Rollup merge of #131751 - compiler-errors:structurally-resolve, r=lcnr

Rename `can_coerce` to `may_coerce`, and then structurally resolve correctly in the probe

We need to structurally resolve the lhs and rhs of the coercion. Also, renaming the method so it's less ambiguous about what it's doing... the word "may" gives more clear signal that it has false positives imo.

r? lcnr

compiler/rustc_hir_typeck/src/_match.rs

@@ -235,8 +235,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             Some(ret_coercion) => {
                 let ret_ty = ret_coercion.borrow().expected_ty();
                 let ret_ty = self.infcx.shallow_resolve(ret_ty);
-                self.can_coerce(arm_ty, ret_ty)
-                    && prior_arm.is_none_or(|(_, ty, _)| self.can_coerce(ty, ret_ty))
+                self.may_coerce(arm_ty, ret_ty)
+                    && prior_arm.is_none_or(|(_, ty, _)| self.may_coerce(ty, ret_ty))
                     // The match arms need to unify for the case of `impl Trait`.
                     && !matches!(ret_ty.kind(), ty::Alias(ty::Opaque, ..))
             }

compiler/rustc_hir_typeck/src/cast.rs

@@ -409,7 +409,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                 let mut sugg_mutref = false;
                 if let ty::Ref(reg, cast_ty, mutbl) = *self.cast_ty.kind() {
                     if let ty::RawPtr(expr_ty, _) = *self.expr_ty.kind()
-                        && fcx.can_coerce(
+                        && fcx.may_coerce(
                             Ty::new_ref(fcx.tcx, fcx.tcx.lifetimes.re_erased, expr_ty, mutbl),
                             self.cast_ty,
                         )
@@ -418,22 +418,22 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     } else if let ty::Ref(expr_reg, expr_ty, expr_mutbl) = *self.expr_ty.kind()
                         && expr_mutbl == Mutability::Not
                         && mutbl == Mutability::Mut
-                        && fcx.can_coerce(Ty::new_mut_ref(fcx.tcx, expr_reg, expr_ty), self.cast_ty)
+                        && fcx.may_coerce(Ty::new_mut_ref(fcx.tcx, expr_reg, expr_ty), self.cast_ty)
                     {
                         sugg_mutref = true;
                     }
 
                     if !sugg_mutref
                         && sugg == None
-                        && fcx.can_coerce(
+                        && fcx.may_coerce(
                             Ty::new_ref(fcx.tcx, reg, self.expr_ty, mutbl),
                             self.cast_ty,
                         )
                     {
                         sugg = Some((format!("&{}", mutbl.prefix_str()), false));
                     }
                 } else if let ty::RawPtr(_, mutbl) = *self.cast_ty.kind()
-                    && fcx.can_coerce(
+                    && fcx.may_coerce(
                         Ty::new_ref(fcx.tcx, fcx.tcx.lifetimes.re_erased, self.expr_ty, mutbl),
                         self.cast_ty,
                     )

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1084,24 +1084,42 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         })
     }
 
-    /// Same as `coerce()`, but without side-effects.
+    /// Probe whether `expr_ty` can be coerced to `target_ty`. This has no side-effects,
+    /// and may return false positives if types are not yet fully constrained by inference.
     ///
-    /// Returns false if the coercion creates any obligations that result in
-    /// errors.
-    pub(crate) fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
-        // FIXME(-Znext-solver): We need to structurally resolve both types here.
-        let source = self.resolve_vars_with_obligations(expr_ty);
-        debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);
-
+    /// Returns false if the coercion is not possible, or if the coercion creates any
+    /// sub-obligations that result in errors.
+    ///
+    /// This should only be used for diagnostics.
+    pub(crate) fn may_coerce(&self, expr_ty: Ty<'tcx>, target_ty: Ty<'tcx>) -> bool {
         let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
         // We don't ever need two-phase here since we throw out the result of the coercion.
         // We also just always set `coerce_never` to true, since this is a heuristic.
-        let coerce = Coerce::new(self, cause, AllowTwoPhase::No, true);
+        let coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No, true);
         self.probe(|_| {
-            let Ok(ok) = coerce.coerce(source, target) else {
+            // Make sure to structurally resolve the types, since we use
+            // the `TyKind`s heavily in coercion.
+            let ocx = ObligationCtxt::new(self);
+            let structurally_resolve = |ty| {
+                let ty = self.shallow_resolve(ty);
+                if self.next_trait_solver()
+                    && let ty::Alias(..) = ty.kind()
+                {
+                    ocx.structurally_normalize(&cause, self.param_env, ty)
+                } else {
+                    Ok(ty)
+                }
+            };
+            let Ok(expr_ty) = structurally_resolve(expr_ty) else {
+                return false;
+            };
+            let Ok(target_ty) = structurally_resolve(target_ty) else {
+                return false;
+            };
+
+            let Ok(ok) = coerce.coerce(expr_ty, target_ty) else {
                 return false;
             };
-            let ocx = ObligationCtxt::new(self);
             ocx.register_obligations(ok.obligations);
             ocx.select_where_possible().is_empty()
         })
@@ -1370,7 +1388,7 @@ pub fn can_coerce<'tcx>(
 ) -> bool {
     let root_ctxt = crate::typeck_root_ctxt::TypeckRootCtxt::new(tcx, body_id);
     let fn_ctxt = FnCtxt::new(&root_ctxt, param_env, body_id);
-    fn_ctxt.can_coerce(ty, output_ty)
+    fn_ctxt.may_coerce(ty, output_ty)
 }
 
