Fix incorrect suggestion for boxing tail expression in blocks

Author: estebank

compiler/rustc_hir_typeck/src/demand.rs

@@ -44,7 +44,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             || self.suggest_non_zero_new_unwrap(err, expr, expected, expr_ty)
             || self.suggest_calling_boxed_future_when_appropriate(err, expr, expected, expr_ty)
             || self.suggest_no_capture_closure(err, expected, expr_ty)
-            || self.suggest_boxing_when_appropriate(err, expr.span, expr.hir_id, expected, expr_ty)
+            || self.suggest_boxing_when_appropriate(
+                err,
+                expr.peel_blocks().span,
+                expr.hir_id,
+                expected,
+                expr_ty,
+            )
             || self.suggest_block_to_brackets_peeling_refs(err, expr, expr_ty, expected)
             || self.suggest_copied_cloned_or_as_ref(err, expr, expr_ty, expected)
             || self.suggest_clone_for_ref(err, expr, expr_ty, expected)

tests/ui/typeck/suggest-box-on-divergent-if-else-arms.fixed

@@ -0,0 +1,14 @@
+// run-rustfix
+trait Trait {}
+struct Struct;
+impl Trait for Struct {}
+fn foo() -> Box<dyn Trait> {
+    Box::new(Struct)
+}
+fn main() {
+    let _ = if true {
+        foo()
+    } else {
+        Box::new(Struct) //~ ERROR E0308
+    };
+}

tests/ui/typeck/suggest-box-on-divergent-if-else-arms.rs

@@ -0,0 +1,14 @@
+// run-rustfix
+trait Trait {}
+struct Struct;
+impl Trait for Struct {}
+fn foo() -> Box<dyn Trait> {
+    Box::new(Struct)
+}
+fn main() {
+    let _ = if true {
+        foo()
+    } else {
+        Struct //~ ERROR E0308
+    };
+}

tests/ui/typeck/suggest-box-on-divergent-if-else-arms.stderr

@@ -0,0 +1,24 @@
+error[E0308]: `if` and `else` have incompatible types
+  --> $DIR/suggest-box-on-divergent-if-else-arms.rs:12:9
+   |
+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: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.