Rollup merge of #37442 - estebank:cast-deref-hint, r=jonathandturner

Provide hint when cast needs a dereference

For a given code:

``` rust
vec![0.0].iter().map(|s| s as i16).collect::<Vec<i16>>();
```

display:

``` nocode
error: casting `&f64` as `i16` is invalid
 --> file3.rs:2:35
  |
2 |     vec![0.0].iter().map(|s| s as i16).collect::<Vec<i16>>();
  |                              -    ^^^
  |                              |
  |                              did you mean `*s`?
```

instead of:

``` nocode
error: casting `&f64` as `i16` is invalid
 --> <anon>:2:30
  |
2 |     vec![0.0].iter().map(|s| s as i16).collect();
  |                              ^^^^^^^^
  |
  = help: cast through a raw pointer first
```

Fixes #37338.

src/test/compile-fail/cast-rfc0401.rs

@@ -115,4 +115,9 @@ fn main()
     let _ = cf as *const Bar;
     //~^ ERROR casting
     //~^^ NOTE vtable kinds
+
+    vec![0.0].iter().map(|s| s as f32).collect::<Vec<f32>>();
+    //~^ ERROR casting `&{float}` as `f32` is invalid
+    //~| NOTE cannot cast `&{float}` as `f32`
+    //~| NOTE did you mean `*s`?
 }