Add `traits::fully_solve_obligation` that acts like `traits::fully_normalize`

It spawns up a trait engine, registers the single obligation, then fully
solves it

10 files changed, 78 insertions, 97 deletions

diff --git a/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs b/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
index c8a63c9c3f8..45dadcfff2e 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
@@ -5,12 +5,11 @@
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir::LangItem;
 use rustc_infer::infer::TyCtxtInferExt;
-use rustc_infer::traits::TraitEngine;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty};
 use rustc_span::DUMMY_SP;
 use rustc_trait_selection::traits::{
-    self, ImplSource, Obligation, ObligationCause, SelectionContext, TraitEngineExt,
+    self, ImplSource, Obligation, ObligationCause, SelectionContext,
 };
 
 use super::ConstCx;
@@ -189,15 +188,8 @@ impl Qualif for NeedsNonConstDrop {
                 return false;
             }
 
-            // If we successfully found one, then select all of the predicates
-            // implied by our const drop impl.
-            let mut fcx = <dyn TraitEngine<'tcx>>::new(cx.tcx);
-            for nested in impl_src.nested_obligations() {
-                fcx.register_predicate_obligation(&infcx, nested);
-            }
-
             // If we had any errors, then it's bad
-            !fcx.select_all_or_error(&infcx).is_empty()
+            !traits::fully_solve_obligations(&infcx, impl_src.nested_obligations()).is_empty()
         })
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
index 294c81d0b21..6b230210888 100644
--- a/compiler/rustc_trait_selection/src/traits/auto_trait.rs
+++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
@@ -205,10 +205,8 @@ impl<'tcx> AutoTraitFinder<'tcx> {
             // At this point, we already have all of the bounds we need. FulfillmentContext is used
             // to store all of the necessary region/lifetime bounds in the InferContext, as well as
             // an additional sanity check.
-            let mut fulfill = <dyn TraitEngine<'tcx>>::new(tcx);
-            fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy());
-            let errors = fulfill.select_all_or_error(&infcx);
-
+            let errors =
+                super::fully_solve_bound(&infcx, ObligationCause::dummy(), full_env, ty, trait_did);
             if !errors.is_empty() {
                 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
             }
diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index 65afd2aeee3..8ab1aa65d3a 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -10,14 +10,14 @@ use crate::traits::select::IntercrateAmbiguityCause;
 use crate::traits::util::impl_subject_and_oblig;
 use crate::traits::SkipLeakCheck;
 use crate::traits::{
-    self, FulfillmentContext, Normalized, Obligation, ObligationCause, PredicateObligation,
-    PredicateObligations, SelectionContext, TraitEngineExt,
+    self, Normalized, Obligation, ObligationCause, PredicateObligation, PredicateObligations,
+    SelectionContext,
 };
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::Diagnostic;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
-use rustc_infer::traits::{util, TraitEngine};
+use rustc_infer::traits::util;
 use rustc_middle::traits::specialization_graph::OverlapMode;
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::subst::Subst;
@@ -384,16 +384,11 @@ fn resolve_negative_obligation<'cx, 'tcx>(
         return false;
     };
 
-    let mut fulfillment_cx = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
-    fulfillment_cx.register_predicate_obligation(infcx, o);
-
-    let errors = fulfillment_cx.select_all_or_error(infcx);
-
+    let errors = super::fully_solve_obligation(infcx, o);
     if !errors.is_empty() {
         return false;
     }
 
-    // FIXME -- also add "assumed to be well formed" types into the `outlives_env`
     let outlives_env = OutlivesEnvironment::new(param_env);
     infcx.process_registered_region_obligations(outlives_env.region_bound_pairs(), param_env);
 
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index b8cbfcb8b79..5fc0128d8c6 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -30,6 +30,7 @@ use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
+use rustc_infer::traits::TraitEngineExt as _;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
 use rustc_middle::ty::visit::TypeVisitable;
@@ -161,22 +162,20 @@ pub fn type_known_to_meet_bound_modulo_regions<'a, 'tcx>(
         // this function's result remains infallible, we must confirm
         // that guess. While imperfect, I believe this is sound.
 
