Suggest boxed trait objects in tail match and if 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.

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

@@ -617,11 +617,20 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 ref prior_arms,
                 last_ty,
                 scrut_hir_id,
+                suggest_box,
+                arm_span,
                 ..
             }) => match source {
                 hir::MatchSource::IfLetDesugar { .. } => {
                     let msg = "`if let` arms have incompatible types";
                     err.span_label(cause.span, msg);
+                    if let Some(ret_sp) = suggest_box {
+                        self.suggest_boxing_for_return_impl_trait(
+                            err,
+                            ret_sp,
+                            prior_arms.iter().chain(std::iter::once(&arm_span)).map(|s| *s),
+                        );
+                    }
                 }
                 hir::MatchSource::TryDesugar => {
                     if let Some(ty::error::ExpectedFound { expected, .. }) = exp_found {
@@ -675,9 +684,23 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             Applicability::MachineApplicable,
                         );
                     }
+                    if let Some(ret_sp) = suggest_box {
+                        // Get return type span and point to it.
+                        self.suggest_boxing_for_return_impl_trait(
+                            err,
+                            ret_sp,
+                            prior_arms.iter().chain(std::iter::once(&arm_span)).map(|s| *s),
+                        );
+                    }
                 }
             },
-            ObligationCauseCode::IfExpression(box IfExpressionCause { then, outer, semicolon }) => {
+            ObligationCauseCode::IfExpression(box IfExpressionCause {
+                then,
+                else_sp,
+                outer,
+                semicolon,
+                suggest_box,
+            }) => {
                 err.span_label(then, "expected because of this");
                 if let Some(sp) = outer {
                     err.span_label(sp, "`if` and `else` have incompatible types");
@@ -690,11 +713,52 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                         Applicability::MachineApplicable,
                     );
                 }
+                if let Some(ret_sp) = suggest_box {
+                    self.suggest_boxing_for_return_impl_trait(
+                        err,
+                        ret_sp,
+                        vec![then, else_sp].into_iter(),
+                    );
+                }
             }
             _ => (),
         }
     }
 
+    fn suggest_boxing_for_return_impl_trait(
+        &self,
+        err: &mut DiagnosticBuilder<'tcx>,
+        return_sp: Span,
+        arm_spans: impl Iterator<Item = Span>,
+    ) {
+        let snippet = self
+            .tcx
+            .sess
+            .source_map()
+            .span_to_snippet(return_sp)
+            .unwrap_or_else(|_| "dyn Trait".to_string());
+        err.span_suggestion_verbose(
+            return_sp,
+            "you could change the return type to be a boxed trait object",
+            format!("Box<dyn {}>", &snippet[5..]),
+            Applicability::MaybeIncorrect,
+        );
+        let sugg = arm_spans
+            .flat_map(|sp| {
+                vec![
+                    (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,
+        );
+    }
+
     /// Given that `other_ty` is the same as a type argument for `name` in `sub`, populate `value`
     /// highlighting `name` and every type argument that isn't at `pos` (which is `other_ty`), and
     /// populate `other_value` with `other_ty`.

compiler/rustc_middle/src/traits/mod.rs

@@ -350,13 +350,16 @@ pub struct MatchExpressionArmCause<'tcx> {
     pub prior_arms: Vec<Span>,
     pub last_ty: Ty<'tcx>,
     pub scrut_hir_id: hir::HirId,
+    pub suggest_box: 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 suggest_box: Option<Span>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]

compiler/rustc_typeck/src/check/_match.rs

@@ -1,28 +1,31 @@
 use crate::check::coercion::CoerceMany;
 use crate::check::{Diverges, Expectation, FnCtxt, Needs};
-use rustc_hir as hir;
-use rustc_hir::ExprKind;
+use rustc_hir::{self as hir, ExprKind};
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
-use rustc_middle::ty::Ty;
+use rustc_infer::traits::Obligation;
+use rustc_middle::ty::{self, ToPredicate, Ty};
 use rustc_span::Span;
-use rustc_trait_selection::traits::ObligationCauseCode;
-use rustc_trait_selection::traits::{IfExpressionCause, MatchExpressionArmCause, ObligationCause};
+use rustc_trait_selection::opaque_types::InferCtxtExt as _;
+use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _;
+use rustc_trait_selection::traits::{
+    IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
+};
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn check_match(
         &self,
         expr: &'tcx hir::Expr<'tcx>,
         scrut: &'tcx hir::Expr<'tcx>,
         arms: &'tcx [hir::Arm<'tcx>],
-        expected: Expectation<'tcx>,
+        orig_expected: Expectation<'tcx>,
         match_src: hir::MatchSource,
     ) -> Ty<'tcx> {
         let tcx = self.tcx;
 
         use hir::MatchSource::*;
         let (source_if, if_no_else, force_scrutinee_bool) = match match_src {
             IfDesugar { contains_else_clause } => (true, !contains_else_clause, true),
-            IfLetDesugar { contains_else_clause } => (true, !contains_else_clause, false),
+            IfLetDesugar { contains_else_clause, .. } => (true, !contains_else_clause, false),
             WhileDesugar => (false, false, true),
             _ => (false, false, false),
         };
@@ -69,7 +72,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // type in that case)
         let mut all_arms_diverge = Diverges::WarnedAlways;
 
