Auto merge of rust-lang#115996 - lcnr:intercrate_ambiguity_causes-uwu…

…, r=compiler-errors

implement `intercrate_ambiguity_causes` in the new solver

I added some comments but this is still somewhat of a mess. I think we should for the most part be able to treat all of this as a black box, so I can accept that this code isn't too great.

I also believe that some of the weirdness here is unavoidable, as proof trees - and their visitor - hide semantically relevant information, so they cannot perfectly represent the actual solver behavior.

There are some known bugs here when testing with `./x.py test tests/ui --bless -- --target-rustcflags -Ztrait-solver=next-coherence`. While I haven't diagnosed them all in detail I believe we are able to handle them all separately

- `structurally_normalize` currently does not normalize opaque types, resulting in divergence between the solver internal `trait_ref_is_knowable` and the one when computing intercrate ambiguity causes.
- we don't add an `intercrate_ambiguity_cause` for reserved impls
- we should `deeply_normalize` the trait ref before printing it, that requires a "best effort" version of `deeply_normalize`

r? `@compiler-errors`

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

@@ -1,30 +1,70 @@
+//! Data structure used to inspect trait solver behavior.
+//!
+//! During trait solving we optionally build "proof trees", the root of
+//! which is a [GoalEvaluation] with [GoalEvaluationKind::Root]. These
+//! trees are used to improve the debug experience and are also used by
+//! the compiler itself to provide necessary context for error messages.
+//!
+//! Because each nested goal in the solver gets [canonicalized] separately
+//! and we discard inference progress via "probes", we cannot mechanically
+//! use proof trees without somehow "lifting up" data local to the current
+//! `InferCtxt`. Any data used mechanically is therefore canonicalized and
+//! stored as [CanonicalState]. As printing canonicalized data worsens the
+//! debugging dumps, we do not simply canonicalize everything.
+//!
+//! This means proof trees contain inference variables and placeholders
+//! local to a different `InferCtxt` which must not be used with the
+//! current one.
+//!
+//! [canonicalized]: https://rustc-dev-guide.rust-lang.org/solve/canonicalization.html
+
 use super::{
-    CandidateSource, CanonicalInput, Certainty, Goal, IsNormalizesToHack, NoSolution, QueryInput,
-    QueryResult,
+    CandidateSource, Canonical, CanonicalInput, Certainty, Goal, IsNormalizesToHack, NoSolution,
+    QueryInput, QueryResult,
 };
-use crate::ty;
+use crate::{infer::canonical::CanonicalVarValues, ty};
 use format::ProofTreeFormatter;
 use std::fmt::{Debug, Write};
 
 mod format;
 
+/// Some `data` together with information about how they relate to the input
+/// of the canonical query.
+///
+/// This is only ever used as [CanonicalState]. Any type information in proof
+/// trees used mechanically has to be canonicalized as we otherwise leak
+/// inference variables from a nested `InferCtxt`.
+#[derive(Debug, Clone, Copy, Eq, PartialEq, TypeFoldable, TypeVisitable)]
+pub struct State<'tcx, T> {
+    pub var_values: CanonicalVarValues<'tcx>,
+    pub data: T,
+}
+
+pub type CanonicalState<'tcx, T> = Canonical<'tcx, State<'tcx, T>>;
+
 #[derive(Debug, Eq, PartialEq)]
 pub enum CacheHit {
     Provisional,
     Global,
 }
 
+/// When evaluating the root goals we also store the
+/// original values for the `CanonicalVarValues` of the
+/// canonicalized goal. We use this to map any [CanonicalState]
+/// from the local `InferCtxt` of the solver query to
+/// the `InferCtxt` of the caller.
 #[derive(Eq, PartialEq)]
-pub enum GoalEvaluationKind {
-    Root,
+pub enum GoalEvaluationKind<'tcx> {
+    Root { orig_values: Vec<ty::GenericArg<'tcx>> },
     Nested { is_normalizes_to_hack: IsNormalizesToHack },
 }
 
