Auto merge of #101604 - compiler-errors:issue-101465, r=lcnr

Fix ICE in opt_suggest_box_span

We were _totally_ mishandling substs and obligations in `opt_suggest_box_span`, so I reworked that function pretty heavily.

Also some drive-by changes, namely removing `ret_type_span`.

Fixes #101465

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -73,7 +73,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     // for opaque types, and then use that kind to fix the spans for type errors
                     // that we see later on.
                     let ty_var = self.next_ty_var(TypeVariableOrigin {
-                        kind: TypeVariableOriginKind::TypeInference,
+                        kind: TypeVariableOriginKind::OpaqueTypeInference(def_id),
                         span,
                     });
                     obligations.extend(

compiler/rustc_infer/src/infer/type_variable.rs

@@ -122,6 +122,7 @@ pub enum TypeVariableOriginKind {
     MiscVariable,
     NormalizeProjectionType,
     TypeInference,
+    OpaqueTypeInference(DefId),
     TypeParameterDefinition(Symbol, Option<DefId>),
 
     /// One of the upvars or closure kind parameters in a `ClosureSubsts`

compiler/rustc_typeck/src/check/_match.rs

@@ -4,7 +4,7 @@ use rustc_errors::{Applicability, MultiSpan};
 use rustc_hir::{self as hir, ExprKind};
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::traits::Obligation;
-use rustc_middle::ty::{self, ToPredicate, Ty};
+use rustc_middle::ty::{self, Subst, ToPredicate, Ty};
 use rustc_span::Span;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 use rustc_trait_selection::traits::{
@@ -137,9 +137,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 Some(&arm.body),
                 arm_ty,
                 Some(&mut |err| {
-                    let Some(ret) = self.ret_type_span else {
-                        return;
-                    };
+                    let Some(ret) = self
+                        .tcx
+                        .hir()
+                        .find_by_def_id(self.body_id.owner)
+                        .and_then(|owner| owner.fn_decl())
+                        .map(|decl| decl.output.span())
+                    else { return; };
                     let Expectation::IsLast(stmt) = orig_expected else {
                         return
                     };
@@ -468,58 +472,77 @@ 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.
+    /// 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(
         &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)
-                if self.in_tail_expr
-                    && self.return_type_has_opaque
-                    && self.can_coerce(first_ty, expected)
-                    && self.can_coerce(second_ty, expected) =>
-            {
-                let obligations = self.fulfillment_cx.borrow().pending_obligations();
-                let mut suggest_box = !obligations.is_empty();
-                'outer: for o in obligations {
-                    for outer_ty in &[first_ty, second_ty] {
-                        match o.predicate.kind().skip_binder() {
-                            ty::PredicateKind::Trait(t) => {
-                                let pred = ty::Binder::dummy(ty::PredicateKind::Trait(
-                                    ty::TraitPredicate {
-                                        trait_ref: ty::TraitRef {
-                                            def_id: t.def_id(),
-                                            substs: self.tcx.mk_substs_trait(*outer_ty, &[]),
-                                        },
-                                        constness: t.constness,
-                                        polarity: t.polarity,
-                                    },
-                                ));
-                                let obl = Obligation::new(
-                                    o.cause.clone(),
-                                    self.param_env,
-                                    pred.to_predicate(self.tcx),
-                                );
-                                suggest_box &= self.predicate_must_hold_modulo_regions(&obl);
-                                if !suggest_box {
-                                    // We've encountered some obligation that didn't hold, so the
-                                    // return expression can't just be boxed. We don't need to
-                                    // evaluate the rest of the obligations.
-                                    break 'outer;
-                                }
+            Expectation::ExpectHasType(expected) => {
+                let TypeVariableOrigin {
+                    span,
+                    kind: TypeVariableOriginKind::OpaqueTypeInference(rpit_def_id),
+                    ..
+                } = self.type_var_origin(expected)? else { return None; };
+
+                let sig = *self
+                    .typeck_results
+                    .borrow()
+                    .liberated_fn_sigs()
+                    .get(hir::HirId::make_owner(self.body_id.owner))?;
+
+                let substs = sig.output().walk().find_map(|arg| {
+                    if let ty::GenericArgKind::Type(ty) = arg.unpack()
+                        && let ty::Opaque(def_id, substs) = *ty.kind()
+                        && def_id == rpit_def_id
+                    {
+                        Some(substs)
+                    } else {
+                        None
+                    }
+                })?;
+                let opaque_ty = self.tcx.mk_opaque(rpit_def_id, substs);
+
+                if !self.can_coerce(first_ty, expected) || !self.can_coerce(second_ty, expected) {
+                    return None;
+                }
+
+                for ty in [first_ty, second_ty] {
+                    for pred in self.tcx.bound_explicit_item_bounds(rpit_def_id).transpose_iter() {
+                        let pred = pred.map_bound(|(pred, _)| *pred).subst(self.tcx, substs);
+                        let pred = match pred.kind().skip_binder() {
+                            ty::PredicateKind::Trait(mut trait_pred) => {
+                                assert_eq!(trait_pred.trait_ref.self_ty(), opaque_ty);
+                                trait_pred.trait_ref.substs =
+                                    self.tcx.mk_substs_trait(ty, &trait_pred.trait_ref.substs[1..]);
+                                pred.kind().rebind(trait_pred).to_predicate(self.tcx)
                             }
-                            _ => {}
+                            ty::PredicateKind::Projection(mut proj_pred) => {
+                                assert_eq!(proj_pred.projection_ty.self_ty(), opaque_ty);
+                                proj_pred.projection_ty.substs = self
+                                    .tcx
+                                    .mk_substs_trait(ty, &proj_pred.projection_ty.substs[1..]);
+                                pred.kind().rebind(proj_pred).to_predicate(self.tcx)
+                            }
+                            _ => continue,
+                        };
+                        if !self.predicate_must_hold_modulo_regions(&Obligation::new(
+                            ObligationCause::misc(span, self.body_id),
+                            self.param_env,
+                            pred,
+                        )) {
+                            return None;
                         }
                     }
                 }
-                // If all the obligations hold (or there are no obligations) the tail expression
-                // we can suggest to return a boxed trait object instead of an opaque type.
-                if suggest_box { self.ret_type_span } else { None }
+
+                Some(span)
             }
             _ => None,
         }

