Auto merge of rust-lang#125907 - fmease:rustdoc-synth-blanket-ocx-next, r=<try>

rustdoc: use the next solver for blanket impl synthesis

Furthermore use an `ObligationCtxt` instead of operating on an `InferCtxt` directly and don't drop the obligations generated by the type relating (fixes a FIXME).

Originally I just wanted to submit the infcx→ocx change. However, that regressed `tests/rustdoc-ui/ice-blanket-impl-52873.rs` (pass→overflow error) because with `ocx.select_where_possible` we would no longer suppress / defatalize (canonical) overflow errors on the last obligation we register. CC rust-lang#54199 which introduced that special case. Obviously in the next solver overflows are non-fatal incl. `ice-blanket-impl-52873.rs`. Hence the switch now.

Note that I wanted to hold off on switching to the new solver since it makes rust-lang#114891 go from long I-compiletime to I-compilemem + I-hang + eventual death by the OOM killer. So I don't know maybe we should block this PR on me finding a rustc reproducer for the rustdoc issue rust-lang#114891 (this is on my agenda) to be able to properly investigate the underlying next-solver issue.

Author: bors

src/librustdoc/clean/blanket_impl.rs

@@ -1,10 +1,10 @@
 use rustc_hir as hir;
-use rustc_infer::infer::{DefineOpaqueTypes, InferOk, TyCtxtInferExt};
-use rustc_infer::traits;
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::traits::ObligationCause;
 use rustc_middle::ty::{self, Upcast};
 use rustc_span::def_id::DefId;
 use rustc_span::DUMMY_SP;
-use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
+use rustc_trait_selection::traits;
 
 use thin_vec::ThinVec;
 
@@ -31,53 +31,47 @@ pub(crate) fn synthesize_blanket_impls(
         }
         // NOTE: doesn't use `for_each_relevant_impl` to avoid looking at anything besides blanket impls
         let trait_impls = tcx.trait_impls_of(trait_def_id);
-        'blanket_impls: for &impl_def_id in trait_impls.blanket_impls() {
+        for &impl_def_id in trait_impls.blanket_impls() {
             trace!("considering impl `{impl_def_id:?}` for trait `{trait_def_id:?}`");
 
             let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
                 continue;
             }
-            let infcx = tcx.infer_ctxt().build();
+            let infcx = tcx.infer_ctxt().with_next_trait_solver(true).build();
+            let ocx = traits::ObligationCtxt::new(&infcx);
+
             let args = infcx.fresh_args_for_item(DUMMY_SP, item_def_id);
             let impl_ty = ty.instantiate(tcx, args);
             let param_env = ty::ParamEnv::empty();
+            let cause = ObligationCause::dummy();
 
             let impl_args = infcx.fresh_args_for_item(DUMMY_SP, impl_def_id);
             let impl_trait_ref = trait_ref.instantiate(tcx, impl_args);
 
             // Require the type the impl is implemented on to match
             // our type, and ignore the impl if there was a mismatch.
-            let Ok(eq_result) = infcx.at(&traits::ObligationCause::dummy(), param_env).eq(
-                DefineOpaqueTypes::Yes,
-                impl_trait_ref.self_ty(),
-                impl_ty,
-            ) else {
+            if ocx.eq(&cause, param_env, impl_trait_ref.self_ty(), impl_ty).is_err() {
                 continue;
-            };
-            let InferOk { value: (), obligations } = eq_result;
-            // FIXME(eddyb) ignoring `obligations` might cause false positives.
-            drop(obligations);
+            }
 
-            let predicates = tcx
-                .predicates_of(impl_def_id)
-                .instantiate(tcx, impl_args)
-                .predicates
-                .into_iter()
-                .chain(Some(impl_trait_ref.upcast(tcx)));
-            for predicate in predicates {
-                let obligation = traits::Obligation::new(
-                    tcx,
-                    traits::ObligationCause::dummy(),
-                    param_env,
-                    predicate,
-                );
-                match infcx.evaluate_obligation(&obligation) {
-                    Ok(eval_result) if eval_result.may_apply() => {}
-                    Err(traits::OverflowError::Canonical) => {}
-                    _ => continue 'blanket_impls,
-                }
+            ocx.register_obligations(traits::predicates_for_generics(
+                |_, _| cause.clone(),
+                param_env,
+                tcx.predicates_of(impl_def_id).instantiate(tcx, impl_args),
+            ));
+
+            ocx.register_obligation(traits::Obligation {
+                cause,
+                recursion_depth: 0,
+                param_env,
+                predicate: impl_trait_ref.upcast(tcx),
+            });
+
+            if !ocx.select_where_possible().is_empty() {
+                continue;
             }
+
             debug!("found applicable impl for trait ref {trait_ref:?}");
 
             cx.generated_synthetics.insert((ty.skip_binder(), trait_def_id));