Provide structured suggestion to use trait objects in some cases of if arm type divergence

[estebank]

error[E0308]: `if` and `else` have incompatible types
  --> $DIR/suggest-box-on-divergent-if-else-arms.rs:15:9
   |
LL |       let _ = if true {
   |  _____________-
LL | |         Struct
   | |         ------ expected because of this
LL | |     } else {
LL | |         foo()
   | |         ^^^^^ expected `Struct`, found `Box<dyn Trait>`
LL | |     };
   | |_____- `if` and `else` have incompatible types
   |
   = note: expected struct `Struct`
              found struct `Box<dyn Trait>`
help: `Struct` implements `Trait` so you can box it to coerce to the trait object `Box<dyn Trait>`
   |
LL |         Box::new(Struct)
   |         +++++++++      +

error[E0308]: `if` and `else` have incompatible types
  --> $DIR/suggest-box-on-divergent-if-else-arms.rs:20:9
   |
LL |       let _ = if true {
   |  _____________-
LL | |         foo()
   | |         ----- expected because of this
LL | |     } else {
LL | |         Struct
   | |         ^^^^^^ expected `Box<dyn Trait>`, found `Struct`
LL | |     };
   | |_____- `if` and `else` have incompatible types
   |
   = note: expected struct `Box<dyn Trait>`
              found struct `Struct`
   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
help: store this in the heap by calling `Box::new`
   |
LL |         Box::new(Struct)
   |         +++++++++      +

error[E0308]: `if` and `else` have incompatible types
  --> $DIR/suggest-box-on-divergent-if-else-arms.rs:25:9
   |
LL |   fn bar() -> impl Trait {
   |               ---------- the found opaque type
...
LL |       let _ = if true {
   |  _____________-
LL | |         Struct
   | |         ------ expected because of this
LL | |     } else {
LL | |         bar()
   | |         ^^^^^ expected `Struct`, found opaque type
LL | |     };
   | |_____- `if` and `else` have incompatible types
   |
   = note:   expected struct `Struct`
           found opaque type `impl Trait`
help: `Struct` implements `Trait` so you can box both arms and coerce to the trait object `Box<dyn Trait>`
   |
LL ~         Box::new(Struct) as Box<dyn Trait>
LL |     } else {
LL ~         Box::new(bar())
   |

error[E0308]: `if` and `else` have incompatible types
  --> $DIR/suggest-box-on-divergent-if-else-arms.rs:30:9
   |
LL |   fn bar() -> impl Trait {
   |               ---------- the expected opaque type
...
LL |       let _ = if true {
   |  _____________-
LL | |         bar()
   | |         ----- expected because of this
LL | |     } else {
LL | |         Struct
   | |         ^^^^^^ expected opaque type, found `Struct`
LL | |     };
   | |_____- `if` and `else` have incompatible types
   |
   = note: expected opaque type `impl Trait`
                   found struct `Struct`
help: `Struct` implements `Trait` so you can box both arms and coerce to the trait object `Box<dyn Trait>`
   |
LL ~         Box::new(bar()) as Box<dyn Trait>
LL |     } else {
LL ~         Box::new(Struct)
   |

Partially address #102629.

[rustbot]

r? @wesleywiser

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[wesleywiser]

Nice! 🏅

[wesleywiser]

@bors r+ rollup

[bors]

📌 Commit 34f4f3d has been approved by wesleywiser

It is now in the queue for this repository.