 #[derive(Eq, PartialEq)]
 pub struct GoalEvaluation<'tcx> {
     pub uncanonicalized_goal: Goal<'tcx, ty::Predicate<'tcx>>,
-    pub kind: GoalEvaluationKind,
+    pub kind: GoalEvaluationKind<'tcx>,
     pub evaluation: CanonicalGoalEvaluation<'tcx>,
+    /// The nested goals from instantiating the query response.
     pub returned_goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 }
 
@@ -66,6 +106,7 @@ pub struct GoalEvaluationStep<'tcx> {
 /// of a goal.
 #[derive(Eq, PartialEq)]
 pub struct Probe<'tcx> {
+    /// What happened inside of this probe in chronological order.
     pub steps: Vec<ProbeStep<'tcx>>,
     pub kind: ProbeKind<'tcx>,
 }
@@ -78,12 +119,21 @@ impl Debug for Probe<'_> {
 
 #[derive(Eq, PartialEq)]
 pub enum ProbeStep<'tcx> {
-    AddGoal(Goal<'tcx, ty::Predicate<'tcx>>),
+    /// We added a goal to the `EvalCtxt` which will get proven
+    /// the next time `EvalCtxt::try_evaluate_added_goals` is called.
+    AddGoal(CanonicalState<'tcx, Goal<'tcx, ty::Predicate<'tcx>>>),
+    /// The inside of a `EvalCtxt::try_evaluate_added_goals` call.
     EvaluateGoals(AddedGoalsEvaluation<'tcx>),
+    /// A call to `probe` while proving the current goal. This is
+    /// used whenever there are multiple candidates to prove the
+    /// current goalby .
     NestedProbe(Probe<'tcx>),
 }
 
-#[derive(Debug, PartialEq, Eq)]
+/// What kind of probe we're in. In case the probe represents a candidate, or
+/// the final result of the current goal - via [ProbeKind::Root] - we also
+/// store the [QueryResult].
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
 pub enum ProbeKind<'tcx> {
     /// The root inference context while proving a goal.
     Root { result: QueryResult<'tcx> },

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

@@ -41,7 +41,7 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
 
     pub(super) fn format_goal_evaluation(&mut self, eval: &GoalEvaluation<'_>) -> std::fmt::Result {
         let goal_text = match eval.kind {
-            GoalEvaluationKind::Root => "ROOT GOAL",
+            GoalEvaluationKind::Root { orig_values: _ } => "ROOT GOAL",
             GoalEvaluationKind::Nested { is_normalizes_to_hack } => match is_normalizes_to_hack {
                 IsNormalizesToHack::No => "GOAL",
                 IsNormalizesToHack::Yes => "NORMALIZES-TO HACK GOAL",

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -344,7 +344,8 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         goal: Goal<'tcx, ty::Predicate<'tcx>>,
     ) -> Result<(bool, Certainty, Vec<Goal<'tcx, ty::Predicate<'tcx>>>), NoSolution> {
         let (orig_values, canonical_goal) = self.canonicalize_goal(goal);
-        let mut goal_evaluation = self.inspect.new_goal_evaluation(goal, goal_evaluation_kind);
+        let mut goal_evaluation =
+            self.inspect.new_goal_evaluation(goal, &orig_values, goal_evaluation_kind);
         let encountered_overflow = self.search_graph.encountered_overflow();
         let canonical_response = EvalCtxt::evaluate_canonical_goal(
             self.tcx(),
@@ -568,7 +569,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 GoalEvaluationKind::Nested { is_normalizes_to_hack: IsNormalizesToHack::Yes },
                 unconstrained_goal,
             )?;
-            self.add_goals(instantiate_goals);
+            self.nested_goals.goals.extend(instantiate_goals);
 
             // Finally, equate the goal's RHS with the unconstrained var.
             // We put the nested goals from this into goals instead of
@@ -605,15 +606,15 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 GoalEvaluationKind::Nested { is_normalizes_to_hack: IsNormalizesToHack::No },
                 goal,
             )?;
-            self.add_goals(instantiate_goals);
+            self.nested_goals.goals.extend(instantiate_goals);
             if has_changed {
                 unchanged_certainty = None;
             }
 
             match certainty {
                 Certainty::Yes => {}
                 Certainty::Maybe(_) => {
-                    self.add_goal(goal);
+                    self.nested_goals.goals.push(goal);
                     unchanged_certainty = unchanged_certainty.map(|c| c.unify_with(certainty));
                 }
             }

