Hint to suggest annotating type to assist unsized coercion

[compiler-errors]

It would be useful if we could suggest for users to add type annotations to assist in places where the compiler is unable to automagically infer an unsized coersion, e.g. when declaration is separated from usage by a let statement like this code (playground link):

struct Wrapper<T>(T);

fn takes_any(x: Wrapper<Box<dyn std::any::Any>>) {}

fn test() {
    let x = Wrapper(Box::new(1i32));
    takes_any(x);
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/lib.rs:9:15
  |
9 |     takes_any(x);
  |               ^ expected trait object `dyn Any`, found `i32`
  |
  = note: expected struct `Wrapper<Box<(dyn Any + 'static)>>`
             found struct `Wrapper<Box<i32>>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

Ideally the output should also suggest adding an explicit type annotation to assist in unsizing, like:

 --> src/lib.rs:8:11
  |
8 |    let x: Wrapper<Box<dyn Any>> = Wrapper(Box::new(1i32));
  |           ^^^^^^^^^^^^^^^^^^^^^
help: Annotate the type when the value is created

This error can show up when users are refactoring their code to make lines shorter or more readable, and they might otherwise be confused why an innocuous change can lead to a pretty esoteric compiler error that doesn't suggest a clear fix. Unsure what the best "help" message should be, though!