-        // The handling of regions in this area of the code is terrible,
-        // see issue #29149. We should be able to improve on this with
-        // NLL.
-        let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
-
         // We can use a dummy node-id here because we won't pay any mind
         // to region obligations that arise (there shouldn't really be any
         // anyhow).
         let cause = ObligationCause::misc(span, hir::CRATE_HIR_ID);
 
-        fulfill_cx.register_bound(infcx, param_env, ty, def_id, cause);
+        // The handling of regions in this area of the code is terrible,
+        // see issue #29149. We should be able to improve on this with
+        // NLL.
+        let errors = fully_solve_bound(infcx, cause, param_env, ty, def_id);
 
         // Note: we only assume something is `Copy` if we can
         // *definitively* show that it implements `Copy`. Otherwise,
         // assume it is move; linear is always ok.
-        match fulfill_cx.select_all_or_error(infcx).as_slice() {
+        match &errors[..] {
             [] => {
                 debug!(
                     "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} success",
@@ -413,6 +412,36 @@ where
     Ok(resolved_value)
 }
 
+pub fn fully_solve_obligation<'a, 'tcx>(
+    infcx: &InferCtxt<'a, 'tcx>,
+    obligation: PredicateObligation<'tcx>,
+) -> Vec<FulfillmentError<'tcx>> {
+    let mut engine = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
+    engine.register_predicate_obligation(infcx, obligation);
+    engine.select_all_or_error(infcx)
+}
+
+pub fn fully_solve_obligations<'a, 'tcx>(
+    infcx: &InferCtxt<'a, 'tcx>,
+    obligations: impl IntoIterator<Item = PredicateObligation<'tcx>>,
+) -> Vec<FulfillmentError<'tcx>> {
+    let mut engine = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
+    engine.register_predicate_obligations(infcx, obligations);
+    engine.select_all_or_error(infcx)
+}
+
+pub fn fully_solve_bound<'a, 'tcx>(
+    infcx: &InferCtxt<'a, 'tcx>,
+    cause: ObligationCause<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    ty: Ty<'tcx>,
+    bound: DefId,
+) -> Vec<FulfillmentError<'tcx>> {
+    let mut engine = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
+    engine.register_bound(infcx, param_env, ty, bound, cause);
+    engine.select_all_or_error(infcx)
+}
+
 /// Normalizes the predicates and checks whether they hold in an empty environment. If this
 /// returns true, then either normalize encountered an error or one of the predicates did not
 /// hold. Used when creating vtables to check for unsatisfiable methods.
diff --git a/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs b/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
index c99564936aa..f6e196e3141 100644
--- a/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
@@ -1,11 +1,9 @@
 use crate::infer::canonical::query_response;
 use crate::infer::{InferCtxt, InferOk};
-use crate::traits::engine::TraitEngineExt as _;
+use crate::traits;
 use crate::traits::query::type_op::TypeOpOutput;
 use crate::traits::query::Fallible;
-use crate::traits::TraitEngine;
 use rustc_infer::infer::region_constraints::RegionConstraintData;
-use rustc_infer::traits::TraitEngineExt as _;
 use rustc_span::source_map::DUMMY_SP;
 
 use std::fmt;
@@ -62,8 +60,6 @@ pub fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
     infcx: &InferCtxt<'_, 'tcx>,
     op: impl FnOnce() -> Fallible<InferOk<'tcx, R>>,
 ) -> Fallible<(TypeOpOutput<'tcx, Op>, RegionConstraintData<'tcx>)> {
-    let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
-
     // During NLL, we expect that nobody will register region
     // obligations **except** as part of a custom type op (and, at the
     // end of each custom type op, we scrape out the region
@@ -77,8 +73,7 @@ pub fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
     );
 
