Stop doing expensive work in opt_suggest_box_span eagerly

Author: compiler-errors

compiler/rustc_hir_typeck/messages.ftl

@@ -122,6 +122,10 @@ hir_typeck_return_stmt_outside_of_fn_body =
     .encl_body_label = the {$statement_kind} is part of this body...
     .encl_fn_label = ...not the enclosing function body
 
+hir_typeck_rpit_box_return_expr = if you change the return type to expect trait objects, box the returned expressions
+
+hir_typeck_rpit_change_return_type = you could change the return type to be a boxed trait object
+
 hir_typeck_rustcall_incorrect_args =
     functions with the "rust-call" ABI must take a single non-self tuple argument
 

compiler/rustc_hir_typeck/src/_match.rs

@@ -1,17 +1,13 @@
 use crate::coercion::{AsCoercionSite, CoerceMany};
 use crate::{Diverges, Expectation, FnCtxt, Needs};
 use rustc_errors::{Applicability, Diag};
-use rustc_hir::{
-    self as hir,
-    def::{CtorOf, DefKind, Res},
-    ExprKind, PatKind,
-};
+use rustc_hir::def::{CtorOf, DefKind, Res};
+use rustc_hir::def_id::LocalDefId;
+use rustc_hir::{self as hir, ExprKind, PatKind};
 use rustc_hir_pretty::ty_to_string;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
-use rustc_infer::traits::Obligation;
 use rustc_middle::ty::{self, Ty};
 use rustc_span::Span;
-use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 use rustc_trait_selection::traits::{
     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
 };
@@ -91,10 +87,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             let arm_ty = self.check_expr_with_expectation(arm.body, expected);
             all_arms_diverge &= self.diverges.get();
-
-            let opt_suggest_box_span = prior_arm.and_then(|(_, prior_arm_ty, _)| {
-                self.opt_suggest_box_span(prior_arm_ty, arm_ty, orig_expected)
-            });
+            let tail_defines_return_position_impl_trait =
+                self.return_position_impl_trait_from_match_expectation(orig_expected);
 
