Do not look in ParamEnv for opaque type bounds that could be satisfied

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -40,21 +40,42 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             predicate: self.infcx.resolve_vars_if_possible(obligation.predicate),
         };
 
-        if obligation.predicate.skip_binder().self_ty().is_ty_var() {
-            debug!(ty = ?obligation.predicate.skip_binder().self_ty(), "ambiguous inference var or opaque type");
-            // Self is a type variable (e.g., `_: AsRef<str>`).
-            //
-            // This is somewhat problematic, as the current scheme can't really
-            // handle it turning to be a projection. This does end up as truly
-            // ambiguous in most cases anyway.
-            //
-            // Take the fast path out - this also improves
-            // performance by preventing assemble_candidates_from_impls from
-            // matching every impl for this trait.
-            return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
-        }
-
         let mut candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };
+        let def_id = obligation.predicate.def_id();
+        let tcx = self.tcx();
+
+        match obligation.predicate.skip_binder().self_ty().kind() {
+            // Opaque types in their defining scope are just like inference vars...
+            ty::Alias(ty::Opaque, alias) if self.infcx.can_define_opaque_ty(alias.def_id) => {
+                if tcx.is_lang_item(def_id, LangItem::Unsize) {
+                    self.assemble_candidates_for_unsizing(obligation, &mut candidates);
+                }
+                self.assemble_candidates_from_impls(obligation, &mut candidates);
+                // .. unless we are looking for candidates just on the opaque signature, ...
+                self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
+                // .. or for auto traits, which look at the hidden type.
+                // Auto traits must be collected after projected tys, because opaque types
+                // do not emit auto trait candidates if a projection for the same auto trait
+                // already exists (e.g. due to the bounds on the opaque).
+                self.assemble_candidates_from_auto_impls(obligation, &mut candidates);
+                return Ok(candidates);
+            }
+            ty::Infer(ty::TyVar(vid)) => {
+                debug!(?vid, "ambiguous inference var");
+                // Self is a type variable (e.g., `_: AsRef<str>`).
+                //
+                // This is somewhat problematic, as the current scheme can't really
+                // handle it turning to be a projection. This does end up as truly
+                // ambiguous in most cases anyway.
+                //
+                // Take the fast path out - this also improves
+                // performance by preventing assemble_candidates_from_impls from
+                // matching every impl for this trait.
+                candidates.ambiguous = true;
+                return Ok(candidates);
+            }
+            _ => {}
+        }
 
         // Negative trait predicates have different rules than positive trait predicates.
         if obligation.polarity() == ty::PredicatePolarity::Negative {
@@ -66,8 +87,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
             // Other bounds. Consider both in-scope bounds from fn decl
             // and applicable impls. There is a certain set of precedence rules here.
-            let def_id = obligation.predicate.def_id();
-            let tcx = self.tcx();
 
             if tcx.is_lang_item(def_id, LangItem::Copy) {
                 debug!(obligation_self_ty = ?obligation.predicate.skip_binder().self_ty());

tests/ui/type-alias-impl-trait/in-where-clause.rs

@@ -4,13 +4,13 @@
 #![feature(type_alias_impl_trait)]
 type Bar = impl Sized;
 //~^ ERROR: cycle
-//~| ERROR: cycle
 
 fn foo() -> Bar
 where
     Bar: Send,
 {
     [0; 1 + 2]
+    //~^ ERROR: cannot check whether the hidden type
 }
 
 fn main() {}

tests/ui/type-alias-impl-trait/in-where-clause.stderr

@@ -10,7 +10,7 @@ note: ...which requires computing type of opaque `Bar::{opaque#0}`...
 LL | type Bar = impl Sized;
    |            ^^^^^^^^^^
 note: ...which requires type-checking `foo`...
-  --> $DIR/in-where-clause.rs:9:1
+  --> $DIR/in-where-clause.rs:8:1
    |
 LL | / fn foo() -> Bar
 LL | | where
@@ -25,25 +25,26 @@ LL | type Bar = impl Sized;
    |            ^^^^^^^^^^
    = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
 
-error[E0391]: cycle detected when computing type of opaque `Bar::{opaque#0}`
-  --> $DIR/in-where-clause.rs:5:12
-   |
-LL | type Bar = impl Sized;
-   |            ^^^^^^^^^^
-   |
-note: ...which requires type-checking `foo`...
-  --> $DIR/in-where-clause.rs:13:9
+error: cannot check whether the hidden type of `in_where_clause[cb1b]::Bar::{opaque#0}` satisfies auto traits
+  --> $DIR/in-where-clause.rs:12:9
    |
 LL |     [0; 1 + 2]
    |         ^^^^^
-   = note: ...which requires evaluating trait selection obligation `Bar: core::marker::Send`...
-   = note: ...which again requires computing type of opaque `Bar::{opaque#0}`, completing the cycle
-note: cycle used when computing type of `Bar::{opaque#0}`
+   |
+   = note: fetching the hidden types of an opaque inside of the defining scope is not supported. You can try moving the opaque type and the item that actually registers a hidden type into a new submodule
+note: opaque type is declared here
   --> $DIR/in-where-clause.rs:5:12
    |
 LL | type Bar = impl Sized;
    |            ^^^^^^^^^^
-   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+note: required by a bound in `foo`
+  --> $DIR/in-where-clause.rs:10:10
+   |
+LL | fn foo() -> Bar
+   |    --- required by a bound in this function
+LL | where
+LL |     Bar: Send,
+   |          ^^^^ required by this bound in `foo`
 
 error: aborting due to 2 previous errors