-        let expected = expected.adjust_for_branches(self);
+        let expected = orig_expected.adjust_for_branches(self);
 
         let mut coercion = {
             let coerce_first = match expected {
@@ -112,14 +115,74 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 self.check_expr_with_expectation(&arm.body, expected)
             };
             all_arms_diverge &= self.diverges.get();
+
+            // 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.
+            let suggest_box = match (
+                orig_expected,
+                self.body_id
+                    .owner
+                    .to_def_id()
+                    .as_local()
+                    .and_then(|id| self.ret_coercion_impl_trait.map(|ty| (id, ty))),
+            ) {
+                (Expectation::ExpectHasType(expected), Some((id, ty)))
+                    if self.in_tail_expr && self.can_coerce(arm_ty, expected) =>
+                {
+                    let impl_trait_ret_ty = self.infcx.instantiate_opaque_types(
+                        id,
+                        self.body_id,
+                        self.param_env,
+                        &ty,
+                        arm.body.span,
+                    );
+                    let mut suggest_box = !impl_trait_ret_ty.obligations.is_empty();
+                    for o in impl_trait_ret_ty.obligations {
+                        match o.predicate.skip_binders_unchecked() {
+                            ty::PredicateAtom::Trait(t, constness) => {
+                                let pred = ty::PredicateAtom::Trait(
+                                    ty::TraitPredicate {
+                                        trait_ref: ty::TraitRef {
+                                            def_id: t.def_id(),
+                                            substs: self.infcx.tcx.mk_substs_trait(arm_ty, &[]),
+                                        },
+                                    },
+                                    constness,
+                                );
+                                let obl = Obligation::new(
+                                    o.cause.clone(),
+                                    self.param_env,
+                                    pred.to_predicate(self.infcx.tcx),
+                                );
+                                suggest_box &= self.infcx.predicate_must_hold_modulo_regions(&obl);
+                                if !suggest_box {
+                                    break;
+                                }
+                            }
+                            _ => {}
+                        }
+                    }
+                    if suggest_box { self.ret_type_span.clone() } else { None }
+                }
+                _ => None,
+            };
+
             if source_if {
                 let then_expr = &arms[0].body;
                 match (i, if_no_else) {
                     (0, _) => coercion.coerce(self, &self.misc(expr.span), &arm.body, arm_ty),
                     (_, true) => {} // Handled above to avoid duplicated type errors (#60254).
                     (_, _) => {
                         let then_ty = prior_arm_ty.unwrap();
-                        let cause = self.if_cause(expr.span, then_expr, &arm.body, then_ty, arm_ty);
+                        let cause = self.if_cause(
+                            expr.span,
+                            then_expr,
+                            &arm.body,
+                            then_ty,
+                            arm_ty,
+                            suggest_box,
+                        );
                         coercion.coerce(self, &cause, &arm.body, arm_ty);
                     }
                 }
@@ -142,6 +205,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             prior_arms: other_arms.clone(),
                             last_ty: prior_arm_ty.unwrap(),
                             scrut_hir_id: scrut.hir_id,
+                            suggest_box,
                         }),
                     ),
                 };
@@ -266,6 +330,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         else_expr: &'tcx hir::Expr<'tcx>,
         then_ty: Ty<'tcx>,
         else_ty: Ty<'tcx>,
+        suggest_box: Option<Span>,
     ) -> ObligationCause<'tcx> {
         let mut outer_sp = 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
@@ -353,8 +418,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             error_sp,
             ObligationCauseCode::IfExpression(box IfExpressionCause {
                 then: then_sp,
+                else_sp: error_sp,
                 outer: outer_sp,
                 semicolon: remove_semicolon,
+                suggest_box,
             }),
         )
     }

compiler/rustc_typeck/src/check/mod.rs

@@ -570,6 +570,14 @@ pub struct FnCtxt<'a, 'tcx> {
     /// any).
     ret_coercion: Option<RefCell<DynamicCoerceMany<'tcx>>>,
 