             let (arm_block_id, arm_span) = if let hir::ExprKind::Block(blk, _) = arm.body.kind {
                 (Some(blk.hir_id), self.find_block_span(blk))
@@ -120,7 +114,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         scrut_span: scrut.span,
                         source: match_src,
                         prior_non_diverging_arms: prior_non_diverging_arms.clone(),
-                        opt_suggest_box_span,
+                        tail_defines_return_position_impl_trait,
                     })),
                 ),
             };
@@ -422,7 +416,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         else_expr: &'tcx hir::Expr<'tcx>,
         then_ty: Ty<'tcx>,
         else_ty: Ty<'tcx>,
-        opt_suggest_box_span: Option<Span>,
+        tail_defines_return_position_impl_trait: Option<LocalDefId>,
     ) -> ObligationCause<'tcx> {
         let mut outer_span = if self.tcx.sess.source_map().is_multiline(span) {
             // The `if`/`else` isn't in one line in the output, include some context to make it
@@ -513,7 +507,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 then_ty,
                 else_ty,
                 outer_span,
-                opt_suggest_box_span,
+                tail_defines_return_position_impl_trait,
             })),
         )
     }
@@ -593,96 +587,37 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    /// When we have a `match` as a tail expression in a `fn` with a returned `impl Trait`
-    /// we check if the different arms would work with boxed trait objects instead and
-    /// provide a structured suggestion in that case.
-    pub(crate) fn opt_suggest_box_span(
+    // Does the expectation of the match define an RPIT?
+    // (e.g. we're in the tail of a function body)
+    //
+    // Returns the `LocalDefId` of the RPIT, which is always identity-substituted.
+    pub fn return_position_impl_trait_from_match_expectation(
         &self,
-        first_ty: Ty<'tcx>,
-        second_ty: Ty<'tcx>,
-        orig_expected: Expectation<'tcx>,
-    ) -> Option<Span> {
-        // FIXME(compiler-errors): This really shouldn't need to be done during the
-        // "good" path of typeck, but here we are.
-        match orig_expected {
-            Expectation::ExpectHasType(expected) => {
-                let TypeVariableOrigin {
-                    span,
-                    kind: TypeVariableOriginKind::OpaqueTypeInference(rpit_def_id),
-                    ..
-                } = self.type_var_origin(expected)?
-                else {
-                    return None;
-                };
-
-                let Some(rpit_local_def_id) = rpit_def_id.as_local() else {
-                    return None;
-                };
-                if !matches!(
-                    self.tcx.hir().expect_item(rpit_local_def_id).expect_opaque_ty().origin,
-                    hir::OpaqueTyOrigin::FnReturn(..)
-                ) {
-                    return None;
-                }
-
-                let sig = self.body_fn_sig()?;
-
-                let args = sig.output().walk().find_map(|arg| {
-                    if let ty::GenericArgKind::Type(ty) = arg.unpack()
-                        && let ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) = *ty.kind()
-                        && def_id == rpit_def_id
-                    {
-                        Some(args)
-                    } else {
-                        None
-                    }
-                })?;
-
-                if !self.can_coerce(first_ty, expected) || !self.can_coerce(second_ty, expected) {
-                    return None;
-                }
-
-                for ty in [first_ty, second_ty] {
-                    for (clause, _) in self
-                        .tcx
-                        .explicit_item_super_predicates(rpit_def_id)
-                        .iter_instantiated_copied(self.tcx, args)
-                    {
-                        let pred = clause.kind().rebind(match clause.kind().skip_binder() {
-                            ty::ClauseKind::Trait(trait_pred) => {
-                                assert!(matches!(
-                                    *trait_pred.trait_ref.self_ty().kind(),
-                                    ty::Alias(ty::Opaque, ty::AliasTy { def_id, args: alias_args, .. })
-                                    if def_id == rpit_def_id && args == alias_args
-                                ));
-                                ty::ClauseKind::Trait(trait_pred.with_self_ty(self.tcx, ty))
-                            }
-                            ty::ClauseKind::Projection(mut proj_pred) => {
-                                assert!(matches!(
-                                    *proj_pred.projection_ty.self_ty().kind(),
-                                    ty::Alias(ty::Opaque, ty::AliasTy { def_id, args: alias_args, .. })
-                                    if def_id == rpit_def_id && args == alias_args
-                                ));
-                                proj_pred = proj_pred.with_self_ty(self.tcx, ty);
-                                ty::ClauseKind::Projection(proj_pred)
-                            }
-                            _ => continue,
-                        });
-                        if !self.predicate_must_hold_modulo_regions(&Obligation::new(
-                            self.tcx,
-                            ObligationCause::misc(span, self.body_id),
-                            self.param_env,
-                            pred,
-                        )) {
-                            return None;
-                        }
-                    }
-                }
-
-                Some(span)
-            }
-            _ => None,
+        expectation: Expectation<'tcx>,
+    ) -> Option<LocalDefId> {
+        let expected_ty = expectation.to_option(self)?;
+        let (def_id, args) = match *expected_ty.kind() {
+            // FIXME: Could also check that the RPIT is not defined
+            ty::Alias(ty::Opaque, alias_ty) => (alias_ty.def_id.as_local()?, alias_ty.args),
+            // FIXME(-Znext-solver): Remove this branch once `replace_opaque_types_with_infer` is gone.
+            ty::Infer(ty::TyVar(_)) => self
+                .inner
+                .borrow()
+                .iter_opaque_types()
+                .find(|(_, v)| v.ty == expected_ty)
+                .map(|(k, _)| (k.def_id, k.args))?,
+            _ => return None,
+        };
+        let hir::OpaqueTyOrigin::FnReturn(parent_def_id) = self.tcx.opaque_type_origin(def_id)
+        else {
+            return None;
+        };
+        if &args[0..self.tcx.generics_of(parent_def_id).count()]
+            != ty::GenericArgs::identity_for_item(self.tcx, parent_def_id).as_slice()
+        {
+            return None;
         }
+        Some(def_id)
     }
 }
 

compiler/rustc_hir_typeck/src/coercion.rs

@@ -35,17 +35,18 @@
 //! // and are then unable to coerce `&7i32` to `&mut i32`.
 //! ```
 