     let InferOk { value, obligations } = infcx.commit_if_ok(|_| op())?;
-    fulfill_cx.register_predicate_obligations(infcx, obligations);
-    let errors = fulfill_cx.select_all_or_error(infcx);
+    let errors = traits::fully_solve_obligations(infcx, obligations);
     if !errors.is_empty() {
         infcx.tcx.sess.diagnostic().delay_span_bug(
             DUMMY_SP,
diff --git a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
index 0506c200c61..0f76fef0eee 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
@@ -14,9 +14,7 @@ use specialization_graph::GraphExt;
 
 use crate::infer::{InferCtxt, InferOk, TyCtxtInferExt};
 use crate::traits::select::IntercrateAmbiguityCause;
-use crate::traits::{
-    self, coherence, FutureCompatOverlapErrorKind, ObligationCause, TraitEngine, TraitEngineExt,
-};
+use crate::traits::{self, coherence, FutureCompatOverlapErrorKind, ObligationCause};
 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_errors::{struct_span_err, EmissionGuarantee, LintDiagnosticBuilder};
 use rustc_hir::def_id::{DefId, LocalDefId};
@@ -26,8 +24,8 @@ use rustc_session::lint::builtin::COHERENCE_LEAK_CHECK;
 use rustc_session::lint::builtin::ORDER_DEPENDENT_TRAIT_OBJECTS;
 use rustc_span::{Span, DUMMY_SP};
 
+use super::util;
 use super::SelectionContext;
-use super::{util, FulfillmentContext};
 
 /// Information pertinent to an overlapping impl error.
 #[derive(Debug)]
@@ -210,11 +208,8 @@ fn fulfill_implication<'a, 'tcx>(
     // (which are packed up in penv)
 
     infcx.save_and_restore_in_snapshot_flag(|infcx| {
-        let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(infcx.tcx);
-        for oblig in obligations.chain(more_obligations) {
-            fulfill_cx.register_predicate_obligation(&infcx, oblig);
-        }
-        match fulfill_cx.select_all_or_error(infcx).as_slice() {
+        let errors = traits::fully_solve_obligations(&infcx, obligations.chain(more_obligations));
+        match &errors[..] {
             [] => {
                 debug!(
                     "fulfill_implication: an impl for {:?} specializes {:?}",
diff --git a/compiler/rustc_typeck/src/coherence/builtin.rs b/compiler/rustc_typeck/src/coherence/builtin.rs
index 50946cc1def..043e21fc1e3 100644
--- a/compiler/rustc_typeck/src/coherence/builtin.rs
+++ b/compiler/rustc_typeck/src/coherence/builtin.rs
@@ -15,7 +15,7 @@ use rustc_middle::ty::{self, suggest_constraining_type_params, Ty, TyCtxt, TypeV
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
 use rustc_trait_selection::traits::misc::{can_type_implement_copy, CopyImplementationError};
 use rustc_trait_selection::traits::predicate_for_trait_def;
-use rustc_trait_selection::traits::{self, ObligationCause, TraitEngine, TraitEngineExt};
+use rustc_trait_selection::traits::{self, ObligationCause};
 use std::collections::BTreeMap;
 
