Use a proper probe for shadowing impl

compiler/rustc_middle/src/traits/solve/inspect.rs

@@ -153,6 +153,8 @@ pub enum ProbeKind<'tcx> {
     /// do a probe to find out what projection type(s) may be used to prove that
     /// the source type upholds all of the target type's object bounds.
     UpcastProjectionCompatibility,
+    /// Looking for param-env candidates that satisfy the trait ref for a projection.
+    ShadowedEnvProbing,
     /// Try to unify an opaque type with an existing key in the storage.
     OpaqueTypeStorageLookup { result: QueryResult<'tcx> },
 }

compiler/rustc_middle/src/traits/solve/inspect/format.rs

@@ -118,6 +118,9 @@ pub(super) fn format_probe(&mut self, probe: &Probe<'_>) -> std::fmt::Result {
             ProbeKind::TraitCandidate { source, result } => {
                 write!(self.f, "CANDIDATE {source:?}: {result:?}")
             }
+            ProbeKind::ShadowedEnvProbing => {
+                write!(self.f, "PROBING FOR IMPLS SHADOWED BY PARAM-ENV CANDIDATE:")
+            }
         }?;
 
         self.nested(|this| {

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -1,7 +1,7 @@
 //! Code shared by trait and projection goals for candidate assembly.
 
 use crate::solve::GoalSource;
-use crate::solve::{inspect, EvalCtxt, SolverMode};
+use crate::solve::{EvalCtxt, SolverMode};
 use rustc_hir::def_id::DefId;
 use rustc_infer::traits::query::NoSolution;
 use rustc_middle::bug;
@@ -16,7 +16,6 @@
 use rustc_middle::ty::{ToPredicate, TypeVisitableExt};
 use rustc_span::{ErrorGuaranteed, DUMMY_SP};
 use std::fmt::Debug;
-use std::mem;
 
 pub(super) mod structural_traits;
 
@@ -792,17 +791,16 @@ fn discard_impls_shadowed_by_env<G: GoalKind<'tcx>>(
         goal: Goal<'tcx, G>,
         candidates: &mut Vec<Candidate<'tcx>>,
     ) {
-        // HACK: We temporarily remove the `ProofTreeBuilder` to
-        // avoid adding `Trait` candidates to the candidates used
-        // to prove the current goal.
-        let inspect = mem::replace(&mut self.inspect, inspect::ProofTreeBuilder::new_noop());
-
         let tcx = self.tcx();
         let trait_goal: Goal<'tcx, ty::TraitPredicate<'tcx>> =
             goal.with(tcx, goal.predicate.trait_ref(tcx));
-        let mut trait_candidates_from_env = Vec::new();
+
-        self.assemble_param_env_candidates(trait_goal, &mut trait_candidates_from_env);
+        let mut trait_candidates_from_env = vec![];
-        self.assemble_alias_bound_candidates(trait_goal, &mut trait_candidates_from_env);
+        self.probe(|_| ProbeKind::ShadowedEnvProbing).enter(|ecx| {
+            ecx.assemble_param_env_candidates(trait_goal, &mut trait_candidates_from_env);
+            ecx.assemble_alias_bound_candidates(trait_goal, &mut trait_candidates_from_env);
+        });
+
         if !trait_candidates_from_env.is_empty() {
             let trait_env_result = self.merge_candidates(trait_candidates_from_env);
             match trait_env_result.unwrap().value.certainty {
@@ -831,7 +829,6 @@ fn discard_impls_shadowed_by_env<G: GoalKind<'tcx>>(
                 }
             }
         }
-        self.inspect = inspect;
     }
 
     /// If there are multiple ways to prove a trait or projection goal, we have

compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs

@@ -176,7 +176,8 @@ fn to_selection<'tcx>(
         | ProbeKind::UnsizeAssembly
         | ProbeKind::UpcastProjectionCompatibility
         | ProbeKind::OpaqueTypeStorageLookup { result: _ }
-        | ProbeKind::Root { result: _ } => {
+        | ProbeKind::Root { result: _ }
+        | ProbeKind::ShadowedEnvProbing => {
             span_bug!(span, "didn't expect to assemble trait candidate from {:#?}", cand.kind())
         }
     })

compiler/rustc_trait_selection/src/solve/inspect/analyse.rs

@@ -18,8 +18,8 @@
 use rustc_middle::traits::solve::{inspect, QueryResult};
 use rustc_middle::traits::solve::{Certainty, Goal};
 use rustc_middle::traits::ObligationCause;
-use rustc_middle::ty;
 use rustc_middle::ty::TypeFoldable;
+use rustc_middle::{bug, ty};
 use rustc_span::{Span, DUMMY_SP};
 
 use crate::solve::eval_ctxt::canonical;
@@ -290,6 +290,17 @@ fn candidates_recur(
             match *step {
                 inspect::ProbeStep::AddGoal(source, goal) => nested_goals.push((source, goal)),
                 inspect::ProbeStep::NestedProbe(ref probe) => {
+                    match probe.kind {
+                        // These never assemble candidates for the goal we're trying to solve.
+                        inspect::ProbeKind::UpcastProjectionCompatibility
+                        | inspect::ProbeKind::ShadowedEnvProbing => continue,
+
+                        inspect::ProbeKind::NormalizedSelfTyAssembly
+                        | inspect::ProbeKind::UnsizeAssembly
+                        | inspect::ProbeKind::Root { .. }
+                        | inspect::ProbeKind::TryNormalizeNonRigid { .. }
+                        | inspect::ProbeKind::TraitCandidate { .. }
+                        | inspect::ProbeKind::OpaqueTypeStorageLookup { .. } => {
                             // Nested probes have to prove goals added in their parent
                             // but do not leak them, so we truncate the added goals
                             // afterwards.
@@ -297,6 +308,8 @@ fn candidates_recur(
                             self.candidates_recur(candidates, nested_goals, probe);
                             nested_goals.truncate(num_goals);
                         }
+                    }
+                }
                 inspect::ProbeStep::MakeCanonicalResponse { shallow_certainty: c } => {
                     assert_eq!(shallow_certainty.replace(c), None);
                 }
@@ -308,9 +321,10 @@ fn candidates_recur(
         }
 
         match probe.kind {
-            inspect::ProbeKind::NormalizedSelfTyAssembly
+            inspect::ProbeKind::UpcastProjectionCompatibility
-            | inspect::ProbeKind::UnsizeAssembly
+            | inspect::ProbeKind::ShadowedEnvProbing => bug!(),
-            | inspect::ProbeKind::UpcastProjectionCompatibility => (),
+
+            inspect::ProbeKind::NormalizedSelfTyAssembly | inspect::ProbeKind::UnsizeAssembly => {}
 
             // We add a candidate even for the root evaluation if there
             // is only one way to prove a given goal, e.g. for `WellFormed`.