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

@@ -10,17 +10,21 @@
 //! [c]: https://rustc-dev-guide.rust-lang.org/solve/canonicalization.html
 use super::{CanonicalInput, Certainty, EvalCtxt, Goal};
 use crate::solve::canonicalize::{CanonicalizeMode, Canonicalizer};
-use crate::solve::{response_no_constraints_raw, CanonicalResponse, QueryResult, Response};
+use crate::solve::{
+    inspect, response_no_constraints_raw, CanonicalResponse, QueryResult, Response,
+};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_index::IndexVec;
 use rustc_infer::infer::canonical::query_response::make_query_region_constraints;
 use rustc_infer::infer::canonical::CanonicalVarValues;
 use rustc_infer::infer::canonical::{CanonicalExt, QueryRegionConstraints};
-use rustc_infer::infer::InferCtxt;
+use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, InferOk};
+use rustc_middle::infer::canonical::Canonical;
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::solve::{
     ExternalConstraintsData, MaybeCause, PredefinedOpaquesData, QueryInput,
 };
+use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::{
     self, BoundVar, GenericArgKind, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable,
     TypeVisitableExt,
@@ -29,6 +33,22 @@ use rustc_span::DUMMY_SP;
 use std::iter;
 use std::ops::Deref;
 
+trait ResponseT<'tcx> {
+    fn var_values(&self) -> CanonicalVarValues<'tcx>;
+}
+
+impl<'tcx> ResponseT<'tcx> for Response<'tcx> {
+    fn var_values(&self) -> CanonicalVarValues<'tcx> {
+        self.var_values
+    }
+}
+
+impl<'tcx, T> ResponseT<'tcx> for inspect::State<'tcx, T> {
+    fn var_values(&self) -> CanonicalVarValues<'tcx> {
+        self.var_values
+    }
+}
+
 impl<'tcx> EvalCtxt<'_, 'tcx> {
     /// Canonicalizes the goal remembering the original values
     /// for each bound variable.
@@ -188,12 +208,14 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         original_values: Vec<ty::GenericArg<'tcx>>,
         response: CanonicalResponse<'tcx>,
     ) -> Result<(Certainty, Vec<Goal<'tcx, ty::Predicate<'tcx>>>), NoSolution> {
-        let substitution = self.compute_query_response_substitution(&original_values, &response);
+        let substitution =
+            Self::compute_query_response_substitution(self.infcx, &original_values, &response);
 
         let Response { var_values, external_constraints, certainty } =
             response.substitute(self.tcx(), &substitution);
 
-        let nested_goals = self.unify_query_var_values(param_env, &original_values, var_values)?;
+        let nested_goals =
+            Self::unify_query_var_values(self.infcx, param_env, &original_values, var_values)?;
 
         let ExternalConstraintsData { region_constraints, opaque_types } =
             external_constraints.deref();
@@ -206,21 +228,21 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     /// This returns the substitutions to instantiate the bound variables of
     /// the canonical response. This depends on the `original_values` for the
     /// bound variables.
-    fn compute_query_response_substitution(
-        &self,
+    fn compute_query_response_substitution<T: ResponseT<'tcx>>(
+        infcx: &InferCtxt<'tcx>,
         original_values: &[ty::GenericArg<'tcx>],
-        response: &CanonicalResponse<'tcx>,
+        response: &Canonical<'tcx, T>,
     ) -> CanonicalVarValues<'tcx> {
         // FIXME: Longterm canonical queries should deal with all placeholders
         // created inside of the query directly instead of returning them to the
         // caller.
-        let prev_universe = self.infcx.universe();
+        let prev_universe = infcx.universe();
         let universes_created_in_query = response.max_universe.index();
         for _ in 0..universes_created_in_query {
-            self.infcx.create_next_universe();
+            infcx.create_next_universe();
         }
 
