Rollup merge of #109462 - compiler-errors:alias-relate, r=BoxyUwU,lcnr

Make alias-eq have a relation direction (and rename it to alias-relate)

Emitting an "alias-eq" is too strict in some situations, since we don't always want strict equality between a projection and rigid ty. Adds a relation direction.

* I could probably just reuse this [`RelationDir`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/combine/enum.RelationDir.html) -- happy to uplift that struct into middle and use that instead, but I didn't feel compelled to... 🤷
* Some of the matching in `compute_alias_relate_goal` is a bit verbose -- I guess I could simplify it by using [`At::relate`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/at/struct.At.html#method.relate) and mapping the relation-dir to a variance.
* Alternatively, I coulld simplify things by making more helper functions on `EvalCtxt` (e.g. `EvalCtxt::relate_with_direction(T, T)` that also does the nested goal registration). No preference.

r? ```@lcnr``` cc ```@BoxyUwU``` though boxy can claim it if she wants
NOTE: first commit is all the changes, the second is just renaming stuff

7 files changed, 19 insertions, 14 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
index dbf6775afc2..6b3a59b1ed5 100644
--- a/compiler/rustc_trait_selection/src/traits/auto_trait.rs
+++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
@@ -832,7 +832,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                 // the `ParamEnv`.
                 ty::PredicateKind::WellFormed(..)
                 | ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(..))
-                | ty::PredicateKind::AliasEq(..)
+                | ty::PredicateKind::AliasRelate(..)
                 | ty::PredicateKind::ObjectSafe(..)
                 | ty::PredicateKind::ClosureKind(..)
                 | ty::PredicateKind::Subtype(..)
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
index 1174efdbfa8..13607b9079a 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
@@ -92,6 +92,11 @@ impl<'a, 'tcx> TypeRelation<'tcx> for CollectAllMismatches<'a, 'tcx> {
 }
 
 impl<'tcx> ObligationEmittingRelation<'tcx> for CollectAllMismatches<'_, 'tcx> {
+    fn alias_relate_direction(&self) -> ty::AliasRelationDirection {
+        // FIXME(deferred_projection_equality): We really should get rid of this relation.
+        ty::AliasRelationDirection::Equate
+    }
+
     fn register_obligations(&mut self, _obligations: PredicateObligations<'tcx>) {
         // FIXME(deferred_projection_equality)
     }
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 41ffaeeac1c..617d53b609d 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -1276,9 +1276,9 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         "TypeWellFormedFromEnv predicate should only exist in the environment"
                     ),
 
-                    ty::PredicateKind::AliasEq(..) => span_bug!(
+                    ty::PredicateKind::AliasRelate(..) => span_bug!(
                         span,
-                        "AliasEq predicate should never be the predicate cause of a SelectionError"
+                        "AliasRelate predicate should never be the predicate cause of a SelectionError"
                     ),
 
                     ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(ct, ty)) => {
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index 23754480fcf..07e31e87bfb 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -361,8 +361,8 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for Chalk")
                 }
-                ty::PredicateKind::AliasEq(..) => {
-                    bug!("AliasEq is only used for new solver")
+                ty::PredicateKind::AliasRelate(..) => {
+                    bug!("AliasRelate is only used for new solver")
                 }
             },
             Some(pred) => match pred {
@@ -630,8 +630,8 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for Chalk")
                 }
-                ty::PredicateKind::AliasEq(..) => {
-                    bug!("AliasEq is only used for new solver")
+                ty::PredicateKind::AliasRelate(..) => {
+                    bug!("AliasRelate is only used for new solver")
                 }
                 ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(ct, ty)) => {
                     match self.selcx.infcx.at(&obligation.cause, obligation.param_env).eq(
diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index 038f8964471..5d2af5ff33c 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -335,7 +335,7 @@ fn predicate_references_self<'tcx>(
             has_self_ty(&ty.into()).then_some(sp)
         }
 
-        ty::PredicateKind::AliasEq(..) => bug!("`AliasEq` not allowed as assumption"),
+        ty::PredicateKind::AliasRelate(..) => bug!("`AliasRelate` not allowed as assumption"),
 
         ty::PredicateKind::WellFormed(..)
         | ty::PredicateKind::ObjectSafe(..)
@@ -395,7 +395,7 @@ fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
             | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..))
             | ty::PredicateKind::ConstEvaluatable(..)
             | ty::PredicateKind::ConstEquate(..)
-            | ty::PredicateKind::AliasEq(..)
+            | ty::PredicateKind::AliasRelate(..)
             | ty::PredicateKind::Ambiguous
             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
         }
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index b8758ad9323..bffefdf359a 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -977,8 +977,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for chalk")
                 }
-                ty::PredicateKind::AliasEq(..) => {
-                    bug!("AliasEq is only used for new solver")
+                ty::PredicateKind::AliasRelate(..) => {
+                    bug!("AliasRelate is only used for new solver")
                 }
                 ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
                 ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(ct, ty)) => {
diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index d498af359c5..ec5bd982a3c 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -191,8 +191,8 @@ pub fn predicate_obligations<'tcx>(
         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
             bug!("TypeWellFormedFromEnv is only used for Chalk")
         }
-        ty::PredicateKind::AliasEq(..) => {
-            bug!("We should only wf check where clauses and `AliasEq` is not a `Clause`")
+        ty::PredicateKind::AliasRelate(..) => {
+            bug!("We should only wf check where clauses and `AliasRelate` is not a `Clause`")
         }
     }
 
@@ -936,7 +936,7 @@ pub(crate) fn required_region_bounds<'tcx>(
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..)
                 | ty::PredicateKind::Ambiguous
-                | ty::PredicateKind::AliasEq(..)
+                | ty::PredicateKind::AliasRelate(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
                     ref t,