Make hidden type registration opt-in, so that each site can be reviewed on its own and we have the right defaults for trait solvers

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -273,7 +273,8 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
         // FIXME(oli-obk): Also do region checks here and then consider removing `check_opaque_meets_bounds` entirely.
         let param_env = self.tcx.param_env(def_id);
         // HACK This bubble is required for this tests to pass:
-        // type-alias-impl-trait/issue-67844-nested-opaque.rs
+        // nested-return-type2-tait2.rs
+        // nested-return-type2-tait3.rs
         let infcx =
             self.tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).build();
         let ocx = ObligationCtxt::new(&infcx);

compiler/rustc_hir_typeck/src/closure.rs

@@ -561,8 +561,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ) {
                 // Check that E' = S'.
                 let cause = self.misc(hir_ty.span);
-                let InferOk { value: (), obligations } =
-                    self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
+                let InferOk { value: (), obligations } = self
+                    .at(&cause, self.param_env)
+                    .define_opaque_types(true)
+                    .eq(*expected_ty, supplied_ty)?;
                 all_obligations.extend(obligations);
             }
 
@@ -574,6 +576,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let cause = &self.misc(decl.output.span());
             let InferOk { value: (), obligations } = self
                 .at(cause, self.param_env)
+                .define_opaque_types(true)
                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
             all_obligations.extend(obligations);
 

compiler/rustc_hir_typeck/src/coercion.rs

@@ -143,11 +143,11 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
     fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
         debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
         self.commit_if_ok(|_| {
+            let at = self.at(&self.cause, self.fcx.param_env).define_opaque_types(true);
             if self.use_lub {
-                self.at(&self.cause, self.fcx.param_env).lub(b, a)
+                at.lub(b, a)
             } else {
-                self.at(&self.cause, self.fcx.param_env)
-                    .sup(b, a)
+                at.sup(b, a)
                     .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
             }
         })
@@ -174,7 +174,9 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             // Best-effort try to unify these types -- we're already on the error path,
             // so this will have the side-effect of making sure we have no ambiguities
             // due to `[type error]` and `_` not coercing together.
-            let _ = self.commit_if_ok(|_| self.at(&self.cause, self.param_env).eq(a, b));
+            let _ = self.commit_if_ok(|_| {
+                self.at(&self.cause, self.param_env).define_opaque_types(true).eq(a, b)
+            });
             return success(vec![], self.fcx.tcx.ty_error(), vec![]);
         }
 
@@ -1484,6 +1486,8 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
             // Another example is `break` with no argument expression.
             assert!(expression_ty.is_unit(), "if let hack without unit type");
             fcx.at(cause, fcx.param_env)
+                // needed for tests/ui/type-alias-impl-trait/issue-65679-inst-opaque-ty-from-val-twice.rs
+                .define_opaque_types(true)
                 .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
                 .map(|infer_ok| {
                     fcx.register_infer_ok_obligations(infer_ok);

compiler/rustc_hir_typeck/src/demand.rs

@@ -113,7 +113,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Ty<'tcx>,
         actual: Ty<'tcx>,
     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
-        match self.at(cause, self.param_env).sup(expected, actual) {
+        match self.at(cause, self.param_env).define_opaque_types(true).sup(expected, actual) {
             Ok(InferOk { obligations, value: () }) => {
                 self.register_predicates(obligations);
                 None
@@ -143,7 +143,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Ty<'tcx>,
         actual: Ty<'tcx>,
     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
-        match self.at(cause, self.param_env).eq(expected, actual) {
+        match self.at(cause, self.param_env).define_opaque_types(true).eq(expected, actual) {
             Ok(InferOk { obligations, value: () }) => {
                 self.register_predicates(obligations);
                 None

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -1436,7 +1436,6 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             TraitCandidate(trait_ref) => self.probe(|_| {
                 let _ = self
                     .at(&ObligationCause::dummy(), self.param_env)
-                    .define_opaque_types(false)
                     .sup(candidate.xform_self_ty, self_ty);
                 match self.select_trait_candidate(trait_ref) {
                     Ok(Some(traits::ImplSource::UserDefined(ref impl_data))) => {
@@ -1466,7 +1465,6 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             // First check that the self type can be related.
             let sub_obligations = match self
                 .at(&ObligationCause::dummy(), self.param_env)
-                .define_opaque_types(false)
                 .sup(probe.xform_self_ty, self_ty)
             {
                 Ok(InferOk { obligations, value: () }) => obligations,
@@ -1683,7 +1681,6 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 if let ProbeResult::Match = result
                     && self
                     .at(&ObligationCause::dummy(), self.param_env)
-                    .define_opaque_types(false)
                     .sup(return_ty, xform_ret_ty)
                     .is_err()
                 {

compiler/rustc_infer/src/infer/at.rs

@@ -55,7 +55,7 @@ impl<'tcx> InferCtxt<'tcx> {
         cause: &'a ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> At<'a, 'tcx> {
-        At { infcx: self, cause, param_env, define_opaque_types: true }
+        At { infcx: self, cause, param_env, define_opaque_types: false }
     }
 
