Auto merge of #145396 - compiler-errors:revert-method-pref, r=lcnr

[BETA] Revert "Use DeepRejectCtxt in assemble_inherent_candidates_from_param"

Fixes #145185.

Backporting this to stable and beta in favor of #145262 (comment).

r? lcnr

Author: bors

compiler/rustc_hir_typeck/src/lib.rs

@@ -1,7 +1,6 @@
 // tidy-alphabetical-start
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -1,4 +1,3 @@
-use std::assert_matches::debug_assert_matches;
 use std::cell::{Cell, RefCell};
 use std::cmp::max;
 use std::ops::Deref;
@@ -16,7 +15,7 @@ use rustc_infer::infer::{BoundRegionConversionTime, DefineOpaqueTypes, InferOk,
 use rustc_infer::traits::ObligationCauseCode;
 use rustc_middle::middle::stability;
 use rustc_middle::ty::elaborate::supertrait_def_ids;
-use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams, simplify_type};
+use rustc_middle::ty::fast_reject::{TreatParams, simplify_type};
 use rustc_middle::ty::{
     self, AssocItem, AssocItemContainer, GenericArgs, GenericArgsRef, GenericParamDefKind,
     ParamEnvAnd, Ty, TyCtxt, TypeVisitableExt, Upcast,
@@ -803,8 +802,8 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     );
                 }
             }
-            ty::Param(_) => {
-                self.assemble_inherent_candidates_from_param(raw_self_ty);
+            ty::Param(p) => {
+                self.assemble_inherent_candidates_from_param(p);
             }
             ty::Bool
             | ty::Char
@@ -905,16 +904,18 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     }
 
     #[instrument(level = "debug", skip(self))]
-    fn assemble_inherent_candidates_from_param(&mut self, param_ty: Ty<'tcx>) {
-        debug_assert_matches!(param_ty.kind(), ty::Param(_));
-
-        let tcx = self.tcx;
+    fn assemble_inherent_candidates_from_param(&mut self, param_ty: ty::ParamTy) {
         let bounds = self.param_env.caller_bounds().iter().filter_map(|predicate| {
             let bound_predicate = predicate.kind();
             match bound_predicate.skip_binder() {
-                ty::ClauseKind::Trait(trait_predicate) => DeepRejectCtxt::relate_rigid_rigid(tcx)
-                    .types_may_unify(param_ty, trait_predicate.trait_ref.self_ty())
-                    .then(|| bound_predicate.rebind(trait_predicate.trait_ref)),
+                ty::ClauseKind::Trait(trait_predicate) => {
+                    match *trait_predicate.trait_ref.self_ty().kind() {
+                        ty::Param(p) if p == param_ty => {
+                            Some(bound_predicate.rebind(trait_predicate.trait_ref))
+                        }
+                        _ => None,
+                    }
+                }
                 ty::ClauseKind::RegionOutlives(_)
                 | ty::ClauseKind::TypeOutlives(_)
                 | ty::ClauseKind::Projection(_)

tests/ui/methods/rigid-alias-bound-is-not-inherent.current.stderr

@@ -0,0 +1,30 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/rigid-alias-bound-is-not-inherent.rs:42:7
+   |
+LL |     x.method();
+   |       ^^^^^^ multiple `method` found
+   |
+note: candidate #1 is defined in the trait `Trait1`
+  --> $DIR/rigid-alias-bound-is-not-inherent.rs:21:5
+   |
+LL |     fn method(&self) {
+   |     ^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl of the trait `Trait2` for the type `T`
+  --> $DIR/rigid-alias-bound-is-not-inherent.rs:27:5
+   |
+LL |     fn method(&self) {
+   |     ^^^^^^^^^^^^^^^^
+help: disambiguate the method for candidate #1
+   |
+LL -     x.method();
+LL +     Trait1::method(&x);
+   |
+help: disambiguate the method for candidate #2
+   |
+LL -     x.method();
+LL +     Trait2::method(&x);
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0034`.

tests/ui/methods/rigid-alias-bound-is-not-inherent.next.stderr

@@ -0,0 +1,30 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/rigid-alias-bound-is-not-inherent.rs:42:7
+   |
+LL |     x.method();
+   |       ^^^^^^ multiple `method` found
+   |
+note: candidate #1 is defined in the trait `Trait1`
+  --> $DIR/rigid-alias-bound-is-not-inherent.rs:21:5
+   |
+LL |     fn method(&self) {
+   |     ^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in the trait `Trait2`
+  --> $DIR/rigid-alias-bound-is-not-inherent.rs:27:5
+   |
+LL |     fn method(&self) {
+   |     ^^^^^^^^^^^^^^^^
+help: disambiguate the method for candidate #1
+   |
+LL -     x.method();
+LL +     Trait1::method(&x);
+   |
+help: disambiguate the method for candidate #2
+   |
+LL -     x.method();
+LL +     Trait2::method(&x);
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0034`.

tests/ui/methods/rigid-alias-bound-is-not-inherent.rs

@@ -0,0 +1,46 @@
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+// See the code below.
+//
+// We were using `DeepRejectCtxt` to ensure that `assemble_inherent_candidates_from_param`
+// did not rely on the param-env being eagerly normalized. Since aliases unify with all
+// types, this meant that a rigid param-env candidate like `<T as Deref>::Target: Trait1`
+// would be registered as a "WhereClauseCandidate", which is treated as inherent. Since
+// we evaluate these candidates for all self types in the deref chain, this candidate
+// would be satisfied for `<T as Deref>::Target`, meaning that it would be preferred over
+// an "extension" candidate like `<T as Deref>::Target: Trait2` even though it holds.
+// This is problematic, since it causes ambiguities to be broken somewhat arbitrarily.
+// And as a side-effect, it also caused our computation of "used" traits to be miscalculated
+// since inherent candidates don't count as an import usage.
+
+use std::ops::Deref;
+
+trait Trait1 {
+    fn method(&self) {
+        println!("1");
+    }
+}
+
+trait Trait2 {
+    fn method(&self) {
+        println!("2");
+    }
+}
+impl<T: Other + ?Sized> Trait2 for T {}
+
+trait Other {}
+
+fn foo<T>(x: T)
+where
+    T: Deref,
+    <T as Deref>::Target: Trait1 + Other,
+{
+    // Make sure that we don't prefer methods from where clauses for rigid aliases,
+    // just for params. We could revisit this behavior, but it would be a lang change.
+    x.method();
+    //~^ ERROR multiple applicable items in scope
+}
+
+fn main() {}