Opportunistically resolve region var in canonicalizer (instead of resolving root var)

compiler/rustc_infer/src/infer/mod.rs

@@ -381,17 +381,13 @@ impl<'tcx> ty::InferCtxtLike for InferCtxt<'tcx> {
         self.probe_ty_var(vid).ok()
     }
 
-    fn root_lt_var(&self, vid: ty::RegionVid) -> ty::RegionVid {
-        self.root_region_var(vid)
-    }
-
-    fn probe_lt_var(&self, vid: ty::RegionVid) -> Option<ty::Region<'tcx>> {
+    fn opportunistic_resolve_lt_var(&self, vid: ty::RegionVid) -> Option<ty::Region<'tcx>> {
         let re = self
             .inner
             .borrow_mut()
             .unwrap_region_constraints()
             .opportunistic_resolve_var(self.tcx, vid);
-        if re.is_var() { None } else { Some(re) }
+        if *re == ty::ReVar(vid) { None } else { Some(re) }
     }
 
     fn root_ct_var(&self, vid: ConstVid) -> ConstVid {
@@ -1367,10 +1363,6 @@ impl<'tcx> InferCtxt<'tcx> {
         self.inner.borrow_mut().type_variables().root_var(var)
     }
 
-    pub fn root_region_var(&self, var: ty::RegionVid) -> ty::RegionVid {
-        self.inner.borrow_mut().unwrap_region_constraints().root_var(var)
-    }
-
     pub fn root_const_var(&self, var: ty::ConstVid) -> ty::ConstVid {
         self.inner.borrow_mut().const_unification_table().find(var).vid
     }

compiler/rustc_infer/src/infer/region_constraints/mod.rs

@@ -623,11 +623,6 @@ impl<'tcx> RegionConstraintCollector<'_, 'tcx> {
         }
     }
 
-    pub fn root_var(&mut self, vid: ty::RegionVid) -> ty::RegionVid {
-        let mut ut = self.unification_table_mut(); // FIXME(rust-lang/ena#42): unnecessary mut
-        ut.find(vid).vid
-    }
-
     fn combine_map(&mut self, t: CombineMapType) -> &mut CombineMap<'tcx> {
         match t {
             Glb => &mut self.glbs,

compiler/rustc_next_trait_solver/src/canonicalizer.rs

@@ -242,16 +242,10 @@ impl<Infcx: InferCtxtLike<Interner = I>, I: Interner> TypeFolder<I>
 
             ty::ReVar(vid) => {
                 assert_eq!(
-                    self.infcx.root_lt_var(vid),
-                    vid,
-                    "region vid should have been resolved fully before canonicalization"
-                );
-                assert_eq!(
-                    self.infcx.probe_lt_var(vid),
+                    self.infcx.opportunistic_resolve_lt_var(vid),
                     None,
                     "region vid should have been resolved fully before canonicalization"
                 );
-
                 match self.canonicalize_mode {
                     CanonicalizeMode::Input => CanonicalVarKind::Region(ty::UniverseIndex::ROOT),
                     CanonicalizeMode::Response { .. } => {

compiler/rustc_type_ir/src/debug.rs

@@ -32,11 +32,7 @@ impl<I: Interner> InferCtxtLike for NoInfcx<I> {
         None
     }
 
-    fn root_lt_var(&self, vid: I::InferRegion) -> I::InferRegion {
-        vid
-    }
-
-    fn probe_lt_var(&self, _vid: I::InferRegion) -> Option<I::Region> {
+    fn opportunistic_resolve_lt_var(&self, _vid: I::InferRegion) -> Option<I::Region> {
         None
     }
 

compiler/rustc_type_ir/src/infcx.rs

@@ -18,14 +18,14 @@ pub trait InferCtxtLike {
         lt: <Self::Interner as Interner>::InferRegion,
     ) -> Option<UniverseIndex>;
 
-    /// Resolve `InferRegion` to its root `InferRegion`.
-    fn root_lt_var(
-        &self,
-        vid: <Self::Interner as Interner>::InferRegion,
-    ) -> <Self::Interner as Interner>::InferRegion;
-
-    /// Resolve `InferRegion` to its inferred region, if it has been equated with a non-infer region.
-    fn probe_lt_var(
+    /// Resolve `InferRegion` to its inferred region, if it has been equated with
+    /// a non-infer region.
+    ///
+    /// FIXME: This has slightly different semantics than `{probe,resolve}_{ty,ct}_var`,
+    /// that has to do with the fact unlike `Ty` or `Const` vars, in rustc, we may
+    /// not always be able to *name* the root region var from the universe of the
+    /// var we're trying to resolve. That's why it's called *opportunistic*.
+    fn opportunistic_resolve_lt_var(
         &self,
         vid: <Self::Interner as Interner>::InferRegion,
     ) -> Option<<Self::Interner as Interner>::Region>;

tests/ui/traits/next-solver/issue-118950-root-region.rs

@@ -0,0 +1,22 @@
+// compile-flags: -Znext-solver
+//
+// This is a gnarly test but I don't know how to minimize it, frankly.
+
+#![feature(lazy_type_alias)]
+//~^ WARN the feature `lazy_type_alias` is incomplete
+
+trait ToUnit<'a> {
+    type Unit;
+}
+
+trait Overlap<T> {}
+
+type Assoc<'a, T> = <*const T as ToUnit<'a>>::Unit;
+
+impl<T> Overlap<T> for T {}
+
+impl<T> Overlap<for<'a> fn(Assoc<'a, T>)> for T where Missing: Overlap<T> {}
+//~^ ERROR conflicting implementations of trait `Overlap<fn(_)>` for type `fn(_)`
+//~| ERROR cannot find type `Missing` in this scope
+
+fn main() {}

tests/ui/traits/next-solver/issue-118950-root-region.stderr

@@ -0,0 +1,36 @@
+error[E0412]: cannot find type `Missing` in this scope
+  --> $DIR/issue-118950-root-region.rs:18:55
+   |
+LL | impl<T> Overlap<for<'a> fn(Assoc<'a, T>)> for T where Missing: Overlap<T> {}
+   |                                                       ^^^^^^^ not found in this scope
+
+warning: the feature `lazy_type_alias` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-118950-root-region.rs:5:12
+   |
+LL | #![feature(lazy_type_alias)]
+   |            ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+WARN rustc_infer::infer::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
+WARN rustc_infer::infer::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
+WARN rustc_infer::infer::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
+WARN rustc_infer::infer::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
+WARN rustc_infer::infer::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
+WARN rustc_infer::infer::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
+WARN rustc_infer::infer::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [ReBound(DebruijnIndex(0), BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
+WARN rustc_infer::infer::generalize may incompletely handle alias type: Alias(Weak, AliasTy { args: [RePlaceholder(!1_BoundRegion { var: 0, kind: BrNamed(DefId(0:15 ~ issue_118950_root_region[d54f]::{impl#1}::'a), 'a) }), ?1t], def_id: DefId(0:8 ~ issue_118950_root_region[d54f]::Assoc) })
+error[E0119]: conflicting implementations of trait `Overlap<fn(_)>` for type `fn(_)`
+  --> $DIR/issue-118950-root-region.rs:18:1
+   |
+LL | impl<T> Overlap<T> for T {}
+   | ------------------------ first implementation here
+LL |
+LL | impl<T> Overlap<for<'a> fn(Assoc<'a, T>)> for T where Missing: Overlap<T> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `fn(_)`
+
+error: aborting due to 2 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0119, E0412.
+For more information about an error, try `rustc --explain E0119`.