Add boxing suggestions for impl Trait return type mismatchesAdd boxing suggestions for impl Trait return type mismatches

[Unique-Usman]

A sort of a follow up pr to this -> #155546

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[mejrs]

r? me

We already have a SuggestBoxingForReturnImplTrait suggestion.

Please use it instead. Or ideally, figure out why it's not being emitted in some cases, like here

// suggestion emitted
fn works() -> impl std::fmt::Display {
    match 13 {
        0 => 0i32,
        _ => 1u32,
    }
}

// suggestion not given
fn oh_no() -> impl std::fmt::Display {
    match 13 {
        0 => return 0i32,
        _ => 1u32,
    }
}

error[E0308]: `match` arms have incompatible types
 --> src/lib.rs:5:14
  |
3 | /     match 13 {
4 | |         0 => 0i32,
  | |              ---- this is found to be of type `i32`
5 | |         _ => 1u32,
  | |              ^^^^ expected `i32`, found `u32`
6 | |     }
  | |_____- `match` arms have incompatible types
  |
help: you could change the return type to be a boxed trait object
  |
2 - fn works() -> impl std::fmt::Display {
2 + fn works() -> Box<dyn std::fmt::Display> {
  |
help: if you change the return type to expect trait objects, box the returned expressions
  |
4 ~         0 => Box::new(0i32),
5 ~         _ => Box::new(1u32),
  |
help: change the type of the numeric literal from `u32` to `i32`
  |
5 -         _ => 1u32,
5 +         _ => 1i32,
  |

error[E0308]: mismatched types
  --> src/lib.rs:13:14
   |
10 | fn oh_no() -> impl std::fmt::Display {
   |               ---------------------- expected a single type implementing `std::fmt::Display` because of return type
11 |     match 13 {
12 |         0 => return 0i32,
   |                     ---- return type resolved to be `i32`
13 |         _ => 1u32,
   |              ^^^^ expected `i32`, found `u32`
   |
help: you can convert a `u32` to an `i32` and panic if the converted value doesn't fit
   |
14 |     }.try_into().unwrap()
   |      ++++++++++++++++++++

For more information about this error, try `rustc --explain E0308`.

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