Rollup merge of #123363 - lcnr:normalizes-to-zero-to-inf, r=BoxyUwU

change `NormalizesTo` to fully structurally normalize

notes in https://hackmd.io/wZ016dE4QKGIhrOnHLlThQ

need to also update the dev-guide once this PR lands. in short, the setup is now as follows:

`normalizes-to` internally implements one step normalization, applying that normalization to the `goal.predicate.term` causes the projected term to get recursively normalized. With this `normalizes-to` normalizes until the projected term is rigid, meaning that we normalize as many steps necessary, but at least 1.

To handle rigid aliases, we add another candidate only if the 1 to inf step normalization failed. With this `normalizes-to` is now full structural normalization. We can now change `AliasRelate` to simply emit `normalizes-to` goals for the rhs and lhs.

This avoids the concerns from rust-lang/trait-system-refactor-initiative#103 and generally feels cleaner

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

@@ -121,8 +121,6 @@ pub enum ProbeStep<'tcx> {
     /// used whenever there are multiple candidates to prove the
     /// current goalby .
     NestedProbe(Probe<'tcx>),
-    CommitIfOkStart,
-    CommitIfOkSuccess,
 }
 
 /// What kind of probe we're in. In case the probe represents a candidate, or
@@ -132,6 +130,8 @@ pub enum ProbeStep<'tcx> {
 pub enum ProbeKind<'tcx> {
     /// The root inference context while proving a goal.
     Root { result: QueryResult<'tcx> },
+    /// Trying to normalize an alias by at least one stpe in `NormalizesTo`.
+    TryNormalizeNonRigid { result: QueryResult<'tcx> },
     /// Probe entered when normalizing the self ty during candidate assembly
     NormalizedSelfTyAssembly,
     /// Some candidate to prove the current goal.
@@ -143,9 +143,6 @@ pub enum ProbeKind<'tcx> {
     /// Used in the probe that wraps normalizing the non-self type for the unsize
     /// trait, which is also structurally matched on.
     UnsizeAssembly,
-    /// A call to `EvalCtxt::commit_if_ok` which failed, causing the work
-    /// to be discarded.
-    CommitIfOk,
     /// During upcasting from some source object to target object type, used to
     /// 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.

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

@@ -100,6 +100,9 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
             ProbeKind::Root { result } => {
                 write!(self.f, "ROOT RESULT: {result:?}")
             }
+            ProbeKind::TryNormalizeNonRigid { result } => {
+                write!(self.f, "TRY NORMALIZE NON-RIGID: {result:?}")
+            }
             ProbeKind::NormalizedSelfTyAssembly => {
                 write!(self.f, "NORMALIZING SELF TY FOR ASSEMBLY:")
             }
@@ -109,9 +112,6 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
             ProbeKind::UpcastProjectionCompatibility => {
                 write!(self.f, "PROBING FOR PROJECTION COMPATIBILITY FOR UPCASTING:")
             }
-            ProbeKind::CommitIfOk => {
-                write!(self.f, "COMMIT_IF_OK:")
-            }
             ProbeKind::MiscCandidate { name, result } => {
                 write!(self.f, "CANDIDATE {name}: {result:?}")
             }
@@ -132,8 +132,6 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
                     }
                     ProbeStep::EvaluateGoals(eval) => this.format_added_goals_evaluation(eval)?,
                     ProbeStep::NestedProbe(probe) => this.format_probe(probe)?,
-                    ProbeStep::CommitIfOkStart => writeln!(this.f, "COMMIT_IF_OK START")?,
-                    ProbeStep::CommitIfOkSuccess => writeln!(this.f, "COMMIT_IF_OK SUCCESS")?,
                 }
             }
             Ok(())

compiler/rustc_trait_selection/src/solve/alias_relate.rs

@@ -2,8 +2,8 @@
 //! Doing this via a separate goal is called "deferred alias relation" and part
 //! of our more general approach to "lazy normalization".
 //!
-//! This is done by first normalizing both sides of the goal, ending up in
-//! either a concrete type, rigid alias, or an infer variable.
+//! This is done by first structurally normalizing both sides of the goal, ending
+//! up in either a concrete type, rigid alias, or an infer variable.
 //! These are related further according to the rules below:
 //!
 //! (1.) If we end up with two rigid aliases, then we relate them structurally.
@@ -14,18 +14,10 @@
 //!
 //! (3.) Otherwise, if we end with two rigid (non-projection) or infer types,
 //! relate them structurally.
-//!
-//! Subtle: when relating an opaque to another type, we emit a
-//! `NormalizesTo(opaque, ?fresh_var)` goal when trying to normalize the opaque.
-//! This nested goal starts out as ambiguous and does not actually define the opaque.
-//! However, if `?fresh_var` ends up geteting equated to another type, we retry the
-//! `NormalizesTo` goal, at which point the opaque is actually defined.
 
