Auto merge of #75608 - estebank:suggest-boxed-match-exprs, r=lcnr,varkor

More structured suggestions for boxed trait objects instead of impl Trait on non-coerceable tail expressions

When encountering a `match` or `if` as a tail expression where the
different arms do not have the same type *and* the return type of that
`fn` is an `impl Trait`, check whether those arms can implement `Trait`
and if so, suggest using boxed trait objects.

Use structured suggestion for `impl T` to `Box<dyn T>`.

Fix https://github.com/rust-lang/rust/issues/69107

compiler/rustc_middle/src/traits/mod.rs

@@ -348,13 +348,16 @@ pub struct MatchExpressionArmCause<'tcx> {
     pub prior_arms: Vec<Span>,
     pub last_ty: Ty<'tcx>,
     pub scrut_hir_id: hir::HirId,
+    pub opt_suggest_box_span: Option<Span>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct IfExpressionCause {
     pub then: Span,
+    pub else_sp: Span,
     pub outer: Option<Span>,
     pub semicolon: Option<Span>,
+    pub opt_suggest_box_span: Option<Span>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]