Rollup merge of rust-lang#129078 - lcnr:scrape_region_constraints-use-ocx, r=compiler-errors

`ParamEnvAnd::fully_perform`: we have an `ocx`, use it

cc rust-lang#123669

r? ``@compiler-errors``

Author: workingjubilee

Diff for: compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs

@@ -168,44 +168,12 @@ where
         // collecting region constraints via `region_constraints`.
         let (mut output, _) = scrape_region_constraints(
             infcx,
-            |_ocx| {
-                let (output, ei, mut obligations, _) =
+            |ocx| {
+                let (output, ei, obligations, _) =
                     Q::fully_perform_into(self, infcx, &mut region_constraints, span)?;
                 error_info = ei;
 
-                // Typically, instantiating NLL query results does not
-                // create obligations. However, in some cases there
-                // are unresolved type variables, and unify them *can*
-                // create obligations. In that case, we have to go
-                // fulfill them. We do this via a (recursive) query.
-                while !obligations.is_empty() {
-                    trace!("{:#?}", obligations);
-                    let mut progress = false;
-                    for obligation in std::mem::take(&mut obligations) {
-                        let obligation = infcx.resolve_vars_if_possible(obligation);
-                        match ProvePredicate::fully_perform_into(
-                            obligation.param_env.and(ProvePredicate::new(obligation.predicate)),
-                            infcx,
-                            &mut region_constraints,
-                            span,
-                        ) {
-                            Ok(((), _, new, certainty)) => {
-                                obligations.extend(new);
-                                progress = true;
-                                if let Certainty::Ambiguous = certainty {
-                                    obligations.push(obligation);
-                                }
-                            }
-                            Err(_) => obligations.push(obligation),
-                        }
-                    }
-                    if !progress {
-                        infcx.dcx().span_bug(
-                            span,
-                            format!("ambiguity processing {obligations:?} from {self:?}"),
-                        );
-                    }
-                }
+                ocx.register_obligations(obligations);
                 Ok(output)
             },
             "fully_perform",