 pub fn check_trait(tcx: TyCtxt<'_>, trait_def_id: DefId) {
@@ -109,15 +109,13 @@ fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
                 // it is not immediately clear why Copy is not implemented for a field, since
                 // all we point at is the field itself.
                 tcx.infer_ctxt().ignoring_regions().enter(|infcx| {
-                    let mut fulfill_cx = <dyn TraitEngine<'_>>::new(tcx);
-                    fulfill_cx.register_bound(
+                    for error in traits::fully_solve_bound(
                         &infcx,
+                        traits::ObligationCause::dummy_with_span(field_ty_span),
                         param_env,
                         ty,
                         tcx.lang_items().copy_trait().unwrap(),
-                        traits::ObligationCause::dummy_with_span(field_ty_span),
-                    );
-                    for error in fulfill_cx.select_all_or_error(&infcx) {
+                    ) {
                         let error_predicate = error.obligation.predicate;
                         // Only note if it's not the root obligation, otherwise it's trivial and
                         // should be self-explanatory (i.e. a field literally doesn't implement Copy).
@@ -315,24 +313,20 @@ fn visit_implementation_of_dispatch_from_dyn<'tcx>(tcx: TyCtxt<'tcx>, impl_did:
                     ))
                     .emit();
                 } else {
-                    let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
-
-                    for field in coerced_fields {
-                        let predicate = predicate_for_trait_def(
-                            tcx,
-                            param_env,
-                            cause.clone(),
-                            dispatch_from_dyn_trait,
-                            0,
-                            field.ty(tcx, substs_a),
-                            &[field.ty(tcx, substs_b).into()],
-                        );
-
-                        fulfill_cx.register_predicate_obligation(&infcx, predicate);
-                    }
-
-                    // Check that all transitive obligations are satisfied.
-                    let errors = fulfill_cx.select_all_or_error(&infcx);
+                    let errors = traits::fully_solve_obligations(
+                        &infcx,
+                        coerced_fields.into_iter().map(|field| {
+                            predicate_for_trait_def(
+                                tcx,
+                                param_env,
+                                cause.clone(),
+                                dispatch_from_dyn_trait,
+                                0,
+                                field.ty(tcx, substs_a),
+                                &[field.ty(tcx, substs_b).into()],
+                            )
+                        }),
+                    );
                     if !errors.is_empty() {
                         infcx.report_fulfillment_errors(&errors, None, false);
                     }
@@ -573,8 +567,6 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUn
             }
         };
 
-        let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
-
         // Register an obligation for `A: Trait<B>`.
         let cause = traits::ObligationCause::misc(span, impl_hir_id);
         let predicate = predicate_for_trait_def(
@@ -586,10 +578,7 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUn
             source,
             &[target.into()],
         );
-        fulfill_cx.register_predicate_obligation(&infcx, predicate);
-
-        // Check that all transitive obligations are satisfied.
-        let errors = fulfill_cx.select_all_or_error(&infcx);
+        let errors = traits::fully_solve_obligation(&infcx, predicate);
         if !errors.is_empty() {
             infcx.report_fulfillment_errors(&errors, None, false);
         }
diff --git a/compiler/rustc_typeck/src/hir_wf_check.rs b/compiler/rustc_typeck/src/hir_wf_check.rs
index 55c7a15f9bc..fd9715e6ca3 100644
--- a/compiler/rustc_typeck/src/hir_wf_check.rs
+++ b/compiler/rustc_typeck/src/hir_wf_check.rs
@@ -3,11 +3,10 @@ use rustc_hir as hir;
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{ForeignItem, ForeignItemKind, HirId};
 use rustc_infer::infer::TyCtxtInferExt;
-use rustc_infer::traits::TraitEngine;
 use rustc_infer::traits::{ObligationCause, WellFormedLoc};
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, Region, ToPredicate, TyCtxt, TypeFoldable, TypeFolder};
-use rustc_trait_selection::traits::{self, TraitEngineExt};
+use rustc_trait_selection::traits;
 
