distinguish user-type annotations from other annotations

In particular, we don't want to preserve the universes for the `'_`
variables that appear in there. And we don't expect to find any
placeholders, which justifies this as harmless.

(In particular, if you have a query like `Foo(!1, !2, ?3)`, then you
care about the universe of `?3`, since it may control whether `?3 =
!1` and `?3 = !2` is a valid answer. But without any placeholders, we
don't really care: any placeholders that would appear in the output
must therefore come from some fresh universe anyway.)

Author: nikomatsakis

src/librustc/infer/canonical/canonicalizer.rs

@@ -107,6 +107,20 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
         )
     }
 
+    pub fn canonicalize_user_type_annotation<V>(&self, value: &V) -> Canonicalized<'gcx, V>
+    where
+        V: TypeFoldable<'tcx> + Lift<'gcx>,
+    {
+        let mut query_state = OriginalQueryValues::default();
+        Canonicalizer::canonicalize(
+            value,
+            Some(self),
+            self.tcx,
+            &CanonicalizeUserTypeAnnotation,
+            &mut query_state,
+        )
+    }
+
     /// A hacky variant of `canonicalize_query` that does not
     /// canonicalize `'static`.  Unfortunately, the existing leak
     /// check treaks `'static` differently in some cases (see also
@@ -197,6 +211,29 @@ impl CanonicalizeRegionMode for CanonicalizeQueryResponse {
     }
 }
 
+struct CanonicalizeUserTypeAnnotation;
+
+impl CanonicalizeRegionMode for CanonicalizeUserTypeAnnotation {
+    fn canonicalize_free_region(
+        &self,
+        canonicalizer: &mut Canonicalizer<'_, '_, 'tcx>,
+        r: ty::Region<'tcx>,
+    ) -> ty::Region<'tcx> {
+        match r {
+            ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReErased | ty::ReEmpty | ty::ReStatic => r,
+            ty::ReVar(_) => canonicalizer.canonical_var_for_region_in_root_universe(r),
+            _ => {
+                // We only expect region names that the user can type.
+                bug!("unexpected region in query response: `{:?}`", r)
+            }
+        }
+    }
+
+    fn any(&self) -> bool {
+        false
+    }
+}
+
 struct CanonicalizeAllFreeRegions;
 
 impl CanonicalizeRegionMode for CanonicalizeAllFreeRegions {

src/librustc_typeck/check/mod.rs

@@ -976,7 +976,7 @@ impl<'a, 'gcx, 'tcx> Visitor<'gcx> for GatherLocalsVisitor<'a, 'gcx, 'tcx> {
                     o_ty
                 };
 
-                let c_ty = self.fcx.inh.infcx.canonicalize_response(&revealed_ty);
+                let c_ty = self.fcx.inh.infcx.canonicalize_user_type_annotation(&revealed_ty);
                 debug!("visit_local: ty.hir_id={:?} o_ty={:?} revealed_ty={:?} c_ty={:?}",
                        ty.hir_id, o_ty, revealed_ty, c_ty);
                 self.fcx.tables.borrow_mut().user_provided_tys_mut().insert(ty.hir_id, c_ty);
@@ -2137,7 +2137,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                             method.substs[i]
                         }
                     });
-                    self.infcx.canonicalize_response(&UserSubsts {
+                    self.infcx.canonicalize_user_type_annotation(&UserSubsts {
                         substs: just_method_substs,
                         user_self_ty: None, // not relevant here
                     })
@@ -2181,7 +2181,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         );
 
         if !substs.is_noop() {
-            let user_substs = self.infcx.canonicalize_response(&UserSubsts {
+            let user_substs = self.infcx.canonicalize_user_type_annotation(&UserSubsts {
                 substs,
                 user_self_ty,
             });