+    ret_coercion_impl_trait: Option<Ty<'tcx>>,
+
+    ret_type_span: Option<Span>,
+
+    /// Used exclusively to reduce cost of advanced evaluation used for
+    /// more helpful diagnostics.
+    in_tail_expr: bool,
+
     /// First span of a return site that we find. Used in error messages.
     ret_coercion_span: RefCell<Option<Span>>,
 
@@ -1302,10 +1310,15 @@ fn check_fn<'a, 'tcx>(
     let hir = tcx.hir();
 
     let declared_ret_ty = fn_sig.output();
+
     let revealed_ret_ty =
         fcx.instantiate_opaque_types_from_value(fn_id, &declared_ret_ty, decl.output.span());
     debug!("check_fn: declared_ret_ty: {}, revealed_ret_ty: {}", declared_ret_ty, revealed_ret_ty);
     fcx.ret_coercion = Some(RefCell::new(CoerceMany::new(revealed_ret_ty)));
+    fcx.ret_type_span = Some(decl.output.span());
+    if let ty::Opaque(..) = declared_ret_ty.kind() {
+        fcx.ret_coercion_impl_trait = Some(declared_ret_ty);
+    }
     fn_sig = tcx.mk_fn_sig(
         fn_sig.inputs().iter().cloned(),
         revealed_ret_ty,
@@ -1366,6 +1379,7 @@ fn check_fn<'a, 'tcx>(
 
     inherited.typeck_results.borrow_mut().liberated_fn_sigs_mut().insert(fn_id, fn_sig);
 
+    fcx.in_tail_expr = true;
     if let ty::Dynamic(..) = declared_ret_ty.kind() {
         // FIXME: We need to verify that the return type is `Sized` after the return expression has
         // been evaluated so that we have types available for all the nodes being returned, but that
@@ -1385,6 +1399,7 @@ fn check_fn<'a, 'tcx>(
         fcx.require_type_is_sized(declared_ret_ty, decl.output.span(), traits::SizedReturnType);
         fcx.check_return_expr(&body.value);
     }
+    fcx.in_tail_expr = false;
 
     // We insert the deferred_generator_interiors entry after visiting the body.
     // This ensures that all nested generators appear before the entry of this generator.
@@ -3084,6 +3099,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             param_env,
             err_count_on_creation: inh.tcx.sess.err_count(),
             ret_coercion: None,
+            ret_coercion_impl_trait: None,
+            ret_type_span: None,
+            in_tail_expr: false,
             ret_coercion_span: RefCell::new(None),
             resume_yield_tys: None,
             ps: RefCell::new(UnsafetyState::function(hir::Unsafety::Normal, hir::CRATE_HIR_ID)),

src/test/ui/point-to-type-err-cause-on-impl-trait-return.rs

@@ -53,7 +53,13 @@ fn cat() -> impl std::fmt::Display {
             1u32 //~ ERROR mismatched types
         }
     }
-        }
+}
+
+fn dog() -> impl std::fmt::Display {
+    match 13 {
+        0 => 0i32,
+        1 => 1u32, //~ ERROR `match` arms have incompatible types
+        _ => 2u32,
     }
 }
 

src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr

@@ -72,6 +72,17 @@ LL | |         1u32
    | |         ^^^^ expected `i32`, found `u32`
 LL | |     }
    | |_____- `if` and `else` have incompatible types
+   |
+help: you could change the return type to be a boxed trait object
+   |
+LL | fn qux() -> Box<dyn std::fmt::Display> {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: if you change the return type to expect trait objects box the returned expressions
+   |
+LL |         Box::new(0i32)
+LL |     } else {
+LL |         Box::new(1u32)
+   |
 
 error[E0308]: mismatched types
   --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:35:14
@@ -136,6 +147,28 @@ help: you could change the return type to be a boxed trait object
 LL | fn cat() -> Box<dyn std::fmt::Display> {
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 7 previous errors
+error[E0308]: `match` arms have incompatible types
+  --> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:61:14
+   |
+LL | /     match 13 {
+LL | |         0 => 0i32,
+   | |              ---- this is found to be of type `i32`
+LL | |         1 => 1u32,
+   | |              ^^^^ expected `i32`, found `u32`
+LL | |         _ => 2u32,
+LL | |     }
+   | |_____- `match` arms have incompatible types
+   |
+help: you could change the return type to be a boxed trait object
+   |
+LL | fn dog() -> Box<dyn std::fmt::Display> {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: if you change the return type to expect trait objects box the returned expressions
+   |
+LL |         0 => Box::new(0i32),
+LL |         1 => Box::new(1u32),
+   |
+
+error: aborting due to 8 previous errors
 
 For more information about this error, try `rustc --explain E0308`.