 use super::EvalCtxt;
-use rustc_infer::traits::query::NoSolution;
-use rustc_infer::traits::solve::GoalSource;
 use rustc_middle::traits::solve::{Certainty, Goal, QueryResult};
-use rustc_middle::ty::{self, Ty};
+use rustc_middle::ty;
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
     #[instrument(level = "debug", skip(self), ret)]
@@ -36,141 +28,58 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         let tcx = self.tcx();
         let Goal { param_env, predicate: (lhs, rhs, direction) } = goal;
 
-        let Some(lhs) = self.try_normalize_term(param_env, lhs)? else {
-            return self
-                .evaluate_added_goals_and_make_canonical_response(Certainty::overflow(true));
+        // Structurally normalize the lhs.
+        let lhs = if let Some(alias) = lhs.to_alias_ty(self.tcx()) {
+            let term = self.next_term_infer_of_kind(lhs);
+            self.add_normalizes_to_goal(goal.with(tcx, ty::NormalizesTo { alias, term }));
+            term
+        } else {
+            lhs
         };
 
-        let Some(rhs) = self.try_normalize_term(param_env, rhs)? else {
-            return self
-                .evaluate_added_goals_and_make_canonical_response(Certainty::overflow(true));
+        // Structurally normalize the rhs.
+        let rhs = if let Some(alias) = rhs.to_alias_ty(self.tcx()) {
+            let term = self.next_term_infer_of_kind(rhs);
+            self.add_normalizes_to_goal(goal.with(tcx, ty::NormalizesTo { alias, term }));
+            term
+        } else {
+            rhs
         };
 
+        // Apply the constraints.
+        self.try_evaluate_added_goals()?;
+        let lhs = self.resolve_vars_if_possible(lhs);
+        let rhs = self.resolve_vars_if_possible(rhs);
+        debug!(?lhs, ?rhs);
+
         let variance = match direction {
             ty::AliasRelationDirection::Equate => ty::Variance::Invariant,
             ty::AliasRelationDirection::Subtype => ty::Variance::Covariant,
         };
-
         match (lhs.to_alias_ty(tcx), rhs.to_alias_ty(tcx)) {
             (None, None) => {
                 self.relate(param_env, lhs, variance, rhs)?;
                 self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
             }
 
             (Some(alias), None) => {
-                self.relate_rigid_alias_non_alias(param_env, alias, variance, rhs)
+                self.relate_rigid_alias_non_alias(param_env, alias, variance, rhs)?;
+                self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+            }
+            (None, Some(alias)) => {
+                self.relate_rigid_alias_non_alias(
+                    param_env,
+                    alias,
+                    variance.xform(ty::Variance::Contravariant),
+                    lhs,
+                )?;
+                self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
             }
-            (None, Some(alias)) => self.relate_rigid_alias_non_alias(
-                param_env,
-                alias,
-                variance.xform(ty::Variance::Contravariant),
-                lhs,
-            ),
 