+use crate::errors::SuggestBoxingForReturnImplTrait;
 use crate::FnCtxt;
 use rustc_errors::{codes::*, struct_span_code_err, Applicability, Diag, MultiSpan};
 use rustc_hir as hir;
-use rustc_hir::def_id::DefId;
+use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::Expr;
 use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::infer::{Coercion, DefineOpaqueTypes, InferOk, InferResult};
-use rustc_infer::traits::TraitEngine;
 use rustc_infer::traits::TraitEngineExt as _;
+use rustc_infer::traits::{IfExpressionCause, MatchExpressionArmCause, TraitEngine};
 use rustc_infer::traits::{Obligation, PredicateObligation};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::traits::BuiltinImplSource;
@@ -59,6 +60,7 @@ use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt};
 use rustc_session::parse::feature_err;
 use rustc_span::symbol::sym;
 use rustc_span::DesugaringKind;
+use rustc_span::{BytePos, Span};
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
@@ -1638,6 +1640,77 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                             unsized_return = self.is_return_ty_definitely_unsized(fcx);
                         }
                     }
+                    ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
+                        arm_span,
+                        arm_ty,
+                        prior_arm_ty,
+                        ref prior_non_diverging_arms,
+                        tail_defines_return_position_impl_trait: Some(rpit_def_id),
+                        ..
+                    }) => {
+                        err = fcx.err_ctxt().report_mismatched_types(
+                            cause,
+                            expected,
+                            found,
+                            coercion_error,
+                        );
+                        // Check that we're actually in the second or later arm
+                        if prior_non_diverging_arms.len() > 0 {
+                            self.suggest_boxing_tail_for_return_position_impl_trait(
+                                fcx,
+                                &mut err,
+                                rpit_def_id,
+                                arm_ty,
+                                prior_arm_ty,
+                                prior_non_diverging_arms
+                                    .iter()
+                                    .chain(std::iter::once(&arm_span))
+                                    .copied(),
+                            );
+                        }
+                    }
+                    ObligationCauseCode::IfExpression(box IfExpressionCause {
+                        then_id,
+                        else_id,
+                        then_ty,
+                        else_ty,
+                        tail_defines_return_position_impl_trait: Some(rpit_def_id),
+                        ..
+                    }) => {
+                        err = fcx.err_ctxt().report_mismatched_types(
+                            cause,
+                            expected,
+                            found,
+                            coercion_error,
+                        );
+                        let then_span = fcx.find_block_span_from_hir_id(then_id);
+                        let else_span = fcx.find_block_span_from_hir_id(else_id);
+                        // don't suggest wrapping either blocks in `if .. {} else {}`
+                        let is_empty_arm = |id| {
+                            let hir::Node::Block(blk) = fcx.tcx.hir_node(id) else {
+                                return false;
+                            };
+                            if blk.expr.is_some() || !blk.stmts.is_empty() {
+                                return false;
+                            }
+                            let Some((_, hir::Node::Expr(expr))) =
+                                fcx.tcx.hir().parent_iter(id).nth(1)
+                            else {
+                                return false;
+                            };
+                            matches!(expr.kind, hir::ExprKind::If(..))
+                        };
+                        if !is_empty_arm(then_id) && !is_empty_arm(else_id) {
+                            self.suggest_boxing_tail_for_return_position_impl_trait(
+                                fcx,
+                                &mut err,
+                                rpit_def_id,
+                                then_ty,
+                                else_ty,
+                                [then_span, else_span].into_iter(),
+                            );
+                        }
+                    }
                     _ => {
                         err = fcx.err_ctxt().report_mismatched_types(
                             cause,
@@ -1677,6 +1750,70 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
         }
     }
 
+    fn suggest_boxing_tail_for_return_position_impl_trait(
+        &self,
+        fcx: &FnCtxt<'_, 'tcx>,
+        err: &mut Diag<'_>,
+        rpit_def_id: LocalDefId,
+        a_ty: Ty<'tcx>,
+        b_ty: Ty<'tcx>,
+        arm_spans: impl Iterator<Item = Span>,
+    ) {
+        let compatible = |ty: Ty<'tcx>| {
+            fcx.probe(|_| {
+                let ocx = ObligationCtxt::new(fcx);
+                ocx.register_obligations(
+                    fcx.tcx
+                        .item_super_predicates(rpit_def_id)
+                        .instantiate_identity_iter()
+                        .filter_map(|clause| {
+                            let predicate = clause
+                                .kind()
+                                .map_bound(|clause| match clause {
+                                    ty::ClauseKind::Trait(trait_pred) => Some(
+                                        ty::ClauseKind::Trait(trait_pred.with_self_ty(fcx.tcx, ty)),
+                                    ),
+                                    ty::ClauseKind::Projection(proj_pred) => {
+                                        Some(ty::ClauseKind::Projection(
+                                            proj_pred.with_self_ty(fcx.tcx, ty),
+                                        ))
+                                    }
+                                    _ => None,
+                                })
+                                .transpose()?;
+                            Some(Obligation::new(
+                                fcx.tcx,
+                                ObligationCause::dummy(),
+                                fcx.param_env,
+                                predicate,
+                            ))
+                        }),
+                );
+                ocx.select_where_possible().is_empty()
+            })
+        };
+
+        if !compatible(a_ty) || !compatible(b_ty) {
+            return;
+        }
+
+        let rpid_def_span = fcx.tcx.def_span(rpit_def_id);
+        err.subdiagnostic(
+            fcx.tcx.dcx(),
+            SuggestBoxingForReturnImplTrait::ChangeReturnType {
+                start_sp: rpid_def_span.with_hi(rpid_def_span.lo() + BytePos(4)),
+                end_sp: rpid_def_span.shrink_to_hi(),
+            },
+        );
+
+        let (starts, ends) =
+            arm_spans.map(|span| (span.shrink_to_lo(), span.shrink_to_hi())).unzip();
+        err.subdiagnostic(
+            fcx.tcx.dcx(),
+            SuggestBoxingForReturnImplTrait::BoxReturnExpr { starts, ends },
+        );
+    }
+
     fn note_unreachable_loop_return(
         &self,
         err: &mut Diag<'_>,

