Improve error message for attempt to statically allocate trait object

[gabcbrown]

Simple example:

trait T {}

struct S;

impl T for S {}

fn foo<X: T>(s: X) -> X {
    s
}

fn main() {
    let x: T = foo(S);
}

This gives the error message:

error[E0277]: the size for values of type `dyn T` cannot be known at compilation time
  --> src/main.rs:12:9
   |
12 |     let x: T = foo(S);
   |         ^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `dyn T`
   = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
   = note: all local variables must have a statically known size
   = help: unsized locals are gated as an unstable feature

The following code works, so perhaps it would be useful for the compiler error to suggest using Box:

trait T {}

struct S;

impl T for S {}

fn foo<X: T>(s: X) -> Box<X> {
    Box::new(s)
}

fn main() {
    let x: Box<T> = foo(S);
}

[estebank]

It should probably also suggest trait objects and link to https://doc.rust-lang.org/book/ch17-02-trait-objects.html

[estebank]

After #106223 the output will be

~/workspace/rust% rustc +dev f57.rs
error[E0308]: mismatched types
  --> f57.rs:11:20
   |
11 |     let x: dyn T = foo(S);
   |            -----   ^^^^^^ expected trait object `dyn T`, found struct `S`
   |            |
   |            expected due to this
   |
   = note: expected trait object `dyn T`
                    found struct `S`

error[E0277]: the size for values of type `dyn T` cannot be known at compilation time
  --> f57.rs:11:9
   |
11 |     let x: dyn T = foo(S);
   |         ^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `dyn T`
   = note: all local variables must have a statically known size
   = help: unsized locals are gated as an unstable feature
help: consider borrowing here
   |
11 |     let x: &dyn T = foo(S);
   |            +

We probably should mention Box<dyn T> as well.

[estebank]

Current output:

error[E0308]: mismatched types
  --> src/main.rs:12:20
   |
12 |     let x: dyn T = foo(S);
   |            -----   ^^^^^^ expected `dyn T`, found `S`
   |            |
   |            expected due to this
   |
   = note: expected trait object `dyn T`
                    found struct `S`
   = help: `S` implements `T` so you could box the found value and coerce it to the trait object `Box<dyn T>`, you will have to change the expected type as well

error[E0277]: the size for values of type `dyn T` cannot be known at compilation time
  --> src/main.rs:12:9
   |
12 |     let x: dyn T = foo(S);
   |         ^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `dyn T`
   = note: all local variables must have a statically known size
   = help: unsized locals are gated as an unstable feature
help: consider borrowing here
   |
12 |     let x: &dyn T = foo(S);
   |            +