Prove obligations to termination instead of ignoring ambiguities.

Sometimes an obligation depends on a later one, so we can't just process them in order like it was done previously.

This is not a problem in our test suite, but there may be ICEs out there and it will definitely be a problem with lazy TAIT.

compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs

@@ -4,7 +4,9 @@ use crate::infer::canonical::{
 use crate::infer::{InferCtxt, InferOk};
 use crate::traits::query::Fallible;
 use crate::traits::ObligationCause;
-use rustc_infer::infer::canonical::Canonical;
+use rustc_infer::infer::canonical::{Canonical, Certainty};
+use rustc_infer::traits::query::NoSolution;
+use rustc_infer::traits::PredicateObligations;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::{ParamEnvAnd, TyCtxt};
 use std::fmt;
@@ -17,7 +19,6 @@ pub mod implied_outlives_bounds;
 pub mod normalize;
 pub mod outlives;
 pub mod prove_predicate;
-use self::prove_predicate::ProvePredicate;
 pub mod subtype;
 
 pub use rustc_middle::traits::query::type_op::*;
@@ -80,9 +81,14 @@ pub trait QueryTypeOp<'tcx>: fmt::Debug + Copy + TypeFoldable<'tcx> + 'tcx {
         query_key: ParamEnvAnd<'tcx, Self>,
         infcx: &InferCtxt<'_, 'tcx>,
         output_query_region_constraints: &mut QueryRegionConstraints<'tcx>,
-    ) -> Fallible<(Self::QueryResponse, Option<Canonical<'tcx, ParamEnvAnd<'tcx, Self>>>)> {
+    ) -> Fallible<(
+        Self::QueryResponse,
+        Option<Canonical<'tcx, ParamEnvAnd<'tcx, Self>>>,
+        PredicateObligations<'tcx>,
+        Certainty,
+    )> {
         if let Some(result) = QueryTypeOp::try_fast_path(infcx.tcx, &query_key) {
-            return Ok((result, None));
+            return Ok((result, None, vec![], Certainty::Proven));
         }
 
         // FIXME(#33684) -- We need to use
@@ -104,20 +110,7 @@ pub trait QueryTypeOp<'tcx>: fmt::Debug + Copy + TypeFoldable<'tcx> + 'tcx {
                 output_query_region_constraints,
             )?;
 
-        // 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.
-        for obligation in obligations {
-            let ((), _) = ProvePredicate::fully_perform_into(
-                obligation.param_env.and(ProvePredicate::new(obligation.predicate)),
-                infcx,
-                output_query_region_constraints,
-            )?;
-        }
-
-        Ok((value, Some(canonical_self)))
+        Ok((value, Some(canonical_self), obligations, canonical_result.value.certainty))
     }
 }
 
@@ -129,9 +122,39 @@ where
 
     fn fully_perform(self, infcx: &InferCtxt<'_, 'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>> {
         let mut region_constraints = QueryRegionConstraints::default();
-        let (output, canonicalized_query) =
+        let (output, canonicalized_query, mut obligations, _) =
             Q::fully_perform_into(self, infcx, &mut region_constraints)?;
 
+        // 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,
+                ) {
+                    Ok(((), _, new, certainty)) => {
+                        obligations.extend(new);
+                        progress = true;
+                        if let Certainty::Ambiguous = certainty {
+                            obligations.push(obligation);
+                        }
+                    }
+                    Err(_) => obligations.push(obligation),
+                }
+            }
+            if !progress {
+                return Err(NoSolution);
+            }
+        }
+
         // Promote the final query-region-constraints into a
         // (optional) ref-counted vector:
         let region_constraints =