compiler/rustc_hir_typeck/src/errors.rs

@@ -621,3 +621,21 @@ pub struct NoteCallerChoosesTyForTyParam<'tcx> {
     pub ty_param_name: Symbol,
     pub found_ty: Ty<'tcx>,
 }
+
+#[derive(Subdiagnostic)]
+pub enum SuggestBoxingForReturnImplTrait {
+    #[multipart_suggestion(hir_typeck_rpit_change_return_type, applicability = "maybe-incorrect")]
+    ChangeReturnType {
+        #[suggestion_part(code = "Box<dyn")]
+        start_sp: Span,
+        #[suggestion_part(code = ">")]
+        end_sp: Span,
+    },
+    #[multipart_suggestion(hir_typeck_rpit_box_return_expr, applicability = "maybe-incorrect")]
+    BoxReturnExpr {
+        #[suggestion_part(code = "Box::new(")]
+        starts: Vec<Span>,
+        #[suggestion_part(code = ")")]
+        ends: Vec<Span>,
+    },
+}

compiler/rustc_hir_typeck/src/expr.rs

@@ -1088,15 +1088,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let else_ty = self.check_expr_with_expectation(else_expr, expected);
             let else_diverges = self.diverges.get();
 
-            let opt_suggest_box_span = self.opt_suggest_box_span(then_ty, else_ty, orig_expected);
+            let tail_defines_return_position_impl_trait =
+                self.return_position_impl_trait_from_match_expectation(orig_expected);
             let if_cause = self.if_cause(
                 sp,
                 cond_expr.span,
                 then_expr,
                 else_expr,
                 then_ty,
                 else_ty,
-                opt_suggest_box_span,
+                tail_defines_return_position_impl_trait,
             );
 
             coerce.coerce(self, &if_cause, else_expr, else_ty);

compiler/rustc_infer/messages.ftl

@@ -270,9 +270,6 @@ infer_ril_introduced_by = requirement introduced by this return type
 infer_ril_introduced_here = `'static` requirement introduced here
 infer_ril_static_introduced_by = "`'static` lifetime requirement introduced by the return type
 
-infer_sbfrit_box_return_expr = if you change the return type to expect trait objects, box the returned expressions
-
-infer_sbfrit_change_return_type = you could change the return type to be a boxed trait object
 infer_source_kind_closure_return =
     try giving this closure an explicit return type
 

compiler/rustc_infer/src/errors/mod.rs

@@ -1262,24 +1262,6 @@ pub enum SuggestAccessingField<'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>,
-    },
-}
-
 #[derive(Subdiagnostic)]
 #[multipart_suggestion(infer_stp_wrap_one, applicability = "maybe-incorrect")]
 pub struct SuggestTuplePatternOne {

