Migrate SuggestBoxingForReturnImplTrait, Fix typo in infer_fn_consider_casting

compiler/rustc_infer/messages.ftl

@@ -362,3 +362,6 @@ infer_sarwa_option = you can convert from `&Option<T>` to `Option<&T>` using `.a
 infer_sarwa_result = you can convert from `&Result<T, E>` to `Result<&T, &E>` using `.as_ref()`
 
 infer_suggest_accessing_field = you might have meant to use field `{$name}` whose type is `{$ty}`
+
+infer_sbfrit_change_return_type = you could change the return type to be a boxed trait object
+infer_sbfrit_box_return_expr = if you change the return type to expect trait objects, box the returned expressions

compiler/rustc_infer/src/errors/mod.rs

@@ -1261,7 +1261,7 @@ pub struct FnItemsAreDistinct;
 pub struct FnUniqTypes;
 
 #[derive(Subdiagnostic)]
-#[help(infer_fn_uniq_types)]
+#[help(infer_fn_consider_casting)]
 pub struct FnConsiderCasting {
     pub casting: String,
 }
@@ -1317,3 +1317,21 @@ pub enum SuggestAccessingField<'a> {
         ty: Ty<'a>,
     },
 }
+
+#[derive(Subdiagnostic)]
+pub enum SuggestBoxingForReturnImplTrait {
+    #[multipart_suggestion(infer_sbfrit_change_return_type, applicability = "maybe-incorrect")]
+    ChangeReturnType {
+        #[suggestion_part(code = "Box<dyn")]
+        start_sp: Span,
+        #[suggestion_part(code = ">")]
+        end_sp: Span,
+    },
+    #[multipart_suggestion(infer_sbfrit_box_return_expr, applicability = "maybe-incorrect")]
+    BoxReturnExpr {
+        #[suggestion_part(code = "Box::new(")]
+        starts: Vec<Span>,
+        #[suggestion_part(code = ")")]
+        ends: Vec<Span>,
+    },
+}

compiler/rustc_infer/src/infer/error_reporting/suggest.rs

@@ -15,7 +15,8 @@ use rustc_target::abi::FieldIdx;
 use crate::errors::{
     ConsiderAddingAwait, DiagArg, FnConsiderCasting, FnItemsAreDistinct, FnUniqTypes,
     FunctionPointerSuggestion, SuggAddLetForLetChains, SuggestAccessingField,
-    SuggestAsRefWhereAppropriate, SuggestRemoveSemiOrReturnBinding,
+    SuggestAsRefWhereAppropriate, SuggestBoxingForReturnImplTrait,
+    SuggestRemoveSemiOrReturnBinding,
 };
 
 use super::TypeErrCtxt;
@@ -80,25 +81,20 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         return_sp: Span,
         arm_spans: impl Iterator<Item = Span>,
     ) {
-        err.multipart_suggestion(
-            "you could change the return type to be a boxed trait object",
-            vec![
-                (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()),
-                (return_sp.shrink_to_hi(), ">".to_string()),
-            ],
-            Applicability::MaybeIncorrect,
-        );
-        let sugg = arm_spans
-            .flat_map(|sp| {
-                [(sp.shrink_to_lo(), "Box::new(".to_string()), (sp.shrink_to_hi(), ")".to_string())]
-                    .into_iter()
-            })
-            .collect::<Vec<_>>();
-        err.multipart_suggestion(
-            "if you change the return type to expect trait objects, box the returned expressions",
-            sugg,
-            Applicability::MaybeIncorrect,
-        );
+        let sugg = SuggestBoxingForReturnImplTrait::ChangeReturnType {
+            start_sp: return_sp.with_hi(return_sp.lo() + BytePos(4)),
+            end_sp: return_sp.shrink_to_hi(),
+        };
+        err.subdiagnostic(sugg);
+
+        let mut starts = Vec::new();
+        let mut ends = Vec::new();
+        for span in arm_spans {
+            starts.push(span.shrink_to_lo());
+            ends.push(span.shrink_to_hi());
+        }
+        let sugg = SuggestBoxingForReturnImplTrait::BoxReturnExpr { starts, ends };
+        err.subdiagnostic(sugg);
     }
 
     pub(super) fn suggest_tuple_pattern(