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

compiler/rustc_middle/src/ty/flags.rs

@@ -288,7 +288,7 @@ impl FlagComputation {
                 self.add_ty(ty);
             }
             ty::PredicateKind::Ambiguous => {}
-            ty::PredicateKind::AliasEq(t1, t2) => {
+            ty::PredicateKind::AliasRelate(t1, t2, _) => {
                 self.add_term(t1);
                 self.add_term(t2);
             }

compiler/rustc_middle/src/ty/mod.rs

@@ -543,7 +543,7 @@ impl<'tcx> Predicate<'tcx> {
             | PredicateKind::Clause(Clause::TypeOutlives(_))
             | PredicateKind::Clause(Clause::Projection(_))
             | PredicateKind::Clause(Clause::ConstArgHasType(..))
-            | PredicateKind::AliasEq(..)
+            | PredicateKind::AliasRelate(..)
             | PredicateKind::ObjectSafe(_)
             | PredicateKind::ClosureKind(_, _, _)
             | PredicateKind::Subtype(_)
@@ -640,7 +640,23 @@ pub enum PredicateKind<'tcx> {
     /// This predicate requires two terms to be equal to eachother.
     ///
     /// Only used for new solver
-    AliasEq(Term<'tcx>, Term<'tcx>),
+    AliasRelate(Term<'tcx>, Term<'tcx>, AliasRelationDirection),
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Hash, TyEncodable, TyDecodable)]
+#[derive(HashStable, Debug)]
+pub enum AliasRelationDirection {
+    Equate,
+    Subtype,
+}
+
+impl std::fmt::Display for AliasRelationDirection {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            AliasRelationDirection::Equate => write!(f, " == "),
+            AliasRelationDirection::Subtype => write!(f, " <: "),
+        }
+    }
 }
 
 /// The crate outlives map is computed during typeck and contains the
@@ -976,11 +992,11 @@ impl<'tcx> Term<'tcx> {
         }
     }
 
-    /// This function returns `None` for `AliasKind::Opaque`.
+    /// This function returns the inner `AliasTy` if this term is a projection.
     ///
     /// FIXME: rename `AliasTy` to `AliasTerm` and make sure we correctly
     /// deal with constants.
-    pub fn to_alias_term_no_opaque(&self, tcx: TyCtxt<'tcx>) -> Option<AliasTy<'tcx>> {
+    pub fn to_projection_term(&self, tcx: TyCtxt<'tcx>) -> Option<AliasTy<'tcx>> {
         match self.unpack() {
             TermKind::Ty(ty) => match ty.kind() {
                 ty::Alias(kind, alias_ty) => match kind {
@@ -1206,7 +1222,7 @@ impl<'tcx> Predicate<'tcx> {
             PredicateKind::Clause(Clause::Trait(t)) => Some(predicate.rebind(t)),
             PredicateKind::Clause(Clause::Projection(..))
             | PredicateKind::Clause(Clause::ConstArgHasType(..))
-            | PredicateKind::AliasEq(..)
+            | PredicateKind::AliasRelate(..)
             | PredicateKind::Subtype(..)
             | PredicateKind::Coerce(..)
             | PredicateKind::Clause(Clause::RegionOutlives(..))
@@ -1227,7 +1243,7 @@ impl<'tcx> Predicate<'tcx> {
             PredicateKind::Clause(Clause::Projection(t)) => Some(predicate.rebind(t)),
             PredicateKind::Clause(Clause::Trait(..))
             | PredicateKind::Clause(Clause::ConstArgHasType(..))
-            | PredicateKind::AliasEq(..)
+            | PredicateKind::AliasRelate(..)
             | PredicateKind::Subtype(..)
             | PredicateKind::Coerce(..)
             | PredicateKind::Clause(Clause::RegionOutlives(..))
@@ -1249,7 +1265,7 @@ impl<'tcx> Predicate<'tcx> {
             PredicateKind::Clause(Clause::Trait(..))
             | PredicateKind::Clause(Clause::ConstArgHasType(..))
             | PredicateKind::Clause(Clause::Projection(..))
-            | PredicateKind::AliasEq(..)
+            | PredicateKind::AliasRelate(..)
             | PredicateKind::Subtype(..)
             | PredicateKind::Coerce(..)
             | PredicateKind::Clause(Clause::RegionOutlives(..))

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -2847,7 +2847,7 @@ define_print_and_forward_display! {
                 p!("the type `", print(ty), "` is found in the environment")
             }
             ty::PredicateKind::Ambiguous => p!("ambiguous"),
-            ty::PredicateKind::AliasEq(t1, t2) => p!(print(t1), " == ", print(t2)),
+            ty::PredicateKind::AliasRelate(t1, t2, dir) => p!(print(t1), write(" {} ", dir), print(t2)),
         }
     }
 

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -177,7 +177,9 @@ impl<'tcx> fmt::Debug for ty::PredicateKind<'tcx> {
                 write!(f, "TypeWellFormedFromEnv({:?})", ty)
             }
             ty::PredicateKind::Ambiguous => write!(f, "Ambiguous"),
-            ty::PredicateKind::AliasEq(t1, t2) => write!(f, "AliasEq({t1:?}, {t2:?})"),
+            ty::PredicateKind::AliasRelate(t1, t2, dir) => {
+                write!(f, "AliasRelate({t1:?}, {dir:?}, {t2:?})")
+            }
         }
     }
 }
@@ -250,6 +252,7 @@ TrivialTypeTraversalAndLiftImpls! {
     crate::ty::AssocItem,
     crate::ty::AssocKind,
     crate::ty::AliasKind,
+    crate::ty::AliasRelationDirection,
     crate::ty::Placeholder<crate::ty::BoundRegionKind>,
     crate::ty::Placeholder<crate::ty::BoundTyKind>,
     crate::ty::ClosureKind,