nll-relate: improve hr opaque types support

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -124,8 +124,13 @@ impl<'a, 'b, 'tcx> NllTypeRelating<'a, 'b, 'tcx> {
         // by using `ty_vid rel B` and then finally and end by equating `ty_vid` to
         // the opaque.
         let mut enable_subtyping = |ty, opaque_is_expected| {
-            let ty_vid = infcx.next_ty_var_id_in_universe(self.span(), ty::UniverseIndex::ROOT);
-
+            // We create the fresh inference variable in the highest universe.
+            // In theory we could limit it to the highest universe in the args of
+            // the opaque but that isn't really worth the effort.
+            //
+            // We'll make sure that the opaque type can actually name everything
+            // in its hidden type later on.
+            let ty_vid = infcx.next_ty_vid(self.span());
             let variance = if opaque_is_expected {
                 self.ambient_variance
             } else {

compiler/rustc_infer/src/infer/mod.rs

@@ -782,22 +782,30 @@ impl<'tcx> InferCtxt<'tcx> {
         self.inner.borrow_mut().type_variables().num_vars()
     }
 
-    pub fn next_ty_var(&self, span: Span) -> Ty<'tcx> {
-        self.next_ty_var_with_origin(TypeVariableOrigin { span, param_def_id: None })
+    pub fn next_ty_vid(&self, span: Span) -> TyVid {
+        self.next_ty_vid_with_origin(TypeVariableOrigin { span, param_def_id: None })
     }
 
-    pub fn next_ty_var_with_origin(&self, origin: TypeVariableOrigin) -> Ty<'tcx> {
-        let vid = self.inner.borrow_mut().type_variables().new_var(self.universe(), origin);
-        Ty::new_var(self.tcx, vid)
+    pub fn next_ty_vid_with_origin(&self, origin: TypeVariableOrigin) -> TyVid {
+        self.inner.borrow_mut().type_variables().new_var(self.universe(), origin)
     }
 
-    pub fn next_ty_var_id_in_universe(&self, span: Span, universe: ty::UniverseIndex) -> TyVid {
+    pub fn next_ty_vid_in_universe(&self, span: Span, universe: ty::UniverseIndex) -> TyVid {
         let origin = TypeVariableOrigin { span, param_def_id: None };
         self.inner.borrow_mut().type_variables().new_var(universe, origin)
     }
 
+    pub fn next_ty_var(&self, span: Span) -> Ty<'tcx> {
+        self.next_ty_var_with_origin(TypeVariableOrigin { span, param_def_id: None })
+    }
+
+    pub fn next_ty_var_with_origin(&self, origin: TypeVariableOrigin) -> Ty<'tcx> {
+        let vid = self.next_ty_vid_with_origin(origin);
+        Ty::new_var(self.tcx, vid)
+    }
+
     pub fn next_ty_var_in_universe(&self, span: Span, universe: ty::UniverseIndex) -> Ty<'tcx> {
-        let vid = self.next_ty_var_id_in_universe(span, universe);
+        let vid = self.next_ty_vid_in_universe(span, universe);
         Ty::new_var(self.tcx, vid)
     }
 

tests/ui/type-alias-impl-trait/higher_kinded_params3.rs

@@ -24,8 +24,7 @@ type Successors<'a> = impl std::fmt::Debug + 'a;
 impl Terminator {
     #[define_opaque(Successors, Tait)]
     fn successors(&self, mut f: for<'x> fn(&'x ()) -> <&'x A as B>::C) -> Successors<'_> {
-        f = g;
-        //~^ ERROR mismatched types
+        f = g; //~ ERROR expected generic lifetime parameter, found `'x`
     }
 }
 

tests/ui/type-alias-impl-trait/higher_kinded_params3.stderr

@@ -1,15 +1,12 @@
-error[E0308]: mismatched types
+error[E0792]: expected generic lifetime parameter, found `'x`
   --> $DIR/higher_kinded_params3.rs:27:9
    |
 LL | type Tait<'a> = impl std::fmt::Debug + 'a;
-   |                 ------------------------- the expected opaque type
+   |           -- this generic parameter must be used with a generic lifetime parameter
 ...
 LL |         f = g;
-   |         ^^^^^ one type is more general than the other
-   |
-   = note: expected fn pointer `for<'x> fn(&'x ()) -> Tait<'x>`
-              found fn pointer `for<'a> fn(&'a ()) -> &'a ()`
+   |         ^^^^^
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0792`.

tests/ui/type-alias-impl-trait/hkl_forbidden3.rs

@@ -8,7 +8,7 @@ fn foo<'a>(x: &'a ()) -> &'a () {
 
 #[define_opaque(Opaque)]
 fn test() -> for<'a> fn(&'a ()) -> Opaque<'a> {
-    foo //~ ERROR: mismatched types
+    foo //~ ERROR: expected generic lifetime parameter, found `'a`
 }
 
 fn main() {}

tests/ui/type-alias-impl-trait/hkl_forbidden3.stderr

@@ -1,15 +1,12 @@
-error[E0308]: mismatched types
+error[E0792]: expected generic lifetime parameter, found `'a`
   --> $DIR/hkl_forbidden3.rs:11:5
    |
 LL | type Opaque<'a> = impl Sized + 'a;
-   |                   --------------- the expected opaque type
+   |             -- this generic parameter must be used with a generic lifetime parameter
 ...
 LL |     foo
-   |     ^^^ one type is more general than the other
-   |
-   = note: expected fn pointer `for<'a> fn(&'a ()) -> Opaque<'a>`
-              found fn pointer `for<'a> fn(&'a ()) -> &'a ()`
+   |     ^^^
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0792`.