Rollup merge of #120261 - estebank:issue-102629, r=wesleywiser

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

```
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.

3 files changed, 158 insertions, 0 deletions

diff --git a/tests/ui/typeck/suggest-box-on-divergent-if-else-arms.fixed b/tests/ui/typeck/suggest-box-on-divergent-if-else-arms.fixed
new file mode 100644
index 00000000000..9ce46bc1a65
--- /dev/null
+++ b/tests/ui/typeck/suggest-box-on-divergent-if-else-arms.fixed
@@ -0,0 +1,32 @@
+// run-rustfix
+trait Trait {}
+struct Struct;
+impl Trait for Struct {}
+fn foo() -> Box<dyn Trait> {
+    Box::new(Struct)
+}
+fn bar() -> impl Trait {
+    Struct
+}
+fn main() {
+    let _ = if true {
+        Box::new(Struct)
+    } else {
+        foo() //~ ERROR E0308
+    };
+    let _ = if true {
+        foo()
+    } else {
+        Box::new(Struct) //~ ERROR E0308
+    };
+    let _ = if true {
+        Box::new(Struct) as Box<dyn Trait>
+    } else {
+        Box::new(bar()) //~ ERROR E0308
+    };
+    let _ = if true {
+        Box::new(bar()) as Box<dyn Trait>
+    } else {
+        Box::new(Struct) //~ ERROR E0308
+    };
+}
diff --git a/tests/ui/typeck/suggest-box-on-divergent-if-else-arms.rs b/tests/ui/typeck/suggest-box-on-divergent-if-else-arms.rs
new file mode 100644
index 00000000000..7f65a3bb59d
--- /dev/null
+++ b/tests/ui/typeck/suggest-box-on-divergent-if-else-arms.rs
@@ -0,0 +1,32 @@
+// run-rustfix
+trait Trait {}
+struct Struct;
+impl Trait for Struct {}
+fn foo() -> Box<dyn Trait> {
+    Box::new(Struct)
+}
+fn bar() -> impl Trait {
+    Struct
+}
+fn main() {
+    let _ = if true {
+        Struct
+    } else {
+        foo() //~ ERROR E0308
+    };
+    let _ = if true {
+        foo()
+    } else {
+        Struct //~ ERROR E0308
+    };
+    let _ = if true {
+        Struct
+    } else {
+        bar() //~ ERROR E0308
+    };
+    let _ = if true {
+        bar()
+    } else {
+        Struct //~ ERROR E0308
+    };
+}
diff --git a/tests/ui/typeck/suggest-box-on-divergent-if-else-arms.stderr b/tests/ui/typeck/suggest-box-on-divergent-if-else-arms.stderr
new file mode 100644
index 00000000000..c58bf60e7d6
--- /dev/null
+++ b/tests/ui/typeck/suggest-box-on-divergent-if-else-arms.stderr
@@ -0,0 +1,94 @@
+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)
+   |
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.