Auto merge of #123864 - oli-obk:define_opaque_types3, r=<try>

Remove a HACK by instead inferring opaque types during expected/formal type checking

I was wondering why I couldn't come up with a test that hits the code path of the argument check checking the types we inferred from the return type... Turns out we reject those attempts early during fudging.

I have absolutely no information for you as to what kind of type inference changes this may incur, but I think we should just land this out of two reasons:

* had I found the other place to use opaque type inference on before I added the hack, we'd be using that today and this PR would never have happened
* if it is possible to hit this path, it requires some god awful recursive RPIT logic that I doubt anyone would have written without actively trying to write obscure code

r? `@ghost`

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -711,32 +711,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let formal_ret = self.resolve_vars_with_obligations(formal_ret);
         let ret_ty = expected_ret.only_has_type(self)?;
 
-        // HACK(oli-obk): This is a hack to keep RPIT and TAIT in sync wrt their behaviour.
-        // Without it, the inference
-        // variable will get instantiated with the opaque type. The inference variable often
-        // has various helpful obligations registered for it that help closures figure out their
-        // signature. If we infer the inference var to the opaque type, the closure won't be able
-        // to find those obligations anymore, and it can't necessarily find them from the opaque
-        // type itself. We could be more powerful with inference if we *combined* the obligations
-        // so that we got both the obligations from the opaque type and the ones from the inference
-        // variable. That will accept more code than we do right now, so we need to carefully consider
-        // the implications.
-        // Note: this check is pessimistic, as the inference type could be matched with something other
-        // than the opaque type, but then we need a new `TypeRelation` just for this specific case and
-        // can't re-use `sup` below.
-        // See tests/ui/impl-trait/hidden-type-is-opaque.rs and
-        // tests/ui/impl-trait/hidden-type-is-opaque-2.rs for examples that hit this path.
-        if formal_ret.has_infer_types() {
-            for ty in ret_ty.walk() {
-                if let ty::GenericArgKind::Type(ty) = ty.unpack()
-                    && let ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) = *ty.kind()
-                    && self.can_define_opaque_ty(def_id)
-                {
-                    return None;
-                }
-            }
-        }
-
         let expect_args = self
             .fudge_inference_if_ok(|| {
                 let ocx = ObligationCtxt::new(self);

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -297,22 +297,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // 3. Check if the formal type is a supertype of the checked one
             //    and register any such obligations for future type checks
             let supertype_error = self.at(&self.misc(provided_arg.span), self.param_env).sup(
-                DefineOpaqueTypes::No,
+                DefineOpaqueTypes::Yes,
                 formal_input_ty,
                 coerced_ty,
             );
-            let subtyping_error = match supertype_error {
+
+            // If neither check failed, the types are compatible
+            match supertype_error {
                 Ok(InferOk { obligations, value: () }) => {
                     self.register_predicates(obligations);
-                    None
+                    Compatibility::Compatible
                 }
-                Err(err) => Some(err),
-            };
-
-            // If neither check failed, the types are compatible
-            match subtyping_error {
-                None => Compatibility::Compatible,
-                Some(_) => Compatibility::Incompatible(subtyping_error),
+                Err(err) => Compatibility::Incompatible(Some(err)),
             }
         };