-        let var_values = response.value.var_values;
+        let var_values = response.value.var_values();
         assert_eq!(original_values.len(), var_values.len());
 
         // If the query did not make progress with constraining inference variables,
@@ -254,13 +276,13 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             }
         }
 
-        let var_values = self.tcx().mk_args_from_iter(response.variables.iter().enumerate().map(
+        let var_values = infcx.tcx.mk_args_from_iter(response.variables.iter().enumerate().map(
             |(index, info)| {
                 if info.universe() != ty::UniverseIndex::ROOT {
                     // A variable from inside a binder of the query. While ideally these shouldn't
                     // exist at all (see the FIXME at the start of this method), we have to deal with
                     // them for now.
-                    self.infcx.instantiate_canonical_var(DUMMY_SP, info, |idx| {
+                    infcx.instantiate_canonical_var(DUMMY_SP, info, |idx| {
                         ty::UniverseIndex::from(prev_universe.index() + idx.index())
                     })
                 } else if info.is_existential() {
@@ -274,7 +296,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
                     if let Some(v) = opt_values[BoundVar::from_usize(index)] {
                         v
                     } else {
-                        self.infcx.instantiate_canonical_var(DUMMY_SP, info, |_| prev_universe)
+                        infcx.instantiate_canonical_var(DUMMY_SP, info, |_| prev_universe)
                     }
                 } else {
                     // For placeholders which were already part of the input, we simply map this
@@ -287,9 +309,9 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         CanonicalVarValues { var_values }
     }
 
-    #[instrument(level = "debug", skip(self, param_env), ret)]
+    #[instrument(level = "debug", skip(infcx, param_env), ret)]
     fn unify_query_var_values(
-        &self,
+        infcx: &InferCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         original_values: &[ty::GenericArg<'tcx>],
         var_values: CanonicalVarValues<'tcx>,
@@ -298,7 +320,18 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 
         let mut nested_goals = vec![];
         for (&orig, response) in iter::zip(original_values, var_values.var_values) {
-            nested_goals.extend(self.eq_and_get_goals(param_env, orig, response)?);
+            nested_goals.extend(
+                infcx
+                    .at(&ObligationCause::dummy(), param_env)
+                    .eq(DefineOpaqueTypes::No, orig, response)
+                    .map(|InferOk { value: (), obligations }| {
+                        obligations.into_iter().map(|o| Goal::from(o))
+                    })
+                    .map_err(|e| {
+                        debug!(?e, "failed to equate");
+                        NoSolution
+                    })?,
+            );
         }
 
         Ok(nested_goals)
@@ -403,3 +436,35 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for EagerResolver<'_, 'tcx> {
         }
     }
 }
+
+impl<'tcx> inspect::ProofTreeBuilder<'tcx> {
+    pub fn make_canonical_state<T: TypeFoldable<TyCtxt<'tcx>>>(
+        ecx: &EvalCtxt<'_, 'tcx>,
+        data: T,
+    ) -> inspect::CanonicalState<'tcx, T> {
+        let state = inspect::State { var_values: ecx.var_values, data };
+        let state = state.fold_with(&mut EagerResolver { infcx: ecx.infcx });
+        Canonicalizer::canonicalize(
+            ecx.infcx,
+            CanonicalizeMode::Response { max_input_universe: ecx.max_input_universe },
+            &mut vec![],
+            state,
+        )
+    }
+
+    pub fn instantiate_canonical_state<T: TypeFoldable<TyCtxt<'tcx>>>(
+        infcx: &InferCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        original_values: &[ty::GenericArg<'tcx>],
+        state: inspect::CanonicalState<'tcx, T>,
+    ) -> Result<(Vec<Goal<'tcx, ty::Predicate<'tcx>>>, T), NoSolution> {
+        let substitution =
+            EvalCtxt::compute_query_response_substitution(infcx, original_values, &state);
+
+        let inspect::State { var_values, data } = state.substitute(infcx.tcx, &substitution);
+
+        let nested_goals =
+            EvalCtxt::unify_query_var_values(infcx, param_env, original_values, var_values)?;
+        Ok((nested_goals, data))
+    }
+}