     /// Forks the inference context, creating a new inference context with the same inference

compiler/rustc_infer/src/infer/mod.rs

@@ -264,7 +264,7 @@ pub struct InferCtxt<'tcx> {
     /// short lived InferCtxt within queries. The opaque type obligations are forwarded
     /// to the outside until the end up in an `InferCtxt` for typeck or borrowck.
     ///
-    /// It is default value is `DefiningAnchor::Error`, this way it is easier to catch errors that
+    /// Its default value is `DefiningAnchor::Error`, this way it is easier to catch errors that
     /// might come up during inference or typeck.
     pub defining_use_anchor: DefiningAnchor,
 

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -545,8 +545,11 @@ impl<'tcx> InferCtxt<'tcx> {
             origin,
         );
         if let Some(prev) = prev {
-            obligations =
-                self.at(&cause, param_env).eq_exp(a_is_expected, prev, hidden_ty)?.obligations;
+            obligations = self
+                .at(&cause, param_env)
+                .define_opaque_types(true)
+                .eq_exp(a_is_expected, prev, hidden_ty)?
+                .obligations;
         }
 
         let item_bounds = tcx.bound_explicit_item_bounds(def_id.to_def_id());

compiler/rustc_trait_selection/src/solve/infcx_ext.rs

@@ -54,7 +54,6 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
         rhs: T,
     ) -> Result<Vec<Goal<'tcx, ty::Predicate<'tcx>>>, NoSolution> {
         self.at(&ObligationCause::dummy(), param_env)
-            .define_opaque_types(false)
             .eq(lhs, rhs)
             .map(|InferOk { value: (), obligations }| {
                 obligations.into_iter().map(|o| o.into()).collect()

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -217,6 +217,7 @@ fn equate_impl_headers<'cx, 'tcx>(
     selcx
         .infcx
         .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
+        .define_opaque_types(true)
         .eq_impl_headers(impl1_header, impl2_header)
         .map(|infer_ok| infer_ok.obligations)
         .ok()

compiler/rustc_trait_selection/src/traits/engine.rs

@@ -128,6 +128,7 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
     {
         self.infcx
             .at(cause, param_env)
+            .define_opaque_types(true)
             .eq_exp(a_is_expected, a, b)
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
@@ -141,6 +142,7 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
     ) -> Result<(), TypeError<'tcx>> {
         self.infcx
             .at(cause, param_env)
+            .define_opaque_types(true)
             .eq(expected, actual)
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
@@ -155,6 +157,7 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
     ) -> Result<(), TypeError<'tcx>> {
         self.infcx
             .at(cause, param_env)
+            .define_opaque_types(true)
             .sup(expected, actual)
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
@@ -169,6 +172,7 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
     ) -> Result<(), TypeError<'tcx>> {
         self.infcx
             .at(cause, param_env)
+            .define_opaque_types(true)
             .sup(expected, actual)
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }

compiler/rustc_trait_selection/src/traits/project.rs

@@ -286,7 +286,12 @@ fn project_and_unify_type<'cx, 'tcx>(
         );
     obligations.extend(new);
 
-    match infcx.at(&obligation.cause, obligation.param_env).eq(normalized, actual) {
+    match infcx
+        .at(&obligation.cause, obligation.param_env)
+        // This is needed to support nested opaque types like `impl Fn() -> impl Trait`
+        .define_opaque_types(true)
+        .eq(normalized, actual)
+    {
         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
             obligations.extend(inferred_obligations);
             ProjectAndUnifyResult::Holds(obligations)

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -821,6 +821,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         self.infcx
             .at(&obligation.cause, obligation.param_env)
+            // needed for tests/ui/type-alias-impl-trait/assoc-projection-ice.rs
+            .define_opaque_types(true)
             .sup(obligation_trait_ref, expected_trait_ref)
             .map(|InferOk { mut obligations, .. }| {
                 obligations.extend(nested);

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1752,7 +1752,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         });
         self.infcx
             .at(&obligation.cause, obligation.param_env)
-            .define_opaque_types(false)
             .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
             .map(|InferOk { obligations: _, value: () }| {
                 // This method is called within a probe, so we can't have
@@ -1815,7 +1814,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let is_match = self
             .infcx
             .at(&obligation.cause, obligation.param_env)
-            .define_opaque_types(false)
             .sup(obligation.predicate, infer_projection)
             .map_or(false, |InferOk { obligations, value: () }| {
                 self.evaluate_predicates_recursively(
@@ -2507,7 +2505,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let InferOk { obligations, .. } = self
             .infcx
             .at(&cause, obligation.param_env)
-            .define_opaque_types(false)
             .eq(placeholder_obligation_trait_ref, impl_trait_ref)
             .map_err(|e| {
                 debug!("match_impl: failed eq_trait_refs due to `{}`", e.to_string(self.tcx()))
@@ -2558,11 +2555,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
         self.infcx
             .at(&obligation.cause, obligation.param_env)
-            // We don't want predicates for opaque types to just match all other types,
-            // if there is an obligation on the opaque type, then that obligation must be met
-            // opaquely. Otherwise we'd match any obligation to the opaque type and then error
-            // out later.
-            .define_opaque_types(false)
             .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
             .map(|InferOk { obligations, .. }| obligations)
             .map_err(|_| ())

tests/ui/type-alias-impl-trait/match-unification.rs

@@ -0,0 +1,14 @@
+use std::fmt::Debug;
+
+// check-pass
+
+fn bar() -> impl Debug {}
+
+fn baz(b: bool) -> Option<impl Debug> {
+    match b {
+        true => baz(false),
+        false => Some(bar()),
+    }
+}
+
+fn main() {}