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

@@ -784,7 +784,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 prior_arm_ty,
                 source,
                 ref prior_non_diverging_arms,
-                opt_suggest_box_span,
                 scrut_span,
                 ..
             }) => match source {
@@ -853,17 +852,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     ) {
                         err.subdiagnostic(self.dcx(), subdiag);
                     }
-                    if let Some(ret_sp) = opt_suggest_box_span {
-                        // Get return type span and point to it.
-                        self.suggest_boxing_for_return_impl_trait(
-                            err,
-                            ret_sp,
-                            prior_non_diverging_arms
-                                .iter()
-                                .chain(std::iter::once(&arm_span))
-                                .copied(),
-                        );
-                    }
                 }
             },
             ObligationCauseCode::IfExpression(box IfExpressionCause {
@@ -872,7 +860,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 then_ty,
                 else_ty,
                 outer_span,
-                opt_suggest_box_span,
+                ..
             }) => {
                 let then_span = self.find_block_span_from_hir_id(then_id);
                 let else_span = self.find_block_span_from_hir_id(else_id);
@@ -890,30 +878,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 ) {
                     err.subdiagnostic(self.dcx(), subdiag);
                 }
-                // don't suggest wrapping either blocks in `if .. {} else {}`
-                let is_empty_arm = |id| {
-                    let hir::Node::Block(blk) = self.tcx.hir_node(id) else {
-                        return false;
-                    };
-                    if blk.expr.is_some() || !blk.stmts.is_empty() {
-                        return false;
-                    }
-                    let Some((_, hir::Node::Expr(expr))) = self.tcx.hir().parent_iter(id).nth(1)
-                    else {
-                        return false;
-                    };
-                    matches!(expr.kind, hir::ExprKind::If(..))
-                };
-                if let Some(ret_sp) = opt_suggest_box_span
-                    && !is_empty_arm(then_id)
-                    && !is_empty_arm(else_id)
-                {
-                    self.suggest_boxing_for_return_impl_trait(
-                        err,
-                        ret_sp,
-                        [then_span, else_span].into_iter(),
-                    );
-                }
             }
             ObligationCauseCode::LetElse => {
                 err.help("try adding a diverging expression, such as `return` or `panic!(..)`");

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

@@ -19,9 +19,8 @@ use rustc_span::{sym, BytePos, Span};
 
 use crate::errors::{
     ConsiderAddingAwait, FnConsiderCasting, FnItemsAreDistinct, FnUniqTypes,
-    FunctionPointerSuggestion, SuggestAccessingField, SuggestBoxingForReturnImplTrait,
-    SuggestRemoveSemiOrReturnBinding, SuggestTuplePatternMany, SuggestTuplePatternOne,
-    TypeErrorAdditionalDiags,
+    FunctionPointerSuggestion, SuggestAccessingField, SuggestRemoveSemiOrReturnBinding,
+    SuggestTuplePatternMany, SuggestTuplePatternOne, TypeErrorAdditionalDiags,
 };
 
 use super::TypeErrCtxt;
@@ -80,28 +79,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
     }
 
-    pub(super) fn suggest_boxing_for_return_impl_trait(
-        &self,
-        err: &mut Diag<'_>,
-        return_sp: Span,
-        arm_spans: impl Iterator<Item = Span>,
-    ) {
-        let sugg = SuggestBoxingForReturnImplTrait::ChangeReturnType {
-            start_sp: return_sp.with_hi(return_sp.lo() + BytePos(4)),
-            end_sp: return_sp.shrink_to_hi(),
-        };
-        err.subdiagnostic(self.dcx(), 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(self.dcx(), sugg);
-    }
-
     pub(super) fn suggest_tuple_pattern(
         &self,
         cause: &ObligationCause<'tcx>,

compiler/rustc_infer/src/infer/mod.rs

@@ -229,6 +229,15 @@ impl<'tcx> InferCtxtInner<'tcx> {
             .expect("region constraints already solved")
             .with_log(&mut self.undo_log)
     }
+
+    // Iterates through the opaque type definitions without taking them; this holds the
+    // `InferCtxtInner` lock, so make sure to not do anything with `InferCtxt` side-effects
+    // while looping through this.
+    pub fn iter_opaque_types(
+        &self,
+    ) -> impl Iterator<Item = (ty::OpaqueTypeKey<'tcx>, ty::OpaqueHiddenType<'tcx>)> + '_ {
+        self.opaque_type_storage.opaque_types.iter().map(|(&k, v)| (k, v.hidden_type))
+    }
 }
 
 pub struct InferCtxt<'tcx> {

compiler/rustc_middle/src/traits/mod.rs

@@ -571,7 +571,8 @@ pub struct MatchExpressionArmCause<'tcx> {
     pub scrut_span: Span,
     pub source: hir::MatchSource,
     pub prior_non_diverging_arms: Vec<Span>,
-    pub opt_suggest_box_span: Option<Span>,
+    // Is the expectation of this match expression an RPIT?
+    pub tail_defines_return_position_impl_trait: Option<LocalDefId>,
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
@@ -582,7 +583,8 @@ pub struct IfExpressionCause<'tcx> {
     pub then_ty: Ty<'tcx>,
     pub else_ty: Ty<'tcx>,
     pub outer_span: Option<Span>,
-    pub opt_suggest_box_span: Option<Span>,
+    // Is the expectation of this match expression an RPIT?
+    pub tail_defines_return_position_impl_trait: Option<LocalDefId>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]