 /// CoerceMany encapsulates the pattern you should use when you have

compiler/rustc_hir_typeck/src/expr.rs

@@ -1330,9 +1330,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let refs_can_coerce = |lhs: Ty<'tcx>, rhs: Ty<'tcx>| {
                 let lhs = Ty::new_imm_ref(self.tcx, self.tcx.lifetimes.re_erased, lhs.peel_refs());
                 let rhs = Ty::new_imm_ref(self.tcx, self.tcx.lifetimes.re_erased, rhs.peel_refs());
-                self.can_coerce(rhs, lhs)
+                self.may_coerce(rhs, lhs)
             };
-            let (applicability, eq) = if self.can_coerce(rhs_ty, lhs_ty) {
+            let (applicability, eq) = if self.may_coerce(rhs_ty, lhs_ty) {
                 (Applicability::MachineApplicable, true)
             } else if refs_can_coerce(rhs_ty, lhs_ty) {
                 // The lhs and rhs are likely missing some references in either side. Subsequent
@@ -1349,7 +1349,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let actual_lhs_ty = self.check_expr(rhs_expr);
                 (
                     Applicability::MaybeIncorrect,
-                    self.can_coerce(rhs_ty, actual_lhs_ty)
+                    self.may_coerce(rhs_ty, actual_lhs_ty)
                         || refs_can_coerce(rhs_ty, actual_lhs_ty),
                 )
             } else if let ExprKind::Binary(
@@ -1363,7 +1363,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let actual_rhs_ty = self.check_expr(lhs_expr);
                 (
                     Applicability::MaybeIncorrect,
-                    self.can_coerce(actual_rhs_ty, lhs_ty)
+                    self.may_coerce(actual_rhs_ty, lhs_ty)
                         || refs_can_coerce(actual_rhs_ty, lhs_ty),
                 )
             } else {
@@ -1414,7 +1414,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         self.param_env,
                     )
                     .may_apply();
-                if lhs_deref_ty_is_sized && self.can_coerce(rhs_ty, lhs_deref_ty) {
+                if lhs_deref_ty_is_sized && self.may_coerce(rhs_ty, lhs_deref_ty) {
                     err.span_suggestion_verbose(
                         lhs.span.shrink_to_lo(),
                         "consider dereferencing here to assign to the mutably borrowed value",

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -658,7 +658,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 && fn_sig.inputs()[1..]
                     .iter()
                     .zip(input_types.iter())
-                    .all(|(expected, found)| self.can_coerce(*expected, *found))
+                    .all(|(expected, found)| self.may_coerce(*expected, *found))
                 && fn_sig.inputs()[1..].len() == input_types.len()
             {
                 err.span_suggestion_verbose(
@@ -722,7 +722,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             let expectation = Expectation::rvalue_hint(self, expected_input_ty);
             let coerced_ty = expectation.only_has_type(self).unwrap_or(formal_input_ty);
-            let can_coerce = self.can_coerce(arg_ty, coerced_ty);
+            let can_coerce = self.may_coerce(arg_ty, coerced_ty);
             if !can_coerce {
                 return Compatibility::Incompatible(Some(ty::error::TypeError::Sorts(
                     ty::error::ExpectedFound::new(true, coerced_ty, arg_ty),
@@ -802,7 +802,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         provided_arg_tys.iter().map(|(ty, _)| *ty).skip(mismatch_idx + tys.len()),
                     ),
                 ) {
-                    if !self.can_coerce(provided_ty, *expected_ty) {
+                    if !self.may_coerce(provided_ty, *expected_ty) {
                         satisfied = false;
                         break;
                     }
@@ -1023,7 +1023,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 std::iter::zip(formal_and_expected_inputs.iter(), removed_arg_tys.iter()).all(
                     |((expected_ty, _), (provided_ty, _))| {
                         !provided_ty.references_error()
-                            && self.can_coerce(*provided_ty, *expected_ty)
+                            && self.may_coerce(*provided_ty, *expected_ty)
                     },
                 )
             };
@@ -2124,7 +2124,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                     let expr_ty = self.typeck_results.borrow().expr_ty(expr);
                                     let return_ty = fn_sig.output();
                                     if !matches!(expr.kind, hir::ExprKind::Ret(..))
-                                        && self.can_coerce(expr_ty, return_ty)
+                                        && self.may_coerce(expr_ty, return_ty)
                                     {
                                         found_semi = true;
                                     }

