Inline evaluate_obligation instead of going through the query system

There are several reasons I changed this:

- `evaluate_obligation` is only used in one place in
  `rustc_trait_selection`, and in fact warns you against using it
  anywhere else.
- The implementation in `rustc_traits` was considerably more complicated
  than necessary, because it didn't have the context available in
  `rustc_trait_selection`.
- It allows moving OverflowError into rustc_trait_selection, making
  rust-lang#81091 simpler (in particular,
  it allows holding an `Obligation` in OverflowError, which is only
  defined in `rustc_infer` and not available in rustc_middle).

The only reason to keep the previous behavior is if the cache from the
query system made it significantly faster.

compiler/rustc_middle/src/query/mod.rs

@@ -2,9 +2,9 @@ use crate::dep_graph::SerializedDepNodeIndex;
 use crate::mir::interpret::{GlobalId, LitToConstInput};
 use crate::traits;
 use crate::traits::query::{
-    CanonicalPredicateGoal, CanonicalProjectionGoal, CanonicalTyGoal,
-    CanonicalTypeOpAscribeUserTypeGoal, CanonicalTypeOpEqGoal, CanonicalTypeOpNormalizeGoal,
-    CanonicalTypeOpProvePredicateGoal, CanonicalTypeOpSubtypeGoal,
+    CanonicalProjectionGoal, CanonicalTyGoal, CanonicalTypeOpAscribeUserTypeGoal,
+    CanonicalTypeOpEqGoal, CanonicalTypeOpNormalizeGoal, CanonicalTypeOpProvePredicateGoal,
+    CanonicalTypeOpSubtypeGoal,
 };
 use crate::ty::query::queries;
 use crate::ty::subst::{GenericArg, SubstsRef};
@@ -1531,14 +1531,6 @@ rustc_queries! {
             desc { "computing dropck types for `{:?}`", goal }
         }
 
-        /// Do not call this query directly: invoke `infcx.predicate_may_hold()` or
-        /// `infcx.predicate_must_hold()` instead.
-        query evaluate_obligation(
-            goal: CanonicalPredicateGoal<'tcx>
-        ) -> Result<traits::EvaluationResult, traits::OverflowError> {
-            desc { "evaluating trait selection obligation `{}`", goal.value.value }
-        }
-
         query evaluate_goal(
             goal: traits::CanonicalChalkEnvironmentAndGoal<'tcx>
         ) -> Result<

compiler/rustc_middle/src/ty/query/mod.rs

@@ -18,9 +18,9 @@ use crate::mir::interpret::{ConstValue, EvalToAllocationRawResult, EvalToConstVa
 use crate::mir::interpret::{LitToConstError, LitToConstInput};
 use crate::mir::mono::CodegenUnit;
 use crate::traits::query::{
-    CanonicalPredicateGoal, CanonicalProjectionGoal, CanonicalTyGoal,
-    CanonicalTypeOpAscribeUserTypeGoal, CanonicalTypeOpEqGoal, CanonicalTypeOpNormalizeGoal,
-    CanonicalTypeOpProvePredicateGoal, CanonicalTypeOpSubtypeGoal, NoSolution,
+    CanonicalProjectionGoal, CanonicalTyGoal, CanonicalTypeOpAscribeUserTypeGoal,
+    CanonicalTypeOpEqGoal, CanonicalTypeOpNormalizeGoal, CanonicalTypeOpProvePredicateGoal,
+    CanonicalTypeOpSubtypeGoal, NoSolution,
 };
 use crate::traits::query::{
     DropckOutlivesResult, DtorckConstraint, MethodAutoderefStepsResult, NormalizationResult,

compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs

@@ -1,4 +1,3 @@
-use crate::infer::canonical::OriginalQueryValues;
 use crate::infer::InferCtxt;
 use crate::traits::{
     EvaluationResult, OverflowError, PredicateObligation, SelectionContext, TraitQueryMode,
@@ -63,13 +62,8 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
         &self,
         obligation: &PredicateObligation<'tcx>,
     ) -> Result<EvaluationResult, OverflowError> {
-        let mut _orig_values = OriginalQueryValues::default();
-        let c_pred = self
-            .canonicalize_query(obligation.param_env.and(obligation.predicate), &mut _orig_values);
-        // Run canonical query. If overflow occurs, rerun from scratch but this time
-        // in standard trait query mode so that overflow is handled appropriately
-        // within `SelectionContext`.
-        self.tcx.evaluate_obligation(c_pred)
+        let mut selcx = SelectionContext::with_query_mode(self, TraitQueryMode::Canonical);
+        selcx.evaluate_root_obligation(&obligation)
     }
 
     // Helper function that canonicalizes and runs the query. If an

compiler/rustc_traits/src/lib.rs

@@ -14,7 +14,6 @@ extern crate rustc_middle;
 
 mod chalk;
 mod dropck_outlives;
-mod evaluate_obligation;
 mod implied_outlives_bounds;
 mod normalize_erasing_regions;
 mod normalize_projection_ty;
@@ -24,7 +23,6 @@ use rustc_middle::ty::query::Providers;
 
 pub fn provide(p: &mut Providers) {
     dropck_outlives::provide(p);
-    evaluate_obligation::provide(p);
     implied_outlives_bounds::provide(p);
     chalk::provide(p);
     normalize_projection_ty::provide(p);