compiler/rustc_typeck/src/check/check.rs

@@ -106,7 +106,6 @@ pub(super) fn check_fn<'a, 'tcx>(
     fcx.return_type_has_opaque = ret_ty != declared_ret_ty;
 
     fcx.ret_coercion = Some(RefCell::new(CoerceMany::new(ret_ty)));
-    fcx.ret_type_span = Some(decl.output.span());
 
     let span = body.value.span;
 

compiler/rustc_typeck/src/check/fn_ctxt/mod.rs

@@ -68,8 +68,6 @@ pub struct FnCtxt<'a, 'tcx> {
     /// any).
     pub(super) ret_coercion: Option<RefCell<DynamicCoerceMany<'tcx>>>,
 
-    pub(super) ret_type_span: Option<Span>,
-
     /// Used exclusively to reduce cost of advanced evaluation used for
     /// more helpful diagnostics.
     pub(super) in_tail_expr: bool,
@@ -142,7 +140,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             param_env,
             err_count_on_creation: inh.tcx.sess.err_count(),
             ret_coercion: None,
-            ret_type_span: None,
             in_tail_expr: false,
             ret_coercion_span: Cell::new(None),
             resume_yield_tys: None,

src/test/ui/suggestions/issue-101465.rs

@@ -0,0 +1,25 @@
+#![feature(trait_alias)]
+
+struct B;
+struct C;
+
+trait Tr {}
+
+impl Tr for B {}
+impl Tr for C {}
+
+trait Tr2<S> = Into<S>;
+
+fn foo2<T: Tr2<()>>() {}
+
+fn foo() -> impl Tr {
+    let x = foo2::<_>();
+
+    match true {
+        true => B,
+        false => C,
+        //~^ `match` arms have incompatible types
+    }
+}
+
+fn main() {}

src/test/ui/suggestions/issue-101465.stderr

@@ -0,0 +1,25 @@
+error[E0308]: `match` arms have incompatible types
+  --> $DIR/issue-101465.rs:20:18
+   |
+LL | /     match true {
+LL | |         true => B,
+   | |                 - this is found to be of type `B`
+LL | |         false => C,
+   | |                  ^ expected struct `B`, found struct `C`
+LL | |
+LL | |     }
+   | |_____- `match` arms have incompatible types
+   |
+help: you could change the return type to be a boxed trait object
+   |
+LL | fn foo() -> Box<dyn Tr> {
+   |             ~~~~~~~   +
+help: if you change the return type to expect trait objects, box the returned expressions
+   |
+LL ~         true => Box::new(B),
+LL ~         false => Box::new(C),
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.