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -261,7 +261,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if let hir::ExprKind::MethodCall(hir::PathSegment { ident: method, .. }, recv_expr, &[], _) =
             expr.kind
             && let Some(recv_ty) = self.typeck_results.borrow().expr_ty_opt(recv_expr)
-            && self.can_coerce(recv_ty, expected)
+            && self.may_coerce(recv_ty, expected)
             && let name = method.name.as_str()
             && (name.starts_with("to_") || name.starts_with("as_") || name == "into")
         {
@@ -349,7 +349,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return true;
         }
 
-        if self.suggest_fn_call(err, expr, found, |output| self.can_coerce(output, expected))
+        if self.suggest_fn_call(err, expr, found, |output| self.may_coerce(output, expected))
             && let ty::FnDef(def_id, ..) = *found.kind()
             && let Some(sp) = self.tcx.hir().span_if_local(def_id)
         {
@@ -568,7 +568,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if self.tcx.hir().is_inside_const_context(hir_id) || !expected.is_box() || found.is_box() {
             return false;
         }
-        if self.can_coerce(Ty::new_box(self.tcx, found), expected) {
+        if self.may_coerce(Ty::new_box(self.tcx, found), expected) {
             let suggest_boxing = match found.kind() {
                 ty::Tuple(tuple) if tuple.is_empty() => {
                     errors::SuggestBoxing::Unit { start: span.shrink_to_lo(), end: span }
@@ -663,7 +663,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         };
         match expected.kind() {
             ty::Adt(def, _) if Some(def.did()) == pin_did => {
-                if self.can_coerce(pin_box_found, expected) {
+                if self.may_coerce(pin_box_found, expected) {
                     debug!("can coerce {:?} to {:?}, suggesting Box::pin", pin_box_found, expected);
                     match found.kind() {
                         ty::Adt(def, _) if def.is_box() => {
@@ -689,7 +689,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         }
                     }
                     true
-                } else if self.can_coerce(pin_found, expected) {
+                } else if self.may_coerce(pin_found, expected) {
                     match found.kind() {
                         ty::Adt(def, _) if def.is_box() => {
                             err.help("use `Box::pin`");
@@ -701,7 +701,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     false
                 }
             }
-            ty::Adt(def, _) if def.is_box() && self.can_coerce(box_found, expected) => {
+            ty::Adt(def, _) if def.is_box() && self.may_coerce(box_found, expected) => {
                 // Check if the parent expression is a call to Pin::new. If it
                 // is and we were expecting a Box, ergo Pin<Box<expected>>, we
                 // can suggest Box::pin.
@@ -884,7 +884,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     let ty = Binder::bind_with_vars(ty, bound_vars);
                     let ty = self.normalize(hir_ty.span, ty);
                     let ty = self.tcx.instantiate_bound_regions_with_erased(ty);
-                    if self.can_coerce(expected, ty) {
+                    if self.may_coerce(expected, ty) {
                         err.subdiagnostic(errors::ExpectedReturnTypeLabel::Other {
                             span: hir_ty.span,
                             expected,
@@ -1141,12 +1141,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     ty::Asyncness::No => ty,
                 };
                 let ty = self.normalize(expr.span, ty);
-                self.can_coerce(found, ty)
+                self.may_coerce(found, ty)
             }
             hir::FnRetTy::DefaultReturn(_) if in_closure => {
                 self.ret_coercion.as_ref().map_or(false, |ret| {
                     let ret_ty = ret.borrow().expected_ty();
-                    self.can_coerce(found, ret_ty)
+                    self.may_coerce(found, ret_ty)
                 })
             }
             _ => false,
@@ -1510,7 +1510,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             provided_ty
         };
 