 pub fn provide(providers: &mut Providers) {
     *providers = Providers { diagnostic_hir_wf_check, ..*providers };
@@ -66,7 +65,6 @@ fn diagnostic_hir_wf_check<'tcx>(
     impl<'tcx> Visitor<'tcx> for HirWfCheck<'tcx> {
         fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
             self.tcx.infer_ctxt().enter(|infcx| {
-                let mut fulfill = <dyn TraitEngine<'tcx>>::new(self.tcx);
                 let tcx_ty =
                     self.icx.to_ty(ty).fold_with(&mut EraseAllBoundRegions { tcx: self.tcx });
                 let cause = traits::ObligationCause::new(
@@ -74,7 +72,7 @@ fn diagnostic_hir_wf_check<'tcx>(
                     self.hir_id,
                     traits::ObligationCauseCode::WellFormed(None),
                 );
-                fulfill.register_predicate_obligation(
+                let errors = traits::fully_solve_obligation(
                     &infcx,
                     traits::Obligation::new(
                         cause,
@@ -83,8 +81,6 @@ fn diagnostic_hir_wf_check<'tcx>(
                             .to_predicate(self.tcx),
                     ),
                 );
-
-                let errors = fulfill.select_all_or_error(&infcx);
                 if !errors.is_empty() {
                     debug!("Wf-check got errors for {:?}: {:?}", ty, errors);
                     for error in errors {
diff --git a/compiler/rustc_typeck/src/lib.rs b/compiler/rustc_typeck/src/lib.rs
index 04c54589d68..1563f3552c5 100644
--- a/compiler/rustc_typeck/src/lib.rs
+++ b/compiler/rustc_typeck/src/lib.rs
@@ -104,7 +104,6 @@ use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::{Node, CRATE_HIR_ID};
 use rustc_infer::infer::{InferOk, TyCtxtInferExt};
-use rustc_infer::traits::TraitEngineExt as _;
 use rustc_middle::middle;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -113,9 +112,7 @@ use rustc_session::config::EntryFnType;
 use rustc_span::{symbol::sym, Span, DUMMY_SP};
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
-use rustc_trait_selection::traits::{
-    self, ObligationCause, ObligationCauseCode, TraitEngine, TraitEngineExt as _,
-};
+use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
 
 use std::iter;
 
@@ -147,18 +144,15 @@ fn require_same_types<'tcx>(
 ) -> bool {
     tcx.infer_ctxt().enter(|ref infcx| {
         let param_env = ty::ParamEnv::empty();
-        let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
-        match infcx.at(cause, param_env).eq(expected, actual) {
-            Ok(InferOk { obligations, .. }) => {
-                fulfill_cx.register_predicate_obligations(infcx, obligations);
-            }
+        let errors = match infcx.at(cause, param_env).eq(expected, actual) {
+            Ok(InferOk { obligations, .. }) => traits::fully_solve_obligations(infcx, obligations),
             Err(err) => {
                 infcx.report_mismatched_types(cause, expected, actual, err).emit();
                 return false;
             }
-        }
+        };
 
-        match fulfill_cx.select_all_or_error(infcx).as_slice() {
+        match &errors[..] {
             [] => true,
             errors => {
                 infcx.report_fulfillment_errors(errors, None, false);
diff --git a/src/tools/clippy/clippy_lints/src/future_not_send.rs b/src/tools/clippy/clippy_lints/src/future_not_send.rs
index 5c46d6c7df7..ef7d75aa8ed 100644
--- a/src/tools/clippy/clippy_lints/src/future_not_send.rs
+++ b/src/tools/clippy/clippy_lints/src/future_not_send.rs
@@ -9,7 +9,7 @@ use rustc_middle::ty::{EarlyBinder, Opaque, PredicateKind::Trait};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::{sym, Span};
 use rustc_trait_selection::traits::error_reporting::suggestions::InferCtxtExt;
-use rustc_trait_selection::traits::{self, FulfillmentError, TraitEngine};
+use rustc_trait_selection::traits::{self, FulfillmentError};
 
 declare_clippy_lint! {
     /// ### What it does
@@ -80,9 +80,7 @@ impl<'tcx> LateLintPass<'tcx> for FutureNotSend {
                 let span = decl.output.span();
                 let send_errors = cx.tcx.infer_ctxt().enter(|infcx| {
                     let cause = traits::ObligationCause::misc(span, hir_id);
-                    let mut fulfillment_cx = traits::FulfillmentContext::new();
-                    fulfillment_cx.register_bound(&infcx, cx.param_env, ret_ty, send_trait, cause);
-                    fulfillment_cx.select_all_or_error(&infcx)
+                    traits::fully_solve_bound(&infcx, cause, cx.param_env, ret_ty, send_trait)
                 });
                 if !send_errors.is_empty() {
                     span_lint_and_then(