clone: Vec<i32> = ref_items.clone();
  |                                 ^^^^^^^^^^^^^^^^^
  |                                 |
  |                                 expected struct `std::vec::Vec`, found &[i32]
  |                                 help: try using a conversion method: `ref_items.clone().to_vec()`
  |
  = note: expected type `std::vec::Vec<i32>`
             found type `&[i32]`

The suggested fix ref_items.clone().to_vec() will work, but to_vec already copies the data so the extra clone is wasteful. The error message should instead suggest ref_items.to_vec().

This is an error you can run into if ref_items was previously of type &Vec<i32> and you changed it to a slice after writing the rest of the code.

[csmoe]

Note: this kind of improvement applies to &str too:
https://play.rust-lang.org/?gist=187e7bb009a976693176dcb45d3fdc8a&version=stable&mode=debug&edition=2015

fn main() {
    let s = "hi";
    let string: String = s.clone();
}

Err:

error[E0308]: mismatched types
 --> src/main.rs:3:26
  |
3 |     let string: String = s.clone();
  |                          ^^^^^^^^^
  |                          |
  |                          expected struct `std::string::String`, found &str
  |                          help: try using a conversion method: `s.clone().to_string()`
  |
  = note: expected type `std::string::String`
             found type `&str`