-        if !self.can_coerce(expected_ty, dummy_ty) {
+        if !self.may_coerce(expected_ty, dummy_ty) {
             return;
         }
         let msg = format!("use `{adt_name}::map_or` to deref inner value of `{adt_name}`");
@@ -1534,7 +1534,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected_ty: Ty<'tcx>,
     ) {
         if let ty::Slice(elem_ty) | ty::Array(elem_ty, _) = expected_ty.kind() {
-            if self.can_coerce(blk_ty, *elem_ty)
+            if self.may_coerce(blk_ty, *elem_ty)
                 && blk.stmts.is_empty()
                 && blk.rules == hir::BlockCheckMode::DefaultBlock
             {
@@ -1744,7 +1744,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if item_ty.has_param() {
             return false;
         }
-        if self.can_coerce(item_ty, expected_ty) {
+        if self.may_coerce(item_ty, expected_ty) {
             err.span_suggestion_verbose(
                 segment.ident.span,
                 format!("try referring to the associated const `{capitalized_name}` instead",),
@@ -1804,7 +1804,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // diagnostic in cases where we have `(&&T).clone()` and we expect `T`).
             && !results.expr_adjustments(callee_expr).iter().any(|adj| matches!(adj.kind, ty::adjustment::Adjust::Deref(..)))
             // Check that we're in fact trying to clone into the expected type
-            && self.can_coerce(*pointee_ty, expected_ty)
+            && self.may_coerce(*pointee_ty, expected_ty)
             && let trait_ref = ty::TraitRef::new(self.tcx, clone_trait_did, [expected_ty])
             // And the expected type doesn't implement `Clone`
             && !self.predicate_must_hold_considering_regions(&traits::Obligation::new(
@@ -2022,7 +2022,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             } else {
                 return false;
             };
-        if is_ctor || !self.can_coerce(args.type_at(0), expected) {
+        if is_ctor || !self.may_coerce(args.type_at(0), expected) {
             return false;
         }
 
@@ -2293,7 +2293,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         .then(|| " (its field is private, but it's local to this crate and its privacy can be changed)".to_string());
 
                     let sole_field_ty = sole_field.ty(self.tcx, args);
-                    if self.can_coerce(expr_ty, sole_field_ty) {
+                    if self.may_coerce(expr_ty, sole_field_ty) {
                         let variant_path =
                             with_no_trimmed_paths!(self.tcx.def_path_str(variant.def_id));
                         // FIXME #56861: DRYer prelude filtering
@@ -2401,7 +2401,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
 
         let int_type = args.type_at(0);
-        if !self.can_coerce(expr_ty, int_type) {
+        if !self.may_coerce(expr_ty, int_type) {
             return false;
         }
 
@@ -2585,7 +2585,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         Ty::new_imm_ref(self.tcx, self.tcx.lifetimes.re_static, checked_ty)
                     }
                 };
-                if self.can_coerce(ref_ty, expected) {
+                if self.may_coerce(ref_ty, expected) {
                     let mut sugg_sp = sp;
                     if let hir::ExprKind::MethodCall(segment, receiver, args, _) = expr.kind {
                         let clone_trait =

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -1934,7 +1934,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             && fn_sig.inputs()[1..]
                                 .iter()
                                 .zip(args.into_iter())
-                                .all(|(expected, found)| self.can_coerce(*expected, *found))
+                                .all(|(expected, found)| self.may_coerce(*expected, *found))
                             && fn_sig.inputs()[1..].len() == args.len()
                         {
                             err.span_suggestion_verbose(
@@ -4148,7 +4148,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return false;
         };
 
-        if !self.can_coerce(output, expected) {
+        if !self.may_coerce(output, expected) {
             return false;
         }
 

compiler/rustc_hir_typeck/src/pat.rs

@@ -1767,7 +1767,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 } else if inexistent_fields.len() == 1 {
                     match pat_field.pat.kind {
                         PatKind::Lit(expr)
-                            if !self.can_coerce(
+                            if !self.may_coerce(
                                 self.typeck_results.borrow().expr_ty(expr),
                                 self.field_ty(field.span, field_def, args),
                             ) => {}

tests/ui/traits/next-solver/diagnostics/coerce-in-may-coerce.rs

@@ -0,0 +1,19 @@
+//@ compile-flags: -Znext-solver
+
+trait Mirror {
+    type Assoc;
+}
+impl<T> Mirror for T {
+    type Assoc = T;
+}
+
+fn arg() -> &'static [i32; 1] { todo!() }
+
+fn arg_error(x: <fn() as Mirror>::Assoc, y: ()) { todo!() }
+
+fn main() {
+    // Should suggest to reverse the args...
+    // but if we don't normalize the expected, then we don't.
+    arg_error((), || ());
+    //~^ ERROR arguments to this function are incorrect
+}