             (Some(alias_lhs), Some(alias_rhs)) => {
                 self.relate(param_env, alias_lhs, variance, alias_rhs)?;
                 self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
             }
         }
     }
-
-    /// Relate a rigid alias with another type. This is the same as
-    /// an ordinary relate except that we treat the outer most alias
-    /// constructor as rigid.
-    #[instrument(level = "debug", skip(self, param_env), ret)]
-    fn relate_rigid_alias_non_alias(
-        &mut self,
-        param_env: ty::ParamEnv<'tcx>,
-        alias: ty::AliasTy<'tcx>,
-        variance: ty::Variance,
-        term: ty::Term<'tcx>,
-    ) -> QueryResult<'tcx> {
-        // NOTE: this check is purely an optimization, the structural eq would
-        // always fail if the term is not an inference variable.
-        if term.is_infer() {
-            let tcx = self.tcx();
-            // We need to relate `alias` to `term` treating only the outermost
-            // constructor as rigid, relating any contained generic arguments as
-            // normal. We do this by first structurally equating the `term`
-            // with the alias constructor instantiated with unconstrained infer vars,
-            // and then relate this with the whole `alias`.
-            //
-            // Alternatively we could modify `Equate` for this case by adding another
-            // variant to `StructurallyRelateAliases`.
-            let identity_args = self.fresh_args_for_item(alias.def_id);
-            let rigid_ctor = ty::AliasTy::new(tcx, alias.def_id, identity_args);
-            self.eq_structurally_relating_aliases(param_env, term, rigid_ctor.to_ty(tcx).into())?;
-            self.eq(param_env, alias, rigid_ctor)?;
-            self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-        } else {
-            Err(NoSolution)
-        }
-    }
-
-    // FIXME: This needs a name that reflects that it's okay to bottom-out with an inference var.
-    /// Normalize the `term` to equate it later.
-    #[instrument(level = "debug", skip(self, param_env), ret)]
-    fn try_normalize_term(
-        &mut self,
-        param_env: ty::ParamEnv<'tcx>,
-        term: ty::Term<'tcx>,
-    ) -> Result<Option<ty::Term<'tcx>>, NoSolution> {
-        match term.unpack() {
-            ty::TermKind::Ty(ty) => {
-                Ok(self.try_normalize_ty_recur(param_env, 0, ty).map(Into::into))
-            }
-            ty::TermKind::Const(_) => {
-                if let Some(alias) = term.to_alias_ty(self.tcx()) {
-                    let term = self.next_term_infer_of_kind(term);
-                    self.add_normalizes_to_goal(Goal::new(
-                        self.tcx(),
-                        param_env,
-                        ty::NormalizesTo { alias, term },
-                    ));
-                    self.try_evaluate_added_goals()?;
-                    Ok(Some(self.resolve_vars_if_possible(term)))
-                } else {
-                    Ok(Some(term))
-                }
-            }
-        }
-    }
-
-    #[instrument(level = "debug", skip(self, param_env), ret)]
-    fn try_normalize_ty_recur(
-        &mut self,
-        param_env: ty::ParamEnv<'tcx>,
-        depth: usize,
-        ty: Ty<'tcx>,
-    ) -> Option<Ty<'tcx>> {
-        if !self.tcx().recursion_limit().value_within_limit(depth) {
-            return None;
-        }
-
-        let ty::Alias(kind, alias) = *ty.kind() else {
-            return Some(ty);
-        };
-
-        match self.commit_if_ok(|this| {
-            let tcx = this.tcx();
-            let normalized_ty = this.next_ty_infer();
-            let normalizes_to = ty::NormalizesTo { alias, term: normalized_ty.into() };
-            match kind {
-                ty::AliasKind::Opaque => {
-                    // HACK: Unlike for associated types, `normalizes-to` for opaques
-                    // is currently not treated as a function. We do not erase the
-                    // expected term.
-                    this.add_goal(GoalSource::Misc, Goal::new(tcx, param_env, normalizes_to));
-                }
-                ty::AliasKind::Projection | ty::AliasKind::Inherent | ty::AliasKind::Weak => {
-                    this.add_normalizes_to_goal(Goal::new(tcx, param_env, normalizes_to))
-                }
-            }
-            this.try_evaluate_added_goals()?;
-            Ok(this.resolve_vars_if_possible(normalized_ty))
-        }) {
-            Ok(ty) => self.try_normalize_ty_recur(param_env, depth + 1, ty),
-            Err(NoSolution) => Some(ty),
-        }
-    }
 }

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

@@ -332,7 +332,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     /// whether an alias is rigid by using the trait solver. When instantiating a response
     /// from the solver we assume that the solver correctly handled aliases and therefore
     /// always relate them structurally here.
-    #[instrument(level = "debug", skip(infcx), ret)]
+    #[instrument(level = "debug", skip(infcx))]
     fn unify_query_